diff --git a/nanobot/agent/loop.py b/nanobot/agent/loop.py
index 4dcecfc5..6ffade73 100644
--- a/nanobot/agent/loop.py
+++ b/nanobot/agent/loop.py
@@ -858,15 +858,17 @@ class AgentLoop:
                 )
             )
 
-        # Persist the triggering user message immediately, before running the
-        # agent loop. If the process is killed mid-turn (OOM, SIGKILL, self-
-        # restart, etc.), the existing runtime_checkpoint preserves the
-        # in-flight assistant/tool state but NOT the user message itself, so
-        # the user's prompt is silently lost on recovery. Saving it up front
-        # makes recovery possible from the session log alone.
+        # Persist the triggering user message up front so a mid-turn crash
+        # doesn't silently lose the prompt on recovery. ``media`` rides along
+        # as raw on-disk paths — sanitized image blocks are stripped from
+        # JSONL, and webui replay needs the paths to mint signed URLs.
         user_persisted_early = False
-        if isinstance(msg.content, str) and msg.content.strip():
-            session.add_message("user", msg.content)
+        media_paths = [p for p in (msg.media or []) if isinstance(p, str) and p]
+        has_text = isinstance(msg.content, str) and msg.content.strip()
+        if has_text or media_paths:
+            extra: dict[str, Any] = {"media": list(media_paths)} if media_paths else {}
+            text = msg.content if isinstance(msg.content, str) else ""
+            session.add_message("user", text, **extra)
             self._mark_pending_user_turn(session)
             self.sessions.save(session)
             user_persisted_early = True
diff --git a/nanobot/api/server.py b/nanobot/api/server.py
index ebdee557..92e7be90 100644
--- a/nanobot/api/server.py
+++ b/nanobot/api/server.py
@@ -7,13 +7,9 @@ All requests route to a single persistent API session.
 from __future__ import annotations
 
 import asyncio
-import base64
 import json as _json
-import mimetypes
-import re
 import time
 import uuid
-from pathlib import Path
 from typing import Any
 
 from aiohttp import web
@@ -21,14 +17,20 @@ from loguru import logger
 
 from nanobot.config.paths import get_media_dir
 from nanobot.utils.helpers import safe_filename
+from nanobot.utils.media_decode import (
+    FileSizeExceeded as _FileSizeExceeded,
+    MAX_FILE_SIZE,
+    save_base64_data_url as _save_base64_data_url,
+)
 from nanobot.utils.runtime import EMPTY_FINAL_RESPONSE_MESSAGE
 
-MAX_FILE_SIZE = 10 * 1024 * 1024  # 10 MB
-_DATA_URL_RE = re.compile(r"^data:([^;]+);base64,(.+)$", re.DOTALL)
-
-
-class _FileSizeExceeded(Exception):
-    """Raised when an uploaded file exceeds the size limit."""
+__all__ = (
+    "MAX_FILE_SIZE",
+    "_FileSizeExceeded",
+    "_save_base64_data_url",
+    "create_app",
+    "handle_chat_completions",
+)
 
 
 API_SESSION_KEY = "api:default"
@@ -102,25 +104,6 @@ _SSE_DONE = b"data: [DONE]\n\n"
 # ---------------------------------------------------------------------------
 
 
-def _save_base64_data_url(data_url: str, media_dir: Path) -> str | None:
-    """Decode a data:...;base64,... URL and save to disk."""
-    m = _DATA_URL_RE.match(data_url)
-    if not m:
-        return None
-    mime_type, b64_payload = m.group(1), m.group(2)
-    try:
-        raw = base64.b64decode(b64_payload)
-    except Exception:
-        return None
-    if len(raw) > MAX_FILE_SIZE:
-        raise _FileSizeExceeded(f"File exceeds {MAX_FILE_SIZE // (1024 * 1024)}MB limit")
-    ext = mimetypes.guess_extension(mime_type) or ".bin"
-    filename = f"{uuid.uuid4().hex[:12]}{ext}"
-    dest = media_dir / safe_filename(filename)
-    dest.write_bytes(raw)
-    return str(dest)
-
-
 def _parse_json_content(body: dict) -> tuple[str, list[str]]:
     """Parse JSON request body. Returns (text, media_paths)."""
     messages = body.get("messages")
diff --git a/nanobot/channels/websocket.py b/nanobot/channels/websocket.py
index 639519a4..1622a37d 100644
--- a/nanobot/channels/websocket.py
+++ b/nanobot/channels/websocket.py
@@ -3,7 +3,10 @@
 from __future__ import annotations
 
 import asyncio
+import base64
+import binascii
 import email.utils
+import hashlib
 import hmac
 import http
 import json
@@ -28,7 +31,12 @@ from websockets.http11 import Response
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.channels.base import BaseChannel
+from nanobot.config.paths import get_media_dir
 from nanobot.config.schema import Base
+from nanobot.utils.media_decode import (
+    FileSizeExceeded,
+    save_base64_data_url,
+)
 
 if TYPE_CHECKING:
     from nanobot.session.manager import SessionManager
@@ -75,7 +83,11 @@ class WebSocketConfig(Base):
     websocket_requires_token: bool = True
     allow_from: list[str] = Field(default_factory=lambda: ["*"])
     streaming: bool = True
-    max_message_bytes: int = Field(default=1_048_576, ge=1024, le=16_777_216)
+    # Default 36 MB, upper 40 MB: supports up to 4 images at ~6 MB each after
+    # client-side Worker normalization (see webui Composer). 4 × 6 MB × 1.37
+    # (base64 overhead) + envelope framing stays under 36 MB; the 40 MB ceiling
+    # leaves a small margin for sender slop without opening a DoS avenue.
+    max_message_bytes: int = Field(default=37_748_736, ge=1024, le=41_943_040)
     ping_interval_s: float = Field(default=20.0, ge=5.0, le=300.0)
     ping_timeout_s: float = Field(default=20.0, ge=5.0, le=300.0)
     ssl_certfile: str = ""
@@ -206,6 +218,35 @@ def _parse_envelope(raw: str) -> dict[str, Any] | None:
     return data
 
 
+# Per-message image limits. The server-side guard is a touch looser than the
+# client's ``Worker`` normalization target (6 MB) — tolerate client slop, but
+# still cap total ingress at ``_MAX_IMAGES_PER_MESSAGE * _MAX_IMAGE_BYTES``
+# which fits comfortably inside ``max_message_bytes``.
+_MAX_IMAGES_PER_MESSAGE = 4
+_MAX_IMAGE_BYTES = 8 * 1024 * 1024
+
+# Image MIME whitelist — matches the Composer's ``accept`` list. SVG is
+# explicitly excluded to avoid the XSS surface inside embedded scripts.
+_IMAGE_MIME_ALLOWED: frozenset[str] = frozenset({
+    "image/png",
+    "image/jpeg",
+    "image/webp",
+    "image/gif",
+})
+
+_DATA_URL_MIME_RE = re.compile(r"^data:([^;]+);base64,", re.DOTALL)
+
+
+def _extract_data_url_mime(url: str) -> str | None:
+    """Return the MIME type of a ``data:<mime>;base64,...`` URL, else ``None``."""
+    if not isinstance(url, str):
+        return None
+    m = _DATA_URL_MIME_RE.match(url)
+    if not m:
+        return None
+    return m.group(1).strip().lower() or None
+
+
 _LOCALHOSTS = frozenset({"127.0.0.1", "::1", "localhost"})
 
 # Matches the legacy chat-id pattern but allows file-system-safe stems too,
@@ -278,6 +319,29 @@ def _is_websocket_upgrade(request: WsRequest) -> bool:
     return True
 
 
+def _b64url_encode(data: bytes) -> str:
+    """URL-safe base64 without padding — compact + friendly in URL paths."""
+    return base64.urlsafe_b64encode(data).rstrip(b"=").decode("ascii")
+
+
+def _b64url_decode(s: str) -> bytes:
+    """Reverse of :func:`_b64url_encode`; caller handles ``ValueError``."""
+    pad = "=" * (-len(s) % 4)
+    return base64.urlsafe_b64decode(s + pad)
+
+
+# Allowed MIME types we actually serve from the media endpoint. Anything
+# outside this set is degraded to ``application/octet-stream`` so an
+# attacker who somehow gets a signed URL for an unexpected file type can't
+# trick the browser into sniffing executable content.
+_MEDIA_ALLOWED_MIMES: frozenset[str] = frozenset({
+    "image/png",
+    "image/jpeg",
+    "image/webp",
+    "image/gif",
+})
+
+
 def _issue_route_secret_matches(headers: Any, configured_secret: str) -> bool:
     """Return True if the token-issue HTTP request carries credentials matching ``token_issue_secret``."""
     if not configured_secret:
@@ -326,6 +390,11 @@ class WebSocketChannel(BaseChannel):
         self._static_dist_path: Path | None = (
             static_dist_path.resolve() if static_dist_path is not None else None
         )
+        # Process-local secret used to HMAC-sign media URLs. The signed URL is
+        # the capability — anyone who holds a valid URL can fetch that one
+        # file, nothing else. The secret regenerates on restart so links
+        # become self-expiring (callers just refresh the session list).
+        self._media_secret: bytes = secrets.token_bytes(32)
 
     # -- Subscription bookkeeping -------------------------------------------
 
@@ -457,6 +526,14 @@ class WebSocketChannel(BaseChannel):
         if m:
             return self._handle_session_delete(request, m.group(1))
 
+        # Signed media fetch: ``<sig>`` is an HMAC over ``<payload>``; the
+        # payload decodes to a path inside :func:`get_media_dir`. See
+        # :meth:`_sign_media_path` for the inverse direction used to build
+        # these URLs when replaying a session.
+        m = re.match(r"^/api/media/([A-Za-z0-9_-]+)/([A-Za-z0-9_-]+)$", got)
+        if m:
+            return self._handle_media_fetch(m.group(1), m.group(2))
+
         # 4. WebSocket upgrade (the channel's primary purpose). Only run the
         # handshake gate on requests that actually ask to upgrade; otherwise
         # a bare ``GET /`` from the browser would be rejected as an
@@ -568,8 +645,109 @@ class WebSocketChannel(BaseChannel):
         data = self._session_manager.read_session_file(decoded_key)
         if data is None:
             return _http_error(404, "session not found")
+        # Decorate persisted user messages with signed media URLs so the
+        # client can render previews. The raw on-disk ``media`` paths are
+        # stripped on the way out — they leak server filesystem layout and
+        # the client never needs them once it has the signed fetch URL.
+        self._augment_media_urls(data)
         return _http_json_response(data)
 
+    def _augment_media_urls(self, payload: dict[str, Any]) -> None:
+        """Mutate *payload* in place: each message's ``media`` path list is
+        replaced by a parallel ``media_urls`` list of signed fetch URLs.
+
+        Messages without media or with non-string path entries are left
+        untouched. Paths that no longer live inside ``media_dir`` (e.g. the
+        file was deleted, or the dir was relocated) are silently skipped;
+        the client falls back to the historical-replay placeholder tile.
+        """
+        messages = payload.get("messages")
+        if not isinstance(messages, list):
+            return
+        for msg in messages:
+            if not isinstance(msg, dict):
+                continue
+            media = msg.get("media")
+            if not isinstance(media, list) or not media:
+                continue
+            urls: list[dict[str, str]] = []
+            for entry in media:
+                if not isinstance(entry, str) or not entry:
+                    continue
+                signed = self._sign_media_path(Path(entry))
+                if signed is None:
+                    continue
+                urls.append({"url": signed, "name": Path(entry).name})
+            if urls:
+                msg["media_urls"] = urls
+            # Always drop the raw paths from the wire payload.
+            msg.pop("media", None)
+
+    def _sign_media_path(self, abs_path: Path) -> str | None:
+        """Return a ``/api/media/<sig>/<payload>`` URL for *abs_path*, or
+        ``None`` when the path does not resolve inside the media root.
+
+        The URL is self-authenticating: the signature binds the payload to
+        this process's ``_media_secret``, so only paths we chose to sign can
+        be fetched. The returned path is relative to the server origin; the
+        client joins it against the existing webui base.
+        """
+        try:
+            media_root = get_media_dir().resolve()
+            rel = abs_path.resolve().relative_to(media_root)
+        except (OSError, ValueError):
+            return None
+        payload = _b64url_encode(rel.as_posix().encode("utf-8"))
+        mac = hmac.new(
+            self._media_secret, payload.encode("ascii"), hashlib.sha256
+        ).digest()[:16]
+        return f"/api/media/{_b64url_encode(mac)}/{payload}"
+
+    def _handle_media_fetch(self, sig: str, payload: str) -> Response:
+        """Serve a single media file previously signed via
+        :meth:`_sign_media_path`. Validates the signature, decodes the
+        payload to a relative path, and streams the file bytes with a
+        long-lived immutable cache header (the URL already encodes the
+        file identity, so caches can be aggressive)."""
+        try:
+            provided_mac = _b64url_decode(sig)
+        except (ValueError, binascii.Error):
+            return _http_error(401, "invalid signature")
+        expected_mac = hmac.new(
+            self._media_secret, payload.encode("ascii"), hashlib.sha256
+        ).digest()[:16]
+        if not hmac.compare_digest(expected_mac, provided_mac):
+            return _http_error(401, "invalid signature")
+        try:
+            rel_bytes = _b64url_decode(payload)
+            rel_str = rel_bytes.decode("utf-8")
+        except (ValueError, binascii.Error, UnicodeDecodeError):
+            return _http_error(400, "invalid payload")
+        # An attacker who somehow bypassed the HMAC check would still need
+        # the resolved path to escape the media root; guard defensively.
+        try:
+            media_root = get_media_dir().resolve()
+            candidate = (media_root / rel_str).resolve()
+            candidate.relative_to(media_root)
+        except (OSError, ValueError):
+            return _http_error(404, "not found")
+        if not candidate.is_file():
+            return _http_error(404, "not found")
+        try:
+            body = candidate.read_bytes()
+        except OSError:
+            return _http_error(500, "read error")
+        mime, _ = mimetypes.guess_type(candidate.name)
+        if mime not in _MEDIA_ALLOWED_MIMES:
+            mime = "application/octet-stream"
+        return _http_response(
+            body,
+            content_type=mime,
+            extra_headers=[
+                ("Cache-Control", "private, max-age=31536000, immutable"),
+            ],
+        )
+
     def _handle_session_delete(self, request: WsRequest, key: str) -> Response:
         if not self._check_api_token(request):
             return _http_error(401, "Unauthorized")
@@ -755,6 +933,48 @@ class WebSocketChannel(BaseChannel):
         finally:
             self._cleanup_connection(connection)
 
+    @staticmethod
+    def _save_envelope_media(
+        media: list[Any],
+    ) -> tuple[list[str], str | None]:
+        """Decode and persist ``media`` items from a ``message`` envelope.
+
+        Returns ``(paths, None)`` on success or ``([], reason)`` on the first
+        failure — the caller is expected to surface ``reason`` to the client
+        and skip publishing so no half-formed message ever reaches the agent.
+        ``reason`` is a short, stable token suitable for UI localization.
+
+        Shape: ``list[{"data_url": str, "name"?: str | None}]``.
+        """
+        if len(media) > _MAX_IMAGES_PER_MESSAGE:
+            return [], "too_many_images"
+        media_dir = get_media_dir("websocket")
+        paths: list[str] = []
+        for item in media:
+            if not isinstance(item, dict):
+                return [], "malformed"
+            data_url = item.get("data_url")
+            if not isinstance(data_url, str) or not data_url:
+                return [], "malformed"
+            mime = _extract_data_url_mime(data_url)
+            if mime is None:
+                return [], "decode"
+            if mime not in _IMAGE_MIME_ALLOWED:
+                return [], "mime"
+            try:
+                saved = save_base64_data_url(
+                    data_url, media_dir, max_bytes=_MAX_IMAGE_BYTES,
+                )
+            except FileSizeExceeded:
+                return [], "size"
+            except Exception as exc:
+                logger.warning("websocket: media decode failed: {}", exc)
+                return [], "decode"
+            if saved is None:
+                return [], "decode"
+            paths.append(saved)
+        return paths, None
+
     async def _dispatch_envelope(
         self,
         connection: Any,
@@ -782,15 +1002,39 @@ class WebSocketChannel(BaseChannel):
             if not _is_valid_chat_id(cid):
                 await self._send_event(connection, "error", detail="invalid chat_id")
                 return
-            if not isinstance(content, str) or not content.strip():
+            if not isinstance(content, str):
                 await self._send_event(connection, "error", detail="missing content")
                 return
+
+            raw_media = envelope.get("media")
+            media_paths: list[str] = []
+            if raw_media is not None:
+                if not isinstance(raw_media, list):
+                    await self._send_event(
+                        connection, "error",
+                        detail="image_rejected", reason="malformed",
+                    )
+                    return
+                media_paths, reason = self._save_envelope_media(raw_media)
+                if reason is not None:
+                    await self._send_event(
+                        connection, "error",
+                        detail="image_rejected", reason=reason,
+                    )
+                    return
+
+            # Allow image-only turns (content may be empty when media is attached).
+            if not content.strip() and not media_paths:
+                await self._send_event(connection, "error", detail="missing content")
+                return
+
             # Auto-attach on first use so clients can one-shot without a separate attach.
             self._attach(connection, cid)
             await self._handle_message(
                 sender_id=client_id,
                 chat_id=cid,
                 content=content,
+                media=media_paths or None,
                 metadata={"remote": getattr(connection, "remote_address", None)},
             )
             return
diff --git a/nanobot/session/manager.py b/nanobot/session/manager.py
index 436d8225..69509a83 100644
--- a/nanobot/session/manager.py
+++ b/nanobot/session/manager.py
@@ -11,7 +11,12 @@ from typing import Any
 from loguru import logger
 
 from nanobot.config.paths import get_legacy_sessions_dir
-from nanobot.utils.helpers import ensure_dir, find_legal_message_start, safe_filename
+from nanobot.utils.helpers import (
+    ensure_dir,
+    find_legal_message_start,
+    image_placeholder_text,
+    safe_filename,
+)
 
 
 @dataclass
@@ -54,7 +59,19 @@ class Session:
 
         out: list[dict[str, Any]] = []
         for message in sliced:
-            entry: dict[str, Any] = {"role": message["role"], "content": message.get("content", "")}
+            content = message.get("content", "")
+            # Synthesize an ``[image: path]`` breadcrumb from the persisted
+            # ``media`` kwarg so LLM replay still sees *something* where the
+            # image used to be. Without this, an image-only user turn
+            # replays as an empty user message — the assistant's reply then
+            # looks like it's responding to nothing.
+            media = message.get("media")
+            if isinstance(media, list) and media and isinstance(content, str):
+                breadcrumbs = "\n".join(
+                    image_placeholder_text(p) for p in media if isinstance(p, str) and p
+                )
+                content = f"{content}\n{breadcrumbs}" if content else breadcrumbs
+            entry: dict[str, Any] = {"role": message["role"], "content": content}
             for key in ("tool_calls", "tool_call_id", "name", "reasoning_content"):
                 if key in message:
                     entry[key] = message[key]
diff --git a/nanobot/utils/media_decode.py b/nanobot/utils/media_decode.py
new file mode 100644
index 00000000..484613d9
--- /dev/null
+++ b/nanobot/utils/media_decode.py
@@ -0,0 +1,55 @@
+"""Shared helpers for decoding ``data:...;base64,...`` URLs to disk.
+
+Historically lived in ``nanobot.api.server``; now shared by the WebSocket
+channel so the ``api`` + ``websocket`` ingress paths apply the same parsing,
+size guard, and filesystem layout.
+"""
+
+from __future__ import annotations
+
+import base64
+import mimetypes
+import re
+import uuid
+from pathlib import Path
+
+from nanobot.utils.helpers import safe_filename
+
+DEFAULT_MAX_BYTES = 10 * 1024 * 1024
+MAX_FILE_SIZE = DEFAULT_MAX_BYTES
+
+_DATA_URL_RE = re.compile(r"^data:([^;]+);base64,(.+)$", re.DOTALL)
+
+
+class FileSizeExceeded(Exception):
+    """Raised when a decoded payload exceeds the caller's size limit."""
+
+
+def save_base64_data_url(
+    data_url: str,
+    media_dir: Path,
+    *,
+    max_bytes: int | None = None,
+) -> str | None:
+    """Decode a ``data:<mime>;base64,<payload>`` URL and persist it.
+
+    Returns the absolute path on success, ``None`` when the URL shape or the
+    base64 payload itself is malformed. Raises :class:`FileSizeExceeded`
+    when the decoded payload is larger than ``max_bytes`` (default 10 MB).
+    """
+    m = _DATA_URL_RE.match(data_url)
+    if not m:
+        return None
+    mime_type, b64_payload = m.group(1), m.group(2)
+    try:
+        raw = base64.b64decode(b64_payload)
+    except Exception:
+        return None
+    limit = DEFAULT_MAX_BYTES if max_bytes is None else max_bytes
+    if len(raw) > limit:
+        raise FileSizeExceeded(f"File exceeds {limit // (1024 * 1024)}MB limit")
+    ext = mimetypes.guess_extension(mime_type) or ".bin"
+    filename = f"{uuid.uuid4().hex[:12]}{ext}"
+    dest = media_dir / safe_filename(filename)
+    dest.write_bytes(raw)
+    return str(dest)
diff --git a/tests/agent/test_loop_save_turn.py b/tests/agent/test_loop_save_turn.py
index 4f1c1f35..50951824 100644
--- a/tests/agent/test_loop_save_turn.py
+++ b/tests/agent/test_loop_save_turn.py
@@ -234,6 +234,87 @@ async def test_process_message_persists_user_message_before_turn_completes(tmp_p
     assert persisted.updated_at >= persisted.created_at
 
 
+# 1x1 PNG used by the media-persistence tests. ``extract_documents`` runs
+# at the top of ``_process_message`` and filters ``msg.media`` down to
+# paths that magic-byte-sniff as images, so the test fixture needs real
+# bytes on disk (not just placeholder paths).
+_PNG_1X1 = (
+    b"\x89PNG\r\n\x1a\n"
+    b"\x00\x00\x00\rIHDR\x00\x00\x00\x01\x00\x00\x00\x01"
+    b"\x08\x06\x00\x00\x00\x1f\x15\xc4\x89"
+    b"\x00\x00\x00\nIDATx\x9cc\x00\x00\x00\x02\x00\x01"
+    b"\x00\x00\x00\x00IEND\xaeB`\x82"
+)
+
+
+@pytest.mark.asyncio
+async def test_process_message_persists_media_paths_on_user_turn(tmp_path: Path) -> None:
+    """User turns that attach images must record the media paths alongside
+    the text so the webui can rehydrate previews on session replay.
+
+    This is the producer half of the signed-media-URL round-trip: paths are
+    stored here, then :meth:`WebSocketChannel._augment_media_urls` maps them
+    onto signed URLs on the way out.
+    """
+    img_a = tmp_path / "uuid-1.png"
+    img_a.write_bytes(_PNG_1X1)
+    img_b = tmp_path / "uuid-2.png"
+    img_b.write_bytes(_PNG_1X1)
+
+    loop = _make_full_loop(tmp_path)
+    loop.consolidator.maybe_consolidate_by_tokens = AsyncMock(return_value=False)  # type: ignore[method-assign]
+    loop._run_agent_loop = AsyncMock(side_effect=RuntimeError("interrupt"))  # type: ignore[method-assign]
+
+    msg = InboundMessage(
+        channel="websocket",
+        sender_id="u1",
+        chat_id="c-media",
+        content="look",
+        media=[str(img_a), str(img_b)],
+    )
+    with pytest.raises(RuntimeError, match="interrupt"):
+        await loop._process_message(msg)
+
+    loop.sessions.invalidate("websocket:c-media")
+    persisted = loop.sessions.get_or_create("websocket:c-media")
+    assert [m["role"] for m in persisted.messages] == ["user"]
+    assert persisted.messages[0]["content"] == "look"
+    assert persisted.messages[0]["media"] == [str(img_a), str(img_b)]
+
+
+@pytest.mark.asyncio
+async def test_process_message_persists_media_only_turn_without_text(tmp_path: Path) -> None:
+    """A turn with images but no text still persists (previously silent-dropped).
+
+    The old early-persist gate skipped messages without text, leaving pure
+    image turns un-checkpointed. They now materialise as an empty-content
+    user row with ``media`` attached.
+    """
+    img = tmp_path / "only.png"
+    img.write_bytes(_PNG_1X1)
+
+    loop = _make_full_loop(tmp_path)
+    loop.consolidator.maybe_consolidate_by_tokens = AsyncMock(return_value=False)  # type: ignore[method-assign]
+    loop._run_agent_loop = AsyncMock(side_effect=RuntimeError("boom"))  # type: ignore[method-assign]
+
+    msg = InboundMessage(
+        channel="websocket",
+        sender_id="u1",
+        chat_id="c-images-only",
+        content="",
+        media=[str(img)],
+    )
+    with pytest.raises(RuntimeError):
+        await loop._process_message(msg)
+
+    loop.sessions.invalidate("websocket:c-images-only")
+    persisted = loop.sessions.get_or_create("websocket:c-images-only")
+    assert len(persisted.messages) == 1
+    assert persisted.messages[0]["role"] == "user"
+    assert persisted.messages[0]["content"] == ""
+    assert persisted.messages[0]["media"] == [str(img)]
+
+
 @pytest.mark.asyncio
 async def test_process_message_does_not_duplicate_early_persisted_user_message(tmp_path: Path) -> None:
     loop = _make_full_loop(tmp_path)
diff --git a/tests/agent/test_session_manager_history.py b/tests/agent/test_session_manager_history.py
index 1297a587..8b4d0740 100644
--- a/tests/agent/test_session_manager_history.py
+++ b/tests/agent/test_session_manager_history.py
@@ -217,3 +217,55 @@ def test_window_cuts_mid_tool_group():
     # leaving orphan tool results for split_a at the front.
     history = session.get_history(max_messages=6)
     _assert_no_orphans(history)
+
+
+# --- Image breadcrumbs: media kwarg is synthesized into content for replay ---
+
+
+def test_get_history_synthesizes_image_breadcrumb_from_media_kwarg():
+    """Persisted user turns carry image paths as a ``media`` kwarg; LLM
+    replay must still see an ``[image: path]`` breadcrumb so the assistant's
+    follow-up reply has a referent instead of trailing an empty user row."""
+    session = Session(key="test:media")
+    session.messages.append(
+        {"role": "user", "content": "look", "media": ["/m/a.png", "/m/b.png"]}
+    )
+    session.messages.append({"role": "assistant", "content": "nice"})
+
+    history = session.get_history(max_messages=500)
+
+    assert history == [
+        {"role": "user", "content": "look\n[image: /m/a.png]\n[image: /m/b.png]"},
+        {"role": "assistant", "content": "nice"},
+    ]
+
+
+def test_get_history_synthesizes_breadcrumb_for_image_only_turn():
+    """Turns with no text but attached images must not replay as empty
+    strings — the LLM would otherwise see a bare user turn followed by an
+    unexplained assistant answer."""
+    session = Session(key="test:image-only")
+    session.messages.append({"role": "user", "content": "", "media": ["/m/pic.png"]})
+    session.messages.append({"role": "assistant", "content": "I see a cat"})
+
+    history = session.get_history(max_messages=500)
+
+    assert history[0] == {"role": "user", "content": "[image: /m/pic.png]"}
+
+
+def test_get_history_ignores_media_kwarg_on_non_user_rows():
+    """``media`` only ever appears on user entries in practice, but the
+    synthesizer must be defensive: assistants / tools with list content
+    don't get the breadcrumb pasted on top."""
+    session = Session(key="test:defensive")
+    session.messages.append(
+        {
+            "role": "assistant",
+            "content": [{"type": "text", "text": "structured"}],
+            "media": ["/m/x.png"],  # nonsense but shouldn't crash
+        }
+    )
+    history = session.get_history(max_messages=500)
+    # List content is passed through verbatim — the synthesizer only
+    # rewrites plain-string content.
+    assert history[0]["content"] == [{"type": "text", "text": "structured"}]
diff --git a/tests/channels/test_websocket_envelope_media.py b/tests/channels/test_websocket_envelope_media.py
new file mode 100644
index 00000000..4fe5f642
--- /dev/null
+++ b/tests/channels/test_websocket_envelope_media.py
@@ -0,0 +1,416 @@
+"""Tests for WS envelope media handling (client image upload path).
+
+Exercises ``WebSocketChannel._dispatch_envelope`` for the ``message`` branch:
+decoding base64 data URLs, rejecting malformed / oversized / non-whitelisted
+payloads, preserving backward compatibility with media-less frames, and
+forwarding saved paths to ``_handle_message``.
+"""
+
+from __future__ import annotations
+
+import base64
+import json
+from pathlib import Path
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from nanobot.channels.websocket import (
+    WebSocketChannel,
+    _extract_data_url_mime,
+)
+
+
+def _tiny_png_data_url() -> str:
+    """A 1-pixel PNG prefixed as a data URL — just enough for magic-bytes sniffing."""
+    # 1x1 transparent PNG
+    png = (
+        b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x01\x00\x00"
+        b"\x00\x01\x08\x06\x00\x00\x00\x1f\x15\xc4\x89\x00\x00\x00\rIDATx"
+        b"\x9cc\xf8\xcf\xc0\x00\x00\x00\x03\x00\x01\x00\x18\xdd\x8d\xb4\x00"
+        b"\x00\x00\x00IEND\xaeB`\x82"
+    )
+    return f"data:image/png;base64,{base64.b64encode(png).decode()}"
+
+
+def _data_url(mime: str, payload: bytes) -> str:
+    return f"data:{mime};base64,{base64.b64encode(payload).decode()}"
+
+
+def _make_channel() -> WebSocketChannel:
+    bus = MagicMock()
+    bus.publish_inbound = AsyncMock()
+    channel = WebSocketChannel(
+        {"enabled": True, "allowFrom": ["*"], "websocketRequiresToken": False},
+        bus,
+    )
+    channel._handle_message = AsyncMock()  # type: ignore[method-assign]
+    return channel
+
+
+# -- Pure helpers --------------------------------------------------------------
+
+
+@pytest.mark.parametrize(
+    ("url", "expected"),
+    [
+        ("data:image/png;base64,AAAA", "image/png"),
+        ("data:image/jpeg;base64,AAAA", "image/jpeg"),
+        ("data:IMAGE/PNG;base64,AAAA", "image/png"),
+        ("data:image/svg+xml;base64,AAAA", "image/svg+xml"),
+        ("data:text/plain;base64,AAAA", "text/plain"),
+        ("http://evil.example/x.png", None),
+        ("data:image/png,AAAA", None),  # missing `;base64`
+        ("", None),
+        (None, None),
+    ],
+)
+def test_extract_data_url_mime(url: Any, expected: str | None) -> None:
+    assert _extract_data_url_mime(url) == expected
+
+
+# -- max_message_bytes bump ----------------------------------------------------
+
+
+def test_max_message_bytes_default_supports_multi_image_frame() -> None:
+    """Default 36 MB must comfortably hold 4 × 6 MB base64-encoded images."""
+    from nanobot.channels.websocket import WebSocketConfig
+
+    default = WebSocketConfig().max_message_bytes
+    # 4 images × 6 MB × 1.37 base64 overhead ≈ 33 MB
+    assert default >= 33 * 1024 * 1024
+    # Upper bound 40 MB matches plan
+    with pytest.raises(Exception):
+        WebSocketConfig(max_message_bytes=41_943_040 + 1)
+
+
+# -- _dispatch_envelope message branch + media --------------------------------
+
+
+@pytest.mark.asyncio
+async def test_message_without_media_backward_compatible() -> None:
+    """Existing clients that don't send ``media`` keep working unchanged."""
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {"type": "message", "chat_id": "abc123", "content": "hello"}
+
+    await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_awaited_once()
+    call = channel._handle_message.call_args
+    assert call.kwargs["chat_id"] == "abc123"
+    assert call.kwargs["content"] == "hello"
+    # When no media, we pass ``media=None`` so downstream treats it as absent.
+    assert call.kwargs["media"] is None
+
+
+@pytest.mark.asyncio
+async def test_message_with_single_image_forwards_saved_path(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "look at this",
+        "media": [{"data_url": _tiny_png_data_url(), "name": "shot.png"}],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_awaited_once()
+    paths = channel._handle_message.call_args.kwargs["media"]
+    assert isinstance(paths, list) and len(paths) == 1
+    saved = Path(paths[0])
+    assert saved.exists()
+    assert saved.suffix == ".png"
+    assert saved.is_relative_to(tmp_path)
+
+
+@pytest.mark.asyncio
+async def test_message_with_multiple_images(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "a couple",
+        "media": [
+            {"data_url": _tiny_png_data_url()},
+            {"data_url": _tiny_png_data_url()},
+            {"data_url": _tiny_png_data_url()},
+        ],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    paths = channel._handle_message.call_args.kwargs["media"]
+    assert len(paths) == 3
+    # Saved filenames must be unique.
+    assert len({Path(p).name for p in paths}) == 3
+
+
+@pytest.mark.asyncio
+async def test_image_only_message_allows_empty_text(tmp_path) -> None:
+    """When media is attached, empty text is acceptable."""
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "",
+        "media": [{"data_url": _tiny_png_data_url()}],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_awaited_once()
+    # Error event NOT sent.
+    mock_conn.send.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_when_more_than_four_images(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "hi",
+        "media": [{"data_url": _tiny_png_data_url()}] * 5,
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    mock_conn.send.assert_awaited_once()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["event"] == "error"
+    assert err["detail"] == "image_rejected"
+    assert err["reason"] == "too_many_images"
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_on_oversize_payload(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    oversized = b"x" * (9 * 1024 * 1024)  # > 8 MB WS limit
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "big",
+        "media": [{"data_url": _data_url("image/png", oversized)}],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["detail"] == "image_rejected"
+    assert err["reason"] == "size"
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_on_non_image_mime(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "pdf?",
+        "media": [{"data_url": _data_url("application/pdf", b"%PDF-1.4")}],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["detail"] == "image_rejected"
+    assert err["reason"] == "mime"
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_on_svg_mime(tmp_path) -> None:
+    """SVG is explicitly rejected — XSS surface inside embedded scripts."""
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "svg",
+        "media": [{"data_url": _data_url("image/svg+xml", b"<svg/>")}],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["reason"] == "mime"
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_on_malformed_data_url(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "nope",
+        "media": [{"data_url": "http://evil.example/image.png"}],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["reason"] == "decode"
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_on_broken_base64(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "nope",
+        "media": [{"data_url": "data:image/png;base64,not-valid-base64!!!"}],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["reason"] == "decode"
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_when_media_item_shape_wrong(tmp_path) -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "huh",
+        # Not a dict — plain string at the top level.
+        "media": ["data:image/png;base64,XXXX"],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["reason"] == "malformed"
+
+
+@pytest.mark.asyncio
+async def test_message_rejected_when_media_field_is_not_list() -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "huh",
+        "media": "not-a-list",
+    }
+
+    await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["detail"] == "image_rejected"
+    assert err["reason"] == "malformed"
+
+
+@pytest.mark.asyncio
+async def test_failed_media_does_not_partially_persist(tmp_path) -> None:
+    """If the second image is invalid, the first must not be forwarded."""
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "mixed",
+        "media": [
+            {"data_url": _tiny_png_data_url()},
+            {"data_url": _data_url("application/pdf", b"%PDF-1.4")},
+        ],
+    }
+
+    with patch(
+        "nanobot.channels.websocket.get_media_dir", return_value=tmp_path
+    ):
+        await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    # The first image was saved to disk (we don't roll it back — the caller
+    # is expected to not reference it) but the agent never sees the paths.
+    # That's the important invariant: no partial publish.
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["reason"] == "mime"
+
+
+@pytest.mark.asyncio
+async def test_rejects_empty_text_without_media() -> None:
+    """When no media is attached, whitespace-only content is still rejected
+    (matches the existing behavior for backward compat)."""
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": "   ",
+    }
+
+    await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["detail"] == "missing content"
+
+
+@pytest.mark.asyncio
+async def test_non_string_content_still_rejected() -> None:
+    channel = _make_channel()
+    mock_conn = AsyncMock()
+    envelope = {
+        "type": "message",
+        "chat_id": "abc123",
+        "content": 42,
+    }
+
+    await channel._dispatch_envelope(mock_conn, "client-1", envelope)
+
+    channel._handle_message.assert_not_awaited()
+    err = json.loads(mock_conn.send.call_args[0][0])
+    assert err["detail"] == "missing content"
diff --git a/tests/channels/test_websocket_media_route.py b/tests/channels/test_websocket_media_route.py
new file mode 100644
index 00000000..14577355
--- /dev/null
+++ b/tests/channels/test_websocket_media_route.py
@@ -0,0 +1,375 @@
+"""Tests for the signed ``/api/media/<sig>/<payload>`` route and its replay
+integration on ``/api/sessions/<key>/messages``.
+
+The route is the return path for images attached to persisted user turns:
+:meth:`WebSocketChannel._sign_media_path` mints URLs during session reads,
+and :meth:`WebSocketChannel._handle_media_fetch` serves the bytes back.
+These tests cover the two halves end-to-end plus the adversarial edges
+(bad signatures, ``..`` traversal, non-existent files, non-image types).
+"""
+
+from __future__ import annotations
+
+import asyncio
+import functools
+import hashlib
+import hmac
+from pathlib import Path
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import httpx
+import pytest
+
+from nanobot.channels.websocket import (
+    WebSocketChannel,
+    _b64url_decode,
+    _b64url_encode,
+)
+from nanobot.session.manager import Session, SessionManager
+
+
+# PNG magic bytes + a couple of sentinel bytes so we can verify byte-for-byte
+# round-trip of the served payload. Stays under mimetype + size limits.
+_PNG_BYTES = (
+    b"\x89PNG\r\n\x1a\n"
+    b"\x00\x00\x00\rIHDR\x00\x00\x00\x01\x00\x00\x00\x01"
+    b"\x08\x06\x00\x00\x00\x1f\x15\xc4\x89"
+    b"\x00\x00\x00\nIDATx\x9cc\x00\x00\x00\x02\x00\x01"
+    b"\x00\x00\x00\x00IEND\xaeB`\x82"
+)
+
+
+def _ch(
+    bus: Any,
+    *,
+    session_manager: SessionManager | None = None,
+    port: int,
+) -> WebSocketChannel:
+    return WebSocketChannel(
+        {
+            "enabled": True,
+            "allowFrom": ["*"],
+            "host": "127.0.0.1",
+            "port": port,
+            "path": "/",
+            "websocketRequiresToken": False,
+        },
+        bus,
+        session_manager=session_manager,
+    )
+
+
+@pytest.fixture()
+def bus() -> MagicMock:
+    b = MagicMock()
+    b.publish_inbound = AsyncMock()
+    return b
+
+
+async def _http_get(
+    url: str, headers: dict[str, str] | None = None
+) -> httpx.Response:
+    return await asyncio.to_thread(
+        functools.partial(httpx.get, url, headers=headers or {}, timeout=5.0)
+    )
+
+
+# ---------------------------------------------------------------------------
+# _sign_media_path: the URL minter
+# ---------------------------------------------------------------------------
+
+
+def test_sign_media_path_rejects_paths_outside_media_root(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """Paths that resolve outside ``get_media_dir()`` must not be signed.
+
+    This is the single most important invariant of the whole scheme:
+    if the minter ever signed an arbitrary path, the HMAC would legitimise
+    it for the fetch handler and we'd hand out a disk-read primitive.
+    """
+    outside = tmp_path / "secrets" / "cred.txt"
+    outside.parent.mkdir()
+    outside.write_text("nope")
+    media = tmp_path / "media"
+    media.mkdir()
+    channel = _ch(bus, port=0)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        assert channel._sign_media_path(outside) is None
+        # Traversal via the media root is also rejected — the resolve() step
+        # normalises ``..`` out before the relative_to check.
+        assert channel._sign_media_path(media / ".." / "secrets" / "cred.txt") is None
+
+
+def test_sign_media_path_round_trips_via_hmac(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """The signature embeds exactly ``HMAC-SHA256(secret, payload)[:16]``."""
+    media = tmp_path / "media"
+    media.mkdir()
+    (media / "a.png").write_bytes(_PNG_BYTES)
+    channel = _ch(bus, port=0)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        url = channel._sign_media_path(media / "a.png")
+    assert url is not None
+    assert url.startswith("/api/media/")
+    sig, payload = url[len("/api/media/"):].split("/", 1)
+    expected = hmac.new(
+        channel._media_secret, payload.encode("ascii"), hashlib.sha256
+    ).digest()[:16]
+    assert _b64url_decode(sig) == expected
+    # The payload decodes back to the *relative* path — no absolute-path leaks.
+    assert _b64url_decode(payload).decode() == "a.png"
+
+
+# ---------------------------------------------------------------------------
+# /api/media/<sig>/<payload>: the serving handler
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_media_route_serves_signed_file(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """Valid signature + existing file => 200 with correct bytes + MIME."""
+    media = tmp_path / "media"
+    media.mkdir()
+    target = media / "round-trip.png"
+    target.write_bytes(_PNG_BYTES)
+
+    channel = _ch(bus, port=29920)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        url_path = channel._sign_media_path(target)
+        assert url_path is not None
+        server_task = asyncio.create_task(channel.start())
+        await asyncio.sleep(0.3)
+        try:
+            resp = await _http_get(f"http://127.0.0.1:29920{url_path}")
+        finally:
+            await channel.stop()
+            await server_task
+
+    assert resp.status_code == 200
+    assert resp.content == _PNG_BYTES
+    assert resp.headers["content-type"].startswith("image/png")
+    # Immutable cache header lets the browser skip round-trips on replay.
+    assert "immutable" in resp.headers.get("cache-control", "")
+
+
+@pytest.mark.asyncio
+async def test_media_route_rejects_bad_signature(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """A payload re-signed with a different secret must 401.
+
+    Protects against a restart: old URLs baked into a stale tab become
+    un-forgeable once ``_media_secret`` regenerates.
+    """
+    media = tmp_path / "media"
+    media.mkdir()
+    (media / "f.png").write_bytes(_PNG_BYTES)
+
+    channel = _ch(bus, port=29921)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        good = channel._sign_media_path(media / "f.png")
+        assert good is not None
+        _, payload = good[len("/api/media/"):].split("/", 1)
+        # Forge a sig with a *different* secret.
+        forged_mac = hmac.new(
+            b"\x00" * 32, payload.encode("ascii"), hashlib.sha256
+        ).digest()[:16]
+        forged = f"/api/media/{_b64url_encode(forged_mac)}/{payload}"
+
+        server_task = asyncio.create_task(channel.start())
+        await asyncio.sleep(0.3)
+        try:
+            resp = await _http_get(f"http://127.0.0.1:29921{forged}")
+        finally:
+            await channel.stop()
+            await server_task
+    assert resp.status_code == 401
+
+
+@pytest.mark.asyncio
+async def test_media_route_rejects_path_traversal_payload(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """Even a validly-signed ``..`` payload must not escape the media root.
+
+    The signer never *emits* such payloads, but an attacker who somehow
+    obtained the secret (or the channel was misconfigured) must still be
+    stopped by the resolve()+relative_to() guard in the serving path.
+    """
+    media = tmp_path / "media"
+    media.mkdir()
+    secret_file = tmp_path / "secret.txt"
+    secret_file.write_text("classified")
+
+    channel = _ch(bus, port=29922)
+    # Hand-craft a traversal payload the legit signer would refuse to mint.
+    payload = _b64url_encode(b"../secret.txt")
+    mac = hmac.new(
+        channel._media_secret, payload.encode("ascii"), hashlib.sha256
+    ).digest()[:16]
+    url = f"/api/media/{_b64url_encode(mac)}/{payload}"
+
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        server_task = asyncio.create_task(channel.start())
+        await asyncio.sleep(0.3)
+        try:
+            resp = await _http_get(f"http://127.0.0.1:29922{url}")
+        finally:
+            await channel.stop()
+            await server_task
+    assert resp.status_code == 404
+    assert b"classified" not in resp.content
+
+
+@pytest.mark.asyncio
+async def test_media_route_404s_missing_file(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """A signed URL for a file that no longer exists degrades to 404 so the
+    client can fall back to the placeholder tile instead of breaking."""
+    media = tmp_path / "media"
+    media.mkdir()
+    target = media / "gone.png"
+    target.write_bytes(_PNG_BYTES)
+
+    channel = _ch(bus, port=29923)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        url_path = channel._sign_media_path(target)
+        assert url_path is not None
+        target.unlink()  # the file vanishes between signing and fetching
+        server_task = asyncio.create_task(channel.start())
+        await asyncio.sleep(0.3)
+        try:
+            resp = await _http_get(f"http://127.0.0.1:29923{url_path}")
+        finally:
+            await channel.stop()
+            await server_task
+    assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+async def test_media_route_degrades_non_image_to_octet_stream(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """A non-image extension must not be served as its native MIME.
+
+    Defence-in-depth: if media_dir ever contained (say) an HTML file, we
+    do not want the browser to render it as HTML via the signed route.
+    """
+    media = tmp_path / "media"
+    media.mkdir()
+    (media / "scary.html").write_bytes(b"<script>alert(1)</script>")
+
+    channel = _ch(bus, port=29924)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        payload = _b64url_encode(b"scary.html")
+        mac = hmac.new(
+            channel._media_secret, payload.encode("ascii"), hashlib.sha256
+        ).digest()[:16]
+        url = f"/api/media/{_b64url_encode(mac)}/{payload}"
+        server_task = asyncio.create_task(channel.start())
+        await asyncio.sleep(0.3)
+        try:
+            resp = await _http_get(f"http://127.0.0.1:29924{url}")
+        finally:
+            await channel.stop()
+            await server_task
+    assert resp.status_code == 200
+    assert resp.headers["content-type"].startswith("application/octet-stream")
+
+
+# ---------------------------------------------------------------------------
+# /api/sessions/<key>/messages: media_urls hydration on session read
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_session_messages_exposes_signed_media_urls(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """The read path must map persisted ``media`` paths onto signed URLs
+    and strip the raw path — the client never learns the server's layout."""
+    media = tmp_path / "media"
+    media.mkdir()
+    img = media / "u.png"
+    img.write_bytes(_PNG_BYTES)
+
+    sm = SessionManager(tmp_path / "ws_state")
+    sess = Session(key="websocket:media-hydrate")
+    sess.add_message("user", "look at this", media=[str(img)])
+    sess.add_message("assistant", "nice")
+    sm.save(sess)
+
+    channel = _ch(bus, session_manager=sm, port=29925)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        server_task = asyncio.create_task(channel.start())
+        await asyncio.sleep(0.3)
+        try:
+            boot = await _http_get("http://127.0.0.1:29925/webui/bootstrap")
+            token = boot.json()["token"]
+            auth = {"Authorization": f"Bearer {token}"}
+            resp = await _http_get(
+                "http://127.0.0.1:29925/api/sessions/websocket:media-hydrate/messages",
+                headers=auth,
+            )
+            body = resp.json()
+            # The signed URL round-trips end-to-end: fetching it yields the same bytes.
+            user_msg = next(m for m in body["messages"] if m["role"] == "user")
+            urls = user_msg["media_urls"]
+            assert isinstance(urls, list) and len(urls) == 1
+            assert urls[0]["name"] == "u.png"
+            assert urls[0]["url"].startswith("/api/media/")
+            # Raw paths must not leak to the wire.
+            assert "media" not in user_msg
+
+            # And the URL actually works.
+            fetched = await _http_get(f"http://127.0.0.1:29925{urls[0]['url']}")
+            assert fetched.status_code == 200
+            assert fetched.content == _PNG_BYTES
+        finally:
+            await channel.stop()
+            await server_task
+
+
+@pytest.mark.asyncio
+async def test_session_messages_skips_vanished_media(
+    bus: MagicMock, tmp_path: Path
+) -> None:
+    """Paths that no longer resolve inside the media root produce no URL —
+    the message is still delivered, just without the preview."""
+    media = tmp_path / "media"
+    media.mkdir()
+
+    sm = SessionManager(tmp_path / "ws_state")
+    sess = Session(key="websocket:vanished")
+    sess.add_message("user", "missing pic", media=[str(media / "absent.png")])
+    sm.save(sess)
+
+    channel = _ch(bus, session_manager=sm, port=29926)
+    with patch("nanobot.channels.websocket.get_media_dir", return_value=media):
+        server_task = asyncio.create_task(channel.start())
+        await asyncio.sleep(0.3)
+        try:
+            boot = await _http_get("http://127.0.0.1:29926/webui/bootstrap")
+            token = boot.json()["token"]
+            resp = await _http_get(
+                "http://127.0.0.1:29926/api/sessions/websocket:vanished/messages",
+                headers={"Authorization": f"Bearer {token}"},
+            )
+            user_msg = next(m for m in resp.json()["messages"] if m["role"] == "user")
+            # absent.png lives inside the media root so it *does* get a signed
+            # URL (we don't stat the file at signing time — that would slow
+            # the listing). Fetching the URL is where the 404 surfaces.
+            urls = user_msg.get("media_urls") or []
+            assert len(urls) == 1
+            fetched = await _http_get(f"http://127.0.0.1:29926{urls[0]['url']}")
+            assert fetched.status_code == 404
+            assert "media" not in user_msg
+        finally:
+            await channel.stop()
+            await server_task
diff --git a/tests/utils/test_media_decode.py b/tests/utils/test_media_decode.py
new file mode 100644
index 00000000..5926ab2b
--- /dev/null
+++ b/tests/utils/test_media_decode.py
@@ -0,0 +1,75 @@
+"""Tests for ``nanobot.utils.media_decode``."""
+
+from __future__ import annotations
+
+import base64
+
+import pytest
+
+from nanobot.utils.media_decode import (
+    DEFAULT_MAX_BYTES,
+    FileSizeExceeded,
+    MAX_FILE_SIZE,
+    save_base64_data_url,
+)
+
+
+def _data_url(payload: bytes, mime: str = "image/png") -> str:
+    return f"data:{mime};base64,{base64.b64encode(payload).decode()}"
+
+
+def test_saves_png_with_correct_extension(tmp_path) -> None:
+    result = save_base64_data_url(_data_url(b"fake png"), tmp_path)
+    assert result is not None
+    assert result.endswith(".png")
+    assert (tmp_path / result.split("/")[-1]).read_bytes() == b"fake png"
+
+
+def test_returns_none_for_malformed_data_url(tmp_path) -> None:
+    assert save_base64_data_url("not-a-data-url", tmp_path) is None
+
+
+def test_returns_none_for_broken_base64(tmp_path) -> None:
+    # Python's b64decode strips non-alphabet chars by default, so we need a
+    # payload whose alphabet-filtered length breaks padding.
+    assert save_base64_data_url("data:image/png;base64,not-valid-base64!!!", tmp_path) is None
+
+
+def test_unknown_mime_falls_back_to_bin(tmp_path) -> None:
+    result = save_base64_data_url(_data_url(b"xyz", mime="unknown/type"), tmp_path)
+    assert result is not None
+    assert result.endswith(".bin")
+
+
+def test_default_limit_is_10mb(tmp_path) -> None:
+    """Backwards-compatible default — the API path depends on this."""
+    assert DEFAULT_MAX_BYTES == 10 * 1024 * 1024
+    assert MAX_FILE_SIZE == 10 * 1024 * 1024
+
+    oversized = b"x" * (11 * 1024 * 1024)
+    with pytest.raises(FileSizeExceeded, match="10MB limit"):
+        save_base64_data_url(_data_url(oversized), tmp_path)
+
+
+def test_explicit_max_bytes_overrides_default(tmp_path) -> None:
+    """WS channel passes 8 MB; a 9 MB payload should be rejected there even
+    though it would pass the 10 MB API limit."""
+    payload = b"y" * (9 * 1024 * 1024)
+    with pytest.raises(FileSizeExceeded, match="8MB limit"):
+        save_base64_data_url(_data_url(payload), tmp_path, max_bytes=8 * 1024 * 1024)
+
+
+def test_saved_file_lives_under_media_dir(tmp_path) -> None:
+    result = save_base64_data_url(_data_url(b"ok"), tmp_path)
+    assert result is not None
+    assert result.startswith(str(tmp_path))
+
+
+def test_legacy_symbols_reexported_from_api_server() -> None:
+    """Existing tests import ``_save_base64_data_url`` / ``_FileSizeExceeded``
+    from ``nanobot.api.server`` — keep the aliases working."""
+    from nanobot.api import server
+
+    assert server._save_base64_data_url is save_base64_data_url
+    assert server._FileSizeExceeded is FileSizeExceeded
+    assert server.MAX_FILE_SIZE == MAX_FILE_SIZE
diff --git a/webui/src/components/ImageLightbox.tsx b/webui/src/components/ImageLightbox.tsx
new file mode 100644
index 00000000..90819afa
--- /dev/null
+++ b/webui/src/components/ImageLightbox.tsx
@@ -0,0 +1,199 @@
+import { useCallback, useEffect, useMemo } from "react";
+import * as DialogPrimitive from "@radix-ui/react-dialog";
+import { ChevronLeft, ChevronRight, X } from "lucide-react";
+import { useTranslation } from "react-i18next";
+
+import { cn } from "@/lib/utils";
+import type { UIImage } from "@/lib/types";
+
+interface ImageLightboxProps {
+  images: UIImage[];
+  index: number | null;
+  onIndexChange: (index: number) => void;
+  onOpenChange: (open: boolean) => void;
+}
+
+/**
+ * Modal image viewer. Uses the Radix Dialog primitives directly so we can
+ * fill the viewport (the shared `DialogContent` wrapper caps at max-w-lg,
+ * which is much too small for a photo preview).
+ *
+ * Implementation notes:
+ * - `translate3d` + `will-change: transform` promote the image to a GPU
+ *   compositing layer so open/swap stays at 60 FPS on long threads.
+ * - Adjacent images are rendered in hidden `<img>` tags so the browser
+ *   decodes them eagerly; pressing left/right feels instant.
+ * - Radix handles `Escape` + focus trapping; we only wire up ←/→ + Home/End.
+ * - Respects `prefers-reduced-motion` by dropping the fade + zoom-in
+ *   keyframes via `motion-reduce:*` variants.
+ */
+export function ImageLightbox({
+  images,
+  index,
+  onIndexChange,
+  onOpenChange,
+}: ImageLightboxProps) {
+  const { t } = useTranslation();
+  const open = index !== null;
+  const total = images.length;
+  const current = index !== null ? images[index] : null;
+
+  const go = useCallback(
+    (delta: number) => {
+      if (index === null || total <= 1) return;
+      const next = (index + delta + total) % total;
+      onIndexChange(next);
+    },
+    [index, onIndexChange, total],
+  );
+
+  useEffect(() => {
+    if (!open) return;
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === "ArrowLeft") {
+        e.preventDefault();
+        go(-1);
+      } else if (e.key === "ArrowRight") {
+        e.preventDefault();
+        go(1);
+      } else if (e.key === "Home") {
+        e.preventDefault();
+        onIndexChange(0);
+      } else if (e.key === "End") {
+        e.preventDefault();
+        onIndexChange(total - 1);
+      }
+    };
+    window.addEventListener("keydown", onKey);
+    return () => window.removeEventListener("keydown", onKey);
+  }, [go, onIndexChange, open, total]);
+
+  // Neighbours we want the browser to decode eagerly.
+  const preload = useMemo(() => {
+    if (index === null || total <= 1) return [] as UIImage[];
+    const prev = images[(index - 1 + total) % total];
+    const next = images[(index + 1) % total];
+    return [prev, next].filter((i) => i && i.url);
+  }, [images, index, total]);
+
+  if (!current || !current.url) return null;
+
+  const hasMany = total > 1;
+  const counter = hasMany ? `${index! + 1} / ${total}` : null;
+
+  return (
+    <DialogPrimitive.Root open={open} onOpenChange={onOpenChange}>
+      <DialogPrimitive.Portal>
+        <DialogPrimitive.Overlay
+          className={cn(
+            "fixed inset-0 z-50 bg-black/80 backdrop-blur-sm",
+            "data-[state=open]:animate-in data-[state=closed]:animate-out",
+            "data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0",
+            "motion-reduce:data-[state=open]:animate-none motion-reduce:data-[state=closed]:animate-none",
+          )}
+        />
+        <DialogPrimitive.Content
+          aria-label={current.name ?? t("lightbox.title")}
+          className={cn(
+            "fixed inset-0 z-50 flex items-center justify-center",
+            "focus:outline-none",
+            "data-[state=open]:animate-in data-[state=closed]:animate-out",
+            "data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0",
+            "data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95",
+            "motion-reduce:data-[state=open]:animate-none motion-reduce:data-[state=closed]:animate-none",
+          )}
+        >
+          <DialogPrimitive.Title className="sr-only">
+            {current.name ?? t("lightbox.title")}
+          </DialogPrimitive.Title>
+
+          <div
+            className="relative flex max-h-[92vh] max-w-[94vw] items-center justify-center"
+            style={{
+              transform: "translate3d(0,0,0)",
+              willChange: "transform",
+            }}
+          >
+            <img
+              key={current.url}
+              src={current.url}
+              alt={current.name ?? ""}
+              decoding="async"
+              draggable={false}
+              className="max-h-[92vh] max-w-[94vw] select-none rounded-[6px] object-contain shadow-2xl"
+            />
+          </div>
+
+          {hasMany ? (
+            <>
+              <NavButton
+                side="left"
+                label={t("lightbox.prev")}
+                onClick={(e) => {
+                  e.stopPropagation();
+                  go(-1);
+                }}
+              />
+              <NavButton
+                side="right"
+                label={t("lightbox.next")}
+                onClick={(e) => {
+                  e.stopPropagation();
+                  go(1);
+                }}
+              />
+              <div className="pointer-events-none absolute bottom-5 left-1/2 -translate-x-1/2 rounded-full bg-black/55 px-3 py-1 text-xs font-medium text-white/90 tabular-nums">
+                {counter}
+              </div>
+            </>
+          ) : null}
+
+          <DialogPrimitive.Close
+            aria-label={t("lightbox.close")}
+            className={cn(
+              "absolute right-4 top-4 grid h-9 w-9 place-items-center rounded-full",
+              "bg-black/55 text-white/90 hover:bg-black/70 hover:text-white",
+              "focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-white/70",
+              "transition-colors motion-reduce:transition-none",
+            )}
+          >
+            <X className="h-4 w-4" aria-hidden />
+          </DialogPrimitive.Close>
+
+          {/* Invisible preload — browser decodes adjacent images so prev/next swap is instant. */}
+          <div aria-hidden className="hidden">
+            {preload.map((img, i) => (
+              <img key={`${img.url}-${i}`} src={img.url} alt="" />
+            ))}
+          </div>
+        </DialogPrimitive.Content>
+      </DialogPrimitive.Portal>
+    </DialogPrimitive.Root>
+  );
+}
+
+interface NavButtonProps {
+  side: "left" | "right";
+  label: string;
+  onClick: React.MouseEventHandler<HTMLButtonElement>;
+}
+
+function NavButton({ side, label, onClick }: NavButtonProps) {
+  const Icon = side === "left" ? ChevronLeft : ChevronRight;
+  return (
+    <button
+      type="button"
+      onClick={onClick}
+      aria-label={label}
+      className={cn(
+        "absolute top-1/2 -translate-y-1/2 grid h-11 w-11 place-items-center rounded-full",
+        "bg-black/55 text-white/90 hover:bg-black/70 hover:text-white",
+        "focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-white/70",
+        "transition-colors motion-reduce:transition-none",
+        side === "left" ? "left-4" : "right-4",
+      )}
+    >
+      <Icon className="h-5 w-5" aria-hidden />
+    </button>
+  );
+}
diff --git a/webui/src/components/MessageBubble.tsx b/webui/src/components/MessageBubble.tsx
index a6c02e3d..63a43c8d 100644
--- a/webui/src/components/MessageBubble.tsx
+++ b/webui/src/components/MessageBubble.tsx
@@ -1,10 +1,11 @@
 import { useState } from "react";
-import { ChevronRight, Wrench } from "lucide-react";
+import { ChevronRight, ImageIcon, Wrench } from "lucide-react";
 import { useTranslation } from "react-i18next";
 
+import { ImageLightbox } from "@/components/ImageLightbox";
 import { MarkdownText } from "@/components/MarkdownText";
 import { cn } from "@/lib/utils";
-import type { UIMessage } from "@/lib/types";
+import type { UIImage, UIMessage } from "@/lib/types";
 
 interface MessageBubbleProps {
   message: UIMessage;
@@ -27,22 +28,28 @@ export function MessageBubble({ message }: MessageBubbleProps) {
   }
 
   if (message.role === "user") {
+    const images = message.images ?? [];
+    const hasImages = images.length > 0;
+    const hasText = message.content.trim().length > 0;
     return (
       <div
         className={cn(
-          "group ml-auto flex max-w-[min(85%,36rem)] items-center gap-2",
+          "group ml-auto flex max-w-[min(85%,36rem)] flex-col items-end gap-1.5",
           baseAnim,
         )}
       >
-        <p
-          className={cn(
-            "ml-auto w-fit rounded-[18px] border border-border/60 bg-secondary/70 px-4 py-2",
-            "text-right text-[18px]/[1.8] whitespace-pre-wrap break-words",
-            "shadow-[0_10px_24px_-18px_rgba(0,0,0,0.55)]",
-          )}
-        >
-          {message.content}
-        </p>
+        {hasImages ? <UserImages images={images} /> : null}
+        {hasText ? (
+          <p
+            className={cn(
+              "ml-auto w-fit rounded-[18px] border border-border/60 bg-secondary/70 px-4 py-2",
+              "text-right text-[18px]/[1.8] whitespace-pre-wrap break-words",
+              "shadow-[0_10px_24px_-18px_rgba(0,0,0,0.55)]",
+            )}
+          >
+            {message.content}
+          </p>
+        ) : null}
       </div>
     );
   }
@@ -62,6 +69,121 @@ export function MessageBubble({ message }: MessageBubbleProps) {
   );
 }
 
+/**
+ * Right-aligned preview row for images attached to a user turn.
+ *
+ * Visual follows agent-chat-ui: a single wrapping row of fixed-size square
+ * thumbnails that stay modest next to the text pill regardless of how many
+ * images are attached.
+ *
+ * The URL is expected to be a self-contained ``data:`` URL (the Composer
+ * hands the normalized base64 payload to the optimistic bubble so that the
+ * preview survives React StrictMode double-mount — blob URLs would be
+ * revoked by the Composer's cleanup before remount). Historical replays
+ * have no URL (the backend strips data URLs before persisting), so we
+ * render a labelled placeholder tile instead of a broken ``<img>``.
+ */
+function UserImages({ images }: { images: UIImage[] }) {
+  const { t } = useTranslation();
+  // Only real-URL images can open in the lightbox; historical-replay
+  // placeholders (no URL) have nothing to zoom into.
+  const viewable = images
+    .map((img, i) => ({ img, i }))
+    .filter(({ img }) => typeof img.url === "string" && img.url.length > 0);
+  const viewableImages = viewable.map(({ img }) => img);
+  const originalToViewable = new Map<number, number>(
+    viewable.map(({ i }, v) => [i, v]),
+  );
+
+  const [lightboxIndex, setLightboxIndex] = useState<number | null>(null);
+
+  return (
+    <>
+      <div className="ml-auto flex flex-wrap items-end justify-end gap-2">
+        {images.map((img, i) => (
+          <UserImageCell
+            key={`${img.url ?? "placeholder"}-${i}`}
+            image={img}
+            placeholderLabel={t("message.imageAttachment")}
+            openLabel={t("lightbox.open")}
+            onOpen={
+              originalToViewable.has(i)
+                ? () => setLightboxIndex(originalToViewable.get(i)!)
+                : undefined
+            }
+          />
+        ))}
+      </div>
+      <ImageLightbox
+        images={viewableImages}
+        index={lightboxIndex}
+        onIndexChange={setLightboxIndex}
+        onOpenChange={(open) => {
+          if (!open) setLightboxIndex(null);
+        }}
+      />
+    </>
+  );
+}
+
+function UserImageCell({
+  image,
+  placeholderLabel,
+  openLabel,
+  onOpen,
+}: {
+  image: UIImage;
+  placeholderLabel: string;
+  openLabel: string;
+  onOpen?: () => void;
+}) {
+  const hasUrl = typeof image.url === "string" && image.url.length > 0;
+  const tileClasses = cn(
+    "relative h-24 w-24 overflow-hidden rounded-[14px] border border-border/60 bg-muted/40",
+    "shadow-[0_6px_18px_-14px_rgba(0,0,0,0.45)]",
+  );
+
+  if (hasUrl && onOpen) {
+    return (
+      <button
+        type="button"
+        onClick={onOpen}
+        aria-label={image.name ? `${openLabel}: ${image.name}` : openLabel}
+        title={image.name ?? undefined}
+        className={cn(
+          tileClasses,
+          "cursor-zoom-in transition-transform duration-150 motion-reduce:transition-none",
+          "hover:scale-[1.02] hover:ring-2 hover:ring-primary/30",
+          "focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary/50",
+        )}
+      >
+        <img
+          src={image.url}
+          alt={image.name ?? ""}
+          loading="lazy"
+          decoding="async"
+          draggable={false}
+          className="h-full w-full object-cover"
+        />
+      </button>
+    );
+  }
+
+  return (
+    <div className={tileClasses} title={image.name ?? undefined}>
+      <div
+        className="flex h-full w-full flex-col items-center justify-center gap-1 px-2 text-[11px] text-muted-foreground"
+        aria-label={placeholderLabel}
+      >
+        <ImageIcon className="h-4 w-4 flex-none" aria-hidden />
+        <span className="line-clamp-2 text-center leading-tight">
+          {image.name ?? placeholderLabel}
+        </span>
+      </div>
+    </div>
+  );
+}
+
 /** Blinking cursor appended at the end of streaming text. */
 function StreamCursor() {
   const { t } = useTranslation();
diff --git a/webui/src/components/thread/StreamErrorNotice.tsx b/webui/src/components/thread/StreamErrorNotice.tsx
new file mode 100644
index 00000000..06c89815
--- /dev/null
+++ b/webui/src/components/thread/StreamErrorNotice.tsx
@@ -0,0 +1,72 @@
+import { AlertTriangle, X } from "lucide-react";
+import { useTranslation } from "react-i18next";
+
+import { Button } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
+import type { StreamError } from "@/lib/nanobot-client";
+
+interface StreamErrorNoticeProps {
+  error: StreamError;
+  onDismiss: () => void;
+}
+
+/**
+ * Dismissible banner that surfaces transport-level faults the user needs to
+ * know about. Rendered above the composer so the message the fault referred
+ * to remains in view just above. ``role="alert"`` + ``aria-live="assertive"``
+ * ensures screen readers announce the failure.
+ */
+export function StreamErrorNotice({ error, onDismiss }: StreamErrorNoticeProps) {
+  const { t } = useTranslation();
+
+  const { title, body } = resolveCopy(error, t);
+
+  return (
+    <div
+      role="alert"
+      aria-live="assertive"
+      className={cn(
+        "mb-2 flex items-start gap-2 rounded-lg border border-destructive/30",
+        "bg-destructive/10 px-3 py-2 text-[12px] leading-5 text-destructive",
+        "animate-in fade-in-0 slide-in-from-bottom-1",
+      )}
+    >
+      <AlertTriangle
+        className="mt-0.5 h-4 w-4 shrink-0"
+        aria-hidden
+      />
+      <div className="flex-1">
+        <p className="font-medium">{title}</p>
+        <p className="mt-0.5 text-destructive/80">{body}</p>
+      </div>
+      <Button
+        variant="ghost"
+        size="icon"
+        onClick={onDismiss}
+        aria-label={t("common.dismiss")}
+        className="h-6 w-6 shrink-0 text-destructive hover:bg-destructive/15 hover:text-destructive"
+      >
+        <X className="h-3.5 w-3.5" />
+      </Button>
+    </div>
+  );
+}
+
+function resolveCopy(
+  error: StreamError,
+  t: (key: string) => string,
+): { title: string; body: string } {
+  switch (error.kind) {
+    case "message_too_big":
+      return {
+        title: t("errors.messageTooBig.title"),
+        body: t("errors.messageTooBig.body"),
+      };
+    default: {
+      // Exhaustiveness guard: if a new StreamError kind is added, TS will
+      // complain here until we add a corresponding i18n branch.
+      const _exhaustive: never = error.kind;
+      return { title: String(_exhaustive), body: "" };
+    }
+  }
+}
diff --git a/webui/src/components/thread/ThreadComposer.tsx b/webui/src/components/thread/ThreadComposer.tsx
index 35350fef..9b38d903 100644
--- a/webui/src/components/thread/ThreadComposer.tsx
+++ b/webui/src/components/thread/ThreadComposer.tsx
@@ -1,12 +1,43 @@
-import { useCallback, useEffect, useRef, useState } from "react";
-import { ArrowUp } from "lucide-react";
+import {
+  useCallback,
+  useEffect,
+  useMemo,
+  useRef,
+  useState,
+  type KeyboardEvent as ReactKeyboardEvent,
+} from "react";
+import {
+  ArrowUp,
+  ImageIcon,
+  Loader2,
+  Paperclip,
+  X,
+} from "lucide-react";
 import { useTranslation } from "react-i18next";
 
 import { Button } from "@/components/ui/button";
+import {
+  useAttachedImages,
+  type AttachedImage,
+  type AttachmentError,
+  MAX_IMAGES_PER_MESSAGE,
+} from "@/hooks/useAttachedImages";
+import { useClipboardAndDrop } from "@/hooks/useClipboardAndDrop";
+import type { SendImage } from "@/hooks/useNanobotStream";
 import { cn } from "@/lib/utils";
 
+/** ``<input accept>``: aligned with the server's MIME whitelist. SVG is
+ * deliberately excluded to avoid an embedded-script XSS surface. */
+const ACCEPT_ATTR = "image/png,image/jpeg,image/webp,image/gif";
+
+function formatBytes(n: number): string {
+  if (n < 1024) return `${n} B`;
+  if (n < 1024 * 1024) return `${(n / 1024).toFixed(1)} KB`;
+  return `${(n / (1024 * 1024)).toFixed(1)} MB`;
+}
+
 interface ThreadComposerProps {
-  onSend: (content: string) => void;
+  onSend: (content: string, images?: SendImage[]) => void;
   disabled?: boolean;
   placeholder?: string;
   modelLabel?: string | null;
@@ -22,11 +53,47 @@ export function ThreadComposer({
 }: ThreadComposerProps) {
   const { t } = useTranslation();
   const [value, setValue] = useState("");
+  const [inlineError, setInlineError] = useState<string | null>(null);
   const textareaRef = useRef<HTMLTextAreaElement>(null);
+  const fileInputRef = useRef<HTMLInputElement>(null);
+  const chipRefs = useRef(new Map<string, HTMLButtonElement>());
   const isHero = variant === "hero";
   const resolvedPlaceholder =
     placeholder ?? t("thread.composer.placeholderThread");
 
+  const { images, enqueue, remove, clear, encoding, full } =
+    useAttachedImages();
+
+  const formatRejection = useCallback(
+    (reason: AttachmentError): string => {
+      const key = `thread.composer.imageRejected.${reason}`;
+      return t(key, { max: MAX_IMAGES_PER_MESSAGE });
+    },
+    [t],
+  );
+
+  const addFiles = useCallback(
+    (files: File[]) => {
+      if (files.length === 0) return;
+      const { rejected } = enqueue(files);
+      if (rejected.length > 0) {
+        setInlineError(formatRejection(rejected[0].reason));
+      } else {
+        setInlineError(null);
+      }
+    },
+    [enqueue, formatRejection],
+  );
+
+  const {
+    isDragging,
+    onPaste,
+    onDragEnter,
+    onDragOver,
+    onDragLeave,
+    onDrop,
+  } = useClipboardAndDrop(addFiles);
+
   useEffect(() => {
     if (disabled) return;
     const el = textareaRef.current;
@@ -35,11 +102,43 @@ export function ThreadComposer({
     return () => cancelAnimationFrame(id);
   }, [disabled]);
 
+  const readyImages = useMemo(
+    () => images.filter((img): img is AttachedImage & { dataUrl: string } =>
+      img.status === "ready" && typeof img.dataUrl === "string",
+    ),
+    [images],
+  );
+  const hasErrors = images.some((img) => img.status === "error");
+
+  const canSend =
+    !disabled
+    && !encoding
+    && !hasErrors
+    && (value.trim().length > 0 || readyImages.length > 0);
+
   const submit = useCallback(() => {
+    if (!canSend) return;
     const trimmed = value.trim();
-    if (!trimmed || disabled) return;
-    onSend(trimmed);
+    // Share the same normalized ``data:`` URL with both the wire payload and
+    // the optimistic bubble preview: data URLs are self-contained (no blob
+    // lifetime, safe under React StrictMode double-mount) and keep the
+    // bubble in sync with whatever the backend actually sees.
+    const payload: SendImage[] | undefined =
+      readyImages.length > 0
+        ? readyImages.map((img) => ({
+            media: {
+              data_url: img.dataUrl,
+              name: img.file.name,
+            },
+            preview: { url: img.dataUrl, name: img.file.name },
+          }))
+        : undefined;
+    onSend(trimmed, payload);
     setValue("");
+    setInlineError(null);
+    // Bubble owns the data URL copy; safe to revoke every staged blob
+    // preview here without affecting the rendered message.
+    clear();
     requestAnimationFrame(() => {
       const el = textareaRef.current;
       if (el) {
@@ -47,9 +146,9 @@ export function ThreadComposer({
         el.focus();
       }
     });
-  }, [disabled, onSend, value]);
+  }, [canSend, clear, onSend, readyImages, value]);
 
-  const onKeyDown: React.KeyboardEventHandler<HTMLTextAreaElement> = (e) => {
+  const onKeyDown = (e: ReactKeyboardEvent<HTMLTextAreaElement>) => {
     if (e.key === "Enter" && !e.shiftKey && !e.nativeEvent.isComposing) {
       e.preventDefault();
       submit();
@@ -62,12 +161,55 @@ export function ThreadComposer({
     el.style.height = `${Math.min(el.scrollHeight, 260)}px`;
   };
 
+  const onFilePick: React.ChangeEventHandler<HTMLInputElement> = (e) => {
+    const files = Array.from(e.target.files ?? []);
+    e.target.value = "";
+    addFiles(files);
+  };
+
+  const removeChip = useCallback(
+    (id: string) => {
+      const { nextFocusId } = remove(id);
+      setInlineError(null);
+      requestAnimationFrame(() => {
+        const el = nextFocusId ? chipRefs.current.get(nextFocusId) : null;
+        if (el) {
+          el.focus();
+        } else {
+          textareaRef.current?.focus();
+        }
+      });
+    },
+    [remove],
+  );
+
+  const onChipKey = useCallback(
+    (id: string) => (e: ReactKeyboardEvent<HTMLButtonElement>) => {
+      if (
+        e.key === "Delete" ||
+        e.key === "Backspace" ||
+        e.key === "Enter" ||
+        e.key === " "
+      ) {
+        e.preventDefault();
+        removeChip(id);
+      }
+    },
+    [removeChip],
+  );
+
+  const attachButtonDisabled = disabled || full;
+
   return (
     <form
       onSubmit={(e) => {
         e.preventDefault();
         submit();
       }}
+      onDragEnter={onDragEnter}
+      onDragOver={onDragOver}
+      onDragLeave={onDragLeave}
+      onDrop={onDrop}
       className={cn("w-full", isHero ? "px-0" : "px-1 pb-1.5 pt-1 sm:px-0")}
     >
       <div
@@ -78,14 +220,44 @@ export function ThreadComposer({
             : "max-w-[49.5rem] rounded-[16px] border border-border/70 bg-card/55",
           "focus-within:bg-card/70 focus-within:ring-1 focus-within:ring-foreground/8",
           disabled && "opacity-60",
+          isDragging && "ring-2 ring-primary/40 motion-reduce:ring-0 motion-reduce:border-primary",
         )}
       >
+        {images.length > 0 ? (
+          <div
+            className="flex flex-wrap gap-2 px-3 pt-3"
+            aria-label={t("thread.composer.attachImage")}
+          >
+            {images.map((img) => (
+              <AttachmentChip
+                key={img.id}
+                image={img}
+                labelRemove={t("thread.composer.remove")}
+                labelEncoding={t("thread.composer.encoding")}
+                normalizedHint={(orig, current) =>
+                  t("thread.composer.normalizedSizeHint", {
+                    orig: formatBytes(orig),
+                    current: formatBytes(current),
+                  })
+                }
+                formatError={formatRejection}
+                onRemove={() => removeChip(img.id)}
+                onKeyDown={onChipKey(img.id)}
+                registerRef={(el) => {
+                  if (el) chipRefs.current.set(img.id, el);
+                  else chipRefs.current.delete(img.id);
+                }}
+              />
+            ))}
+          </div>
+        ) : null}
         <textarea
           ref={textareaRef}
           value={value}
           onChange={(e) => setValue(e.target.value)}
           onInput={onInput}
           onKeyDown={onKeyDown}
+          onPaste={onPaste}
           rows={1}
           placeholder={resolvedPlaceholder}
           disabled={disabled}
@@ -100,6 +272,17 @@ export function ThreadComposer({
             "disabled:cursor-not-allowed",
           )}
         />
+        {inlineError ? (
+          <div
+            role="alert"
+            className={cn(
+              "mx-3 mb-1 rounded-md border border-destructive/40 bg-destructive/8 px-2.5 py-1",
+              "text-[11.5px] font-medium text-destructive",
+            )}
+          >
+            {inlineError}
+          </div>
+        ) : null}
         <div
           className={cn(
             "flex items-center justify-between gap-2",
@@ -107,6 +290,28 @@ export function ThreadComposer({
           )}
         >
           <div className="flex min-w-0 items-center gap-2">
+            <input
+              ref={fileInputRef}
+              type="file"
+              accept={ACCEPT_ATTR}
+              multiple
+              hidden
+              onChange={onFilePick}
+            />
+            <Button
+              type="button"
+              size="icon"
+              variant="ghost"
+              disabled={attachButtonDisabled}
+              aria-label={t("thread.composer.attachImage")}
+              onClick={() => fileInputRef.current?.click()}
+              className={cn(
+                "rounded-full text-muted-foreground hover:text-foreground",
+                isHero ? "h-8.5 w-8.5" : "h-7.5 w-7.5",
+              )}
+            >
+              <Paperclip className={cn(isHero ? "h-4 w-4" : "h-3.5 w-3.5")} />
+            </Button>
             {modelLabel ? (
               <span
                 title={modelLabel}
@@ -131,12 +336,12 @@ export function ThreadComposer({
           <Button
             type="submit"
             size="icon"
-            disabled={disabled || !value.trim()}
+            disabled={!canSend}
             aria-label={t("thread.composer.send")}
             className={cn(
               "rounded-full border border-border/70 bg-secondary/85 text-secondary-foreground shadow-none transition-transform hover:bg-accent",
               isHero ? "h-8.5 w-8.5" : "h-7.5 w-7.5",
-              value.trim() && !disabled && "hover:scale-[1.03] active:scale-95",
+              canSend && "hover:scale-[1.03] active:scale-95",
             )}
           >
             <ArrowUp className={cn(isHero ? "h-4.5 w-4.5" : "h-4 w-4")} />
@@ -146,3 +351,94 @@ export function ThreadComposer({
     </form>
   );
 }
+
+interface AttachmentChipProps {
+  image: AttachedImage;
+  labelRemove: string;
+  labelEncoding: string;
+  normalizedHint: (origBytes: number, currentBytes: number) => string;
+  formatError: (reason: AttachmentError) => string;
+  onRemove: () => void;
+  onKeyDown: (e: ReactKeyboardEvent<HTMLButtonElement>) => void;
+  registerRef: (el: HTMLButtonElement | null) => void;
+}
+
+function AttachmentChip({
+  image,
+  labelRemove,
+  labelEncoding,
+  normalizedHint,
+  formatError,
+  onRemove,
+  onKeyDown,
+  registerRef,
+}: AttachmentChipProps) {
+  const sizeLabel =
+    image.status === "ready" && image.normalized && image.encodedBytes
+      ? normalizedHint(image.file.size, image.encodedBytes)
+      : formatBytes(image.file.size);
+  const tone =
+    image.status === "error"
+      ? "border-destructive/40 bg-destructive/5 text-destructive"
+      : "border-border/70 bg-muted/60";
+
+  return (
+    <div
+      className={cn(
+        "group relative flex items-center gap-2 rounded-[12px] border px-2 py-1.5",
+        "transition-colors motion-reduce:transition-none",
+        tone,
+      )}
+      data-testid="composer-chip"
+    >
+      <div className="relative h-10 w-10 overflow-hidden rounded-md bg-background">
+        {image.previewUrl ? (
+          <img
+            src={image.previewUrl}
+            alt=""
+            aria-hidden
+            loading="eager"
+            draggable={false}
+            className="h-full w-full object-cover"
+          />
+        ) : (
+          <div className="flex h-full w-full items-center justify-center">
+            <ImageIcon className="h-4 w-4 text-muted-foreground" aria-hidden />
+          </div>
+        )}
+        {image.status === "encoding" ? (
+          <div
+            className="absolute inset-0 flex items-center justify-center bg-background/60"
+            aria-label={labelEncoding}
+          >
+            <Loader2 className="h-4 w-4 animate-spin motion-reduce:animate-none" aria-hidden />
+          </div>
+        ) : null}
+      </div>
+      <div className="flex min-w-0 flex-col text-[11.5px] leading-4">
+        <span className="truncate max-w-[14rem] font-medium" title={image.file.name}>
+          {image.file.name}
+        </span>
+        <span className="truncate text-muted-foreground">
+          {image.status === "error" && image.error
+            ? formatError(image.error)
+            : sizeLabel}
+        </span>
+      </div>
+      <button
+        type="button"
+        ref={registerRef}
+        onClick={onRemove}
+        onKeyDown={onKeyDown}
+        aria-label={labelRemove}
+        className={cn(
+          "ml-1 grid h-5 w-5 flex-none place-items-center rounded-full",
+          "text-muted-foreground/80 hover:bg-foreground/8 hover:text-foreground",
+          "focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-foreground/30",
+        )}
+      >
+        <X className="h-3.5 w-3.5" aria-hidden />
+      </button>
+    </div>
+  );
+}
diff --git a/webui/src/components/thread/ThreadShell.tsx b/webui/src/components/thread/ThreadShell.tsx
index 1b0bd6c8..0355c47e 100644
--- a/webui/src/components/thread/ThreadShell.tsx
+++ b/webui/src/components/thread/ThreadShell.tsx
@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 
 import { ThreadComposer } from "@/components/thread/ThreadComposer";
 import { ThreadHeader } from "@/components/thread/ThreadHeader";
+import { StreamErrorNotice } from "@/components/thread/StreamErrorNotice";
 import { ThreadViewport } from "@/components/thread/ThreadViewport";
 import { useNanobotStream } from "@/hooks/useNanobotStream";
 import { useSessionHistory } from "@/hooks/useSessions";
@@ -47,10 +48,14 @@ export function ThreadShell({
     if (!chatId) return historical;
     return messageCacheRef.current.get(chatId) ?? historical;
   }, [chatId, historical]);
-  const { messages, isStreaming, send, setMessages } = useNanobotStream(
-    chatId,
-    initial,
-  );
+  const {
+    messages,
+    isStreaming,
+    send,
+    setMessages,
+    streamError,
+    dismissStreamError,
+  } = useNanobotStream(chatId, initial);
   const showHeroComposer = messages.length === 0 && !loading;
 
   useEffect(() => {
@@ -140,31 +145,39 @@ export function ThreadShell({
         isStreaming={isStreaming}
         emptyState={emptyState}
         composer={
-          session ? (
-            <ThreadComposer
-              onSend={send}
-              disabled={!chatId}
-              placeholder={
-                showHeroComposer
-                  ? t("thread.composer.placeholderHero")
-                  : t("thread.composer.placeholderThread")
-              }
-              modelLabel={toModelBadgeLabel(modelName)}
-              variant={showHeroComposer ? "hero" : "thread"}
-            />
-          ) : (
-            <ThreadComposer
-              onSend={handleWelcomeSend}
-              disabled={booting}
-              placeholder={
-                booting
-                  ? t("thread.composer.placeholderOpening")
-                  : t("thread.composer.placeholderHero")
-              }
-              modelLabel={toModelBadgeLabel(modelName)}
-              variant="hero"
-            />
-          )
+          <>
+            {streamError ? (
+              <StreamErrorNotice
+                error={streamError}
+                onDismiss={dismissStreamError}
+              />
+            ) : null}
+            {session ? (
+              <ThreadComposer
+                onSend={send}
+                disabled={!chatId}
+                placeholder={
+                  showHeroComposer
+                    ? t("thread.composer.placeholderHero")
+                    : t("thread.composer.placeholderThread")
+                }
+                modelLabel={toModelBadgeLabel(modelName)}
+                variant={showHeroComposer ? "hero" : "thread"}
+              />
+            ) : (
+              <ThreadComposer
+                onSend={handleWelcomeSend}
+                disabled={booting}
+                placeholder={
+                  booting
+                    ? t("thread.composer.placeholderOpening")
+                    : t("thread.composer.placeholderHero")
+                }
+                modelLabel={toModelBadgeLabel(modelName)}
+                variant="hero"
+              />
+            )}
+          </>
         }
       />
     </section>
diff --git a/webui/src/hooks/useAttachedImages.ts b/webui/src/hooks/useAttachedImages.ts
new file mode 100644
index 00000000..6aa679f7
--- /dev/null
+++ b/webui/src/hooks/useAttachedImages.ts
@@ -0,0 +1,233 @@
+import { useCallback, useEffect, useRef, useState } from "react";
+
+import { encodeImage, type EncodeFailure } from "@/lib/imageEncode";
+
+/** Lifecycle stages of one attachment:
+ *
+ * - ``encoding``  — posted to the Worker; chip shows a spinner
+ * - ``ready``     — ``dataUrl`` available; safe to submit
+ * - ``error``     — validation / decode failure; chip shows inline error
+ */
+export type AttachmentStatus = "encoding" | "ready" | "error";
+
+export interface AttachedImage {
+  id: string;
+  file: File;
+  /** Optimistic ``blob:`` preview URL; revoked on ``remove`` / ``clear`` /
+   * unmount. */
+  previewUrl: string;
+  status: AttachmentStatus;
+  /** Populated when ``status === "ready"``. */
+  dataUrl?: string;
+  /** Size of the final encoded payload (base64 bytes decoded). */
+  encodedBytes?: number;
+  /** Whether the Worker re-encoded the image to hit the size budget. */
+  normalized?: boolean;
+  /** Human-readable validation / encoding error when ``status === "error"``. */
+  error?: AttachmentError;
+}
+
+/** Machine-readable rejection reasons surfaced as inline chip errors.
+ *
+ * Callers localize these via the ``composer.imageRejected.*`` i18n table. */
+export type AttachmentError =
+  | "unsupported_type"   // server whitelist excludes this MIME
+  | "too_many_images"    // per-message cap (4) reached before enqueue
+  | "magic_mismatch"     // extension lies about the real content
+  | "decode_failed"      // Worker couldn't decode / re-encode
+  | "too_large"          // even after normalization we exceed the budget
+  | "io";                // file read failed at the browser layer
+
+export const MAX_IMAGES_PER_MESSAGE = 4;
+
+/** MIME whitelist — mirrors the server's and the ``<input accept>`` attr. */
+const ACCEPTED_MIMES: ReadonlySet<string> = new Set([
+  "image/png",
+  "image/jpeg",
+  "image/webp",
+  "image/gif",
+]);
+
+function uuid(): string {
+  if (typeof crypto !== "undefined" && "randomUUID" in crypto) {
+    return (crypto as Crypto).randomUUID();
+  }
+  return `img-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+}
+
+function mapEncodeFailure(reason: EncodeFailure["reason"]): AttachmentError {
+  switch (reason) {
+    case "invalid_mime":
+    case "magic_mismatch":
+      return "magic_mismatch";
+    case "too_large_after_normalize":
+      return "too_large";
+    case "io":
+      return "io";
+    case "decode_failed":
+    default:
+      return "decode_failed";
+  }
+}
+
+export interface UseAttachedImagesApi {
+  images: AttachedImage[];
+  /** Enqueue new files. Returns the list of rejected files so the caller can
+   * surface inline errors. Files rejected client-side (wrong MIME, limit) are
+   * *not* added to ``images`` — only recoverable encoding failures show up as
+   * error chips. */
+  enqueue: (files: Iterable<File>) => {
+    rejected: Array<{ file: File; reason: AttachmentError }>;
+  };
+  remove: (id: string) => { nextFocusId: string | null };
+  /** Revoke every staged blob URL and drop all attachments. Called after a
+   * successful submit — the optimistic bubble holds onto an independent
+   * ``data:`` URL so tearing down blob previews here is safe. */
+  clear: () => void;
+  /** ``true`` when at least one image is still encoding — Send should wait. */
+  encoding: boolean;
+  /** ``true`` when we've hit ``MAX_IMAGES_PER_MESSAGE``. */
+  full: boolean;
+}
+
+/** Manage the lifecycle of images attached to the Composer.
+ *
+ * Responsibilities in one place:
+ *   - validation (MIME whitelist, count cap)
+ *   - blob URL creation + revocation
+ *   - Worker orchestration
+ *   - focus bookkeeping so keyboard delete doesn't strand the user
+ */
+export function useAttachedImages(): UseAttachedImagesApi {
+  const [images, setImages] = useState<AttachedImage[]>([]);
+  // Ref mirror so ``enqueue`` can see the authoritative length when invoked
+  // multiple times in a single tick (rapid file selection, drag of many
+  // files, paste storms). ``state`` is stale for that second + call.
+  const imagesRef = useRef<AttachedImage[]>([]);
+  imagesRef.current = images;
+
+  const setEntry = useCallback((id: string, patch: Partial<AttachedImage>) => {
+    setImages((prev) => {
+      const next = prev.map((img) => (img.id === id ? { ...img, ...patch } : img));
+      imagesRef.current = next;
+      return next;
+    });
+  }, []);
+
+  const enqueue = useCallback(
+    (files: Iterable<File>) => {
+      const rejected: Array<{ file: File; reason: AttachmentError }> = [];
+      const toAdd: AttachedImage[] = [];
+      let slot = MAX_IMAGES_PER_MESSAGE - imagesRef.current.length;
+
+      for (const file of files) {
+        if (!ACCEPTED_MIMES.has(file.type)) {
+          rejected.push({ file, reason: "unsupported_type" });
+          continue;
+        }
+        if (slot <= 0) {
+          rejected.push({ file, reason: "too_many_images" });
+          continue;
+        }
+        slot -= 1;
+        toAdd.push({
+          id: uuid(),
+          file,
+          previewUrl: URL.createObjectURL(file),
+          status: "encoding",
+        });
+      }
+
+      if (toAdd.length > 0) {
+        const next = [...imagesRef.current, ...toAdd];
+        imagesRef.current = next;
+        setImages(next);
+        // Fire the Worker after the commit so chips render first (good INP).
+        for (const entry of toAdd) {
+          queueMicrotask(() => {
+            encodeImage(entry.file).then(
+              (result) => {
+                if (result.ok) {
+                  setEntry(entry.id, {
+                    status: "ready",
+                    dataUrl: result.dataUrl,
+                    encodedBytes: result.bytes,
+                    normalized: result.normalized,
+                  });
+                } else {
+                  setEntry(entry.id, {
+                    status: "error",
+                    error: mapEncodeFailure(result.reason),
+                  });
+                }
+              },
+              () => {
+                setEntry(entry.id, {
+                  status: "error",
+                  error: "decode_failed",
+                });
+              },
+            );
+          });
+        }
+      }
+      return { rejected };
+    },
+    [setEntry],
+  );
+
+  const remove = useCallback((id: string) => {
+    let nextFocusId: string | null = null;
+    setImages((prev) => {
+      const idx = prev.findIndex((img) => img.id === id);
+      if (idx === -1) return prev;
+      const target = prev[idx];
+      try {
+        URL.revokeObjectURL(target.previewUrl);
+      } catch {
+        // No-op: previewUrl revocation is best-effort.
+      }
+      const next = [...prev.slice(0, idx), ...prev.slice(idx + 1)];
+      imagesRef.current = next;
+      // Prefer moving focus to the chip at the same index, else previous.
+      const candidate = next[idx] ?? next[idx - 1];
+      nextFocusId = candidate?.id ?? null;
+      return next;
+    });
+    return { nextFocusId };
+  }, []);
+
+  const clear = useCallback(() => {
+    setImages((prev) => {
+      for (const img of prev) {
+        try {
+          URL.revokeObjectURL(img.previewUrl);
+        } catch {
+          // revoke is best-effort
+        }
+      }
+      imagesRef.current = [];
+      return [];
+    });
+  }, []);
+
+  // Final safety net: revoke any outstanding blob URLs on unmount. Safe
+  // under StrictMode double-invoke because revoked blob URLs are only
+  // referenced from in-hook chip state, which is rebuilt on remount.
+  useEffect(() => {
+    return () => {
+      for (const img of imagesRef.current) {
+        try {
+          URL.revokeObjectURL(img.previewUrl);
+        } catch {
+          // best-effort cleanup on unmount
+        }
+      }
+    };
+  }, []);
+
+  const encoding = images.some((img) => img.status === "encoding");
+  const full = images.length >= MAX_IMAGES_PER_MESSAGE;
+
+  return { images, enqueue, remove, clear, encoding, full };
+}
diff --git a/webui/src/hooks/useClipboardAndDrop.ts b/webui/src/hooks/useClipboardAndDrop.ts
new file mode 100644
index 00000000..39e3a68c
--- /dev/null
+++ b/webui/src/hooks/useClipboardAndDrop.ts
@@ -0,0 +1,111 @@
+import { useCallback, useRef, useState } from "react";
+
+/** Extract image ``File``s from a paste / drop event.
+ *
+ * Deliberate behaviour:
+ *   - Only items whose ``kind === "file"`` and ``type`` starts with
+ *     ``image/`` are returned; ``<img>`` tags inside HTML fragments are
+ *     ignored (defending against remote URL fetch + XSS surfaces).
+ *   - Plain text pasted alongside images is *not* consumed by this helper,
+ *     so the caller can still let the textarea receive it naturally.
+ */
+export function extractImageFilesFromPaste(
+  event: ClipboardEvent | React.ClipboardEvent,
+): File[] {
+  const clipboard = (event as ClipboardEvent).clipboardData
+    ?? (event as React.ClipboardEvent).clipboardData;
+  if (!clipboard) return [];
+  const files: File[] = [];
+  for (const item of Array.from(clipboard.items)) {
+    if (item.kind !== "file") continue;
+    if (!item.type.startsWith("image/")) continue;
+    const file = item.getAsFile();
+    if (file) files.push(file);
+  }
+  return files;
+}
+
+/** Extract dropped image files, mirroring ``extractImageFilesFromPaste``. */
+export function extractImageFilesFromDrop(
+  event: DragEvent | React.DragEvent,
+): File[] {
+  const dt = (event as DragEvent).dataTransfer
+    ?? (event as React.DragEvent).dataTransfer;
+  if (!dt) return [];
+  const files: File[] = [];
+  for (const item of Array.from(dt.files)) {
+    if (item.type.startsWith("image/")) files.push(item);
+  }
+  return files;
+}
+
+export interface UseClipboardAndDropApi {
+  /** Whether a drag is currently hovering the drop zone (toggle dragover UI). */
+  isDragging: boolean;
+  onPaste: (
+    event: React.ClipboardEvent,
+  ) => void;
+  onDragEnter: (event: React.DragEvent) => void;
+  onDragOver: (event: React.DragEvent) => void;
+  onDragLeave: (event: React.DragEvent) => void;
+  onDrop: (event: React.DragEvent) => void;
+}
+
+/** Wire paste + drag-and-drop to a callback.
+ *
+ * The hook owns ``isDragging`` state and the refcount that keeps it accurate
+ * across nested ``dragenter`` / ``dragleave`` events (a known DOM gotcha: the
+ * text cursor inside a textarea fires ``dragleave`` on entry, flicking the
+ * highlight off otherwise). */
+export function useClipboardAndDrop(
+  onImageFiles: (files: File[]) => void,
+): UseClipboardAndDropApi {
+  const [isDragging, setIsDragging] = useState(false);
+  const dragDepth = useRef(0);
+
+  const onPaste = useCallback(
+    (event: React.ClipboardEvent) => {
+      const files = extractImageFilesFromPaste(event);
+      if (files.length === 0) return;
+      // Consume only when an image is actually present; plain-text paste still
+      // reaches the textarea unmolested.
+      event.preventDefault();
+      onImageFiles(files);
+    },
+    [onImageFiles],
+  );
+
+  const onDragEnter = useCallback((event: React.DragEvent) => {
+    if (!Array.from(event.dataTransfer.types ?? []).includes("Files")) return;
+    event.preventDefault();
+    dragDepth.current += 1;
+    setIsDragging(true);
+  }, []);
+
+  const onDragOver = useCallback((event: React.DragEvent) => {
+    if (!Array.from(event.dataTransfer.types ?? []).includes("Files")) return;
+    event.preventDefault();
+    event.dataTransfer.dropEffect = "copy";
+  }, []);
+
+  const onDragLeave = useCallback((event: React.DragEvent) => {
+    if (!Array.from(event.dataTransfer.types ?? []).includes("Files")) return;
+    event.preventDefault();
+    dragDepth.current = Math.max(0, dragDepth.current - 1);
+    if (dragDepth.current === 0) setIsDragging(false);
+  }, []);
+
+  const onDrop = useCallback(
+    (event: React.DragEvent) => {
+      dragDepth.current = 0;
+      setIsDragging(false);
+      const files = extractImageFilesFromDrop(event);
+      if (files.length === 0) return;
+      event.preventDefault();
+      onImageFiles(files);
+    },
+    [onImageFiles],
+  );
+
+  return { isDragging, onPaste, onDragEnter, onDragOver, onDragLeave, onDrop };
+}
diff --git a/webui/src/hooks/useNanobotStream.ts b/webui/src/hooks/useNanobotStream.ts
index 45a7ab01..b89744ed 100644
--- a/webui/src/hooks/useNanobotStream.ts
+++ b/webui/src/hooks/useNanobotStream.ts
@@ -1,7 +1,13 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 
 import { useClient } from "@/providers/ClientProvider";
-import type { InboundEvent, UIMessage } from "@/lib/types";
+import type { StreamError } from "@/lib/nanobot-client";
+import type {
+  InboundEvent,
+  OutboundMedia,
+  UIImage,
+  UIMessage,
+} from "@/lib/types";
 
 interface StreamBuffer {
   /** ID of the assistant message currently receiving deltas. */
@@ -16,24 +22,52 @@ interface StreamBuffer {
  * separately (e.g. via ``fetchSessionMessages``) since the server only replays
  * live events.
  */
+/** Payload passed to ``send`` when the user attaches one or more images.
+ *
+ * ``media`` is handed to the wire client verbatim; ``preview`` powers the
+ * optimistic user bubble (blob URLs so the preview appears before the server
+ * acks the frame). Keeping the two separate lets the bubble re-use the local
+ * blob URL even after the server persists the file under a different name. */
+export interface SendImage {
+  media: OutboundMedia;
+  preview: UIImage;
+}
+
 export function useNanobotStream(
   chatId: string | null,
   initialMessages: UIMessage[] = [],
 ): {
   messages: UIMessage[];
   isStreaming: boolean;
-  send: (content: string) => void;
+  send: (content: string, images?: SendImage[]) => void;
   setMessages: React.Dispatch<React.SetStateAction<UIMessage[]>>;
+  /** Latest transport-level fault raised since the last ``dismissStreamError``.
+   * ``null`` when there is nothing to show. */
+  streamError: StreamError | null;
+  /** Clear the current ``streamError`` (e.g. after the user dismisses the
+   * notification or starts a fresh action). */
+  dismissStreamError: () => void;
 } {
   const { client } = useClient();
   const [messages, setMessages] = useState<UIMessage[]>(initialMessages);
   const [isStreaming, setIsStreaming] = useState(false);
+  const [streamError, setStreamError] = useState<StreamError | null>(null);
   const buffer = useRef<StreamBuffer | null>(null);
 
-  // Reset local state when switching chats.
+  useEffect(() => {
+    return client.onError((err) => setStreamError(err));
+  }, [client]);
+
+  const dismissStreamError = useCallback(() => setStreamError(null), []);
+
+  // Reset local state when switching chats. ``streamError`` is scoped to the
+  // send that triggered it, so a chat swap should wipe it out: a stale
+  // "Message too large" banner on a freshly-opened chat-B would confuse the
+  // user about which send actually failed (and in which chat).
   useEffect(() => {
     setMessages(initialMessages);
     setIsStreaming(false);
+    setStreamError(null);
     buffer.current = null;
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [chatId]);
@@ -145,8 +179,14 @@ export function useNanobotStream(
   }, [chatId, client]);
 
   const send = useCallback(
-    (content: string) => {
-      if (!chatId || !content.trim()) return;
+    (content: string, images?: SendImage[]) => {
+      if (!chatId) return;
+      const hasImages = !!images && images.length > 0;
+      // Text is optional when images are attached — the agent will still see
+      // the image blocks via ``media`` paths.
+      if (!hasImages && !content.trim()) return;
+
+      const previews = hasImages ? images!.map((i) => i.preview) : undefined;
       setMessages((prev) => [
         ...prev,
         {
@@ -154,12 +194,21 @@ export function useNanobotStream(
           role: "user",
           content,
           createdAt: Date.now(),
+          ...(previews ? { images: previews } : {}),
         },
       ]);
-      client.sendMessage(chatId, content);
+      const wireMedia = hasImages ? images!.map((i) => i.media) : undefined;
+      client.sendMessage(chatId, content, wireMedia);
     },
     [chatId, client],
   );
 
-  return { messages, isStreaming, send, setMessages };
+  return {
+    messages,
+    isStreaming,
+    send,
+    setMessages,
+    streamError,
+    dismissStreamError,
+  };
 }
diff --git a/webui/src/hooks/useSessions.ts b/webui/src/hooks/useSessions.ts
index b627f860..ea51c220 100644
--- a/webui/src/hooks/useSessions.ts
+++ b/webui/src/hooks/useSessions.ts
@@ -123,12 +123,25 @@ export function useSessionHistory(key: string | null): {
         const ui: UIMessage[] = body.messages.flatMap((m, idx) => {
           if (m.role !== "user" && m.role !== "assistant") return [];
           if (typeof m.content !== "string") return [];
+          // Hydrate signed media URLs into the bubble's ``images`` slot so
+          // historical user turns render real previews (the live-send path
+          // uses data URLs; both shapes converge on the same ``UIImage``).
+          const images =
+            m.role === "user" &&
+            Array.isArray(m.media_urls) &&
+            m.media_urls.length > 0
+              ? m.media_urls.map((mu) => ({
+                  url: mu.url,
+                  name: mu.name,
+                }))
+              : undefined;
           return [
             {
               id: `hist-${idx}`,
               role: m.role,
               content: m.content,
               createdAt: m.timestamp ? Date.parse(m.timestamp) : Date.now(),
+              ...(images ? { images } : {}),
             },
           ];
         });
diff --git a/webui/src/i18n/locales/en/common.json b/webui/src/i18n/locales/en/common.json
index 0b0f3406..aa6b3165 100644
--- a/webui/src/i18n/locales/en/common.json
+++ b/webui/src/i18n/locales/en/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "Opening a new chat…",
       "inputAria": "Message input",
       "sendHint": "Enter to send · Shift+Enter for newline",
-      "send": "Send message"
+      "send": "Send message",
+      "attachImage": "Attach image",
+      "encoding": "Encoding…",
+      "remove": "Remove attachment",
+      "normalizedSizeHint": "{{orig}} → {{current}} (auto)",
+      "imageRejected": {
+        "unsupported_type": "Unsupported file type",
+        "too_many_images": "Max {{max}} images per message",
+        "magic_mismatch": "File doesn't look like a real image",
+        "decode_failed": "Couldn't decode this image",
+        "too_large": "Image is too large — try a smaller one",
+        "io": "Couldn't read this file"
+      }
     },
     "scrollToBottom": "Scroll to bottom"
   },
@@ -72,12 +84,29 @@
     "streaming": "streaming",
     "assistantTyping": "Assistant is typing",
     "toolSingle": "Using a tool",
-    "toolMany": "Used {{count}} tools"
+    "toolMany": "Used {{count}} tools",
+    "imageAttachment": "Image attachment"
+  },
+  "lightbox": {
+    "title": "Image preview",
+    "open": "View image",
+    "prev": "Previous image",
+    "next": "Next image",
+    "close": "Close image preview"
   },
   "code": {
     "fallbackLanguage": "code",
     "copyAria": "Copy code",
     "copy": "Copy",
     "copied": "Copied"
+  },
+  "common": {
+    "dismiss": "Dismiss"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "Message too large",
+      "body": "The server rejected your last message because it exceeded the size limit. Remove some images or try smaller files, then send again."
+    }
   }
 }
diff --git a/webui/src/i18n/locales/es/common.json b/webui/src/i18n/locales/es/common.json
index 2f11a2fb..93bef843 100644
--- a/webui/src/i18n/locales/es/common.json
+++ b/webui/src/i18n/locales/es/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "Abriendo un nuevo chat…",
       "inputAria": "Entrada de mensaje",
       "sendHint": "Enter para enviar · Shift+Enter para nueva línea",
-      "send": "Enviar mensaje"
+      "send": "Enviar mensaje",
+      "attachImage": "Adjuntar imagen",
+      "encoding": "Procesando…",
+      "remove": "Quitar adjunto",
+      "normalizedSizeHint": "{{orig}} → {{current}} (auto)",
+      "imageRejected": {
+        "unsupported_type": "Tipo de archivo no compatible",
+        "too_many_images": "Máximo {{max}} imágenes por mensaje",
+        "magic_mismatch": "El archivo no parece una imagen real",
+        "decode_failed": "No se pudo decodificar esta imagen",
+        "too_large": "Imagen demasiado grande — prueba una más pequeña",
+        "io": "No se pudo leer este archivo"
+      }
     },
     "scrollToBottom": "Desplazarse al final"
   },
@@ -72,12 +84,29 @@
     "streaming": "transmitiendo",
     "assistantTyping": "El asistente está escribiendo",
     "toolSingle": "Usando una herramienta",
-    "toolMany": "Se usaron {{count}} herramientas"
+    "toolMany": "Se usaron {{count}} herramientas",
+    "imageAttachment": "Imagen adjunta"
+  },
+  "lightbox": {
+    "title": "Vista previa de imagen",
+    "open": "Ver imagen",
+    "prev": "Imagen anterior",
+    "next": "Imagen siguiente",
+    "close": "Cerrar vista previa"
   },
   "code": {
     "fallbackLanguage": "código",
     "copyAria": "Copiar código",
     "copy": "Copiar",
     "copied": "Copiado"
+  },
+  "common": {
+    "dismiss": "Cerrar"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "Mensaje demasiado grande",
+      "body": "El servidor rechazó tu último mensaje por superar el tamaño permitido. Quita algunas imágenes o usa archivos más pequeños y vuelve a enviarlo."
+    }
   }
 }
diff --git a/webui/src/i18n/locales/fr/common.json b/webui/src/i18n/locales/fr/common.json
index 33d59c72..ba9e759b 100644
--- a/webui/src/i18n/locales/fr/common.json
+++ b/webui/src/i18n/locales/fr/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "Ouverture d’une nouvelle discussion…",
       "inputAria": "Champ de message",
       "sendHint": "Entrée pour envoyer · Maj+Entrée pour un retour à la ligne",
-      "send": "Envoyer le message"
+      "send": "Envoyer le message",
+      "attachImage": "Joindre une image",
+      "encoding": "Traitement…",
+      "remove": "Retirer la pièce jointe",
+      "normalizedSizeHint": "{{orig}} → {{current}} (auto)",
+      "imageRejected": {
+        "unsupported_type": "Type de fichier non pris en charge",
+        "too_many_images": "Maximum {{max}} images par message",
+        "magic_mismatch": "Ce fichier n'est pas une image",
+        "decode_failed": "Impossible de décoder cette image",
+        "too_large": "Image trop grande — essayez-en une plus petite",
+        "io": "Impossible de lire ce fichier"
+      }
     },
     "scrollToBottom": "Faire défiler vers le bas"
   },
@@ -72,12 +84,29 @@
     "streaming": "en cours de génération",
     "assistantTyping": "L’assistant est en train d’écrire",
     "toolSingle": "Utilisation d’un outil",
-    "toolMany": "{{count}} outils utilisés"
+    "toolMany": "{{count}} outils utilisés",
+    "imageAttachment": "Pièce jointe image"
+  },
+  "lightbox": {
+    "title": "Aperçu de l’image",
+    "open": "Voir l’image",
+    "prev": "Image précédente",
+    "next": "Image suivante",
+    "close": "Fermer l’aperçu"
   },
   "code": {
     "fallbackLanguage": "code",
     "copyAria": "Copier le code",
     "copy": "Copier",
     "copied": "Copié"
+  },
+  "common": {
+    "dismiss": "Fermer"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "Message trop volumineux",
+      "body": "Le serveur a rejeté votre dernier message car il dépasse la taille autorisée. Retirez des images ou choisissez des fichiers plus légers, puis renvoyez-le."
+    }
   }
 }
diff --git a/webui/src/i18n/locales/id/common.json b/webui/src/i18n/locales/id/common.json
index f37132bd..9775372c 100644
--- a/webui/src/i18n/locales/id/common.json
+++ b/webui/src/i18n/locales/id/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "Membuka obrolan baru…",
       "inputAria": "Input pesan",
       "sendHint": "Enter untuk kirim · Shift+Enter untuk baris baru",
-      "send": "Kirim pesan"
+      "send": "Kirim pesan",
+      "attachImage": "Lampirkan gambar",
+      "encoding": "Memproses…",
+      "remove": "Hapus lampiran",
+      "normalizedSizeHint": "{{orig}} → {{current}} (auto)",
+      "imageRejected": {
+        "unsupported_type": "Tipe file tidak didukung",
+        "too_many_images": "Maksimal {{max}} gambar per pesan",
+        "magic_mismatch": "File ini tampaknya bukan gambar asli",
+        "decode_failed": "Tidak dapat mendekode gambar ini",
+        "too_large": "Gambar terlalu besar — coba yang lebih kecil",
+        "io": "Tidak dapat membaca file ini"
+      }
     },
     "scrollToBottom": "Gulir ke bawah"
   },
@@ -72,12 +84,29 @@
     "streaming": "sedang mengalir",
     "assistantTyping": "Asisten sedang mengetik",
     "toolSingle": "Menggunakan sebuah alat",
-    "toolMany": "Menggunakan {{count}} alat"
+    "toolMany": "Menggunakan {{count}} alat",
+    "imageAttachment": "Lampiran gambar"
+  },
+  "lightbox": {
+    "title": "Pratinjau gambar",
+    "open": "Lihat gambar",
+    "prev": "Gambar sebelumnya",
+    "next": "Gambar berikutnya",
+    "close": "Tutup pratinjau"
   },
   "code": {
     "fallbackLanguage": "kode",
     "copyAria": "Salin kode",
     "copy": "Salin",
     "copied": "Tersalin"
+  },
+  "common": {
+    "dismiss": "Tutup"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "Pesan terlalu besar",
+      "body": "Server menolak pesan terakhir karena melebihi batas ukuran. Hapus beberapa gambar atau gunakan berkas yang lebih kecil, lalu coba lagi."
+    }
   }
 }
diff --git a/webui/src/i18n/locales/ja/common.json b/webui/src/i18n/locales/ja/common.json
index 8529e9a8..6868dec5 100644
--- a/webui/src/i18n/locales/ja/common.json
+++ b/webui/src/i18n/locales/ja/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "新しいチャットを開いています…",
       "inputAria": "メッセージ入力欄",
       "sendHint": "Enter で送信 · Shift+Enter で改行",
-      "send": "メッセージを送信"
+      "send": "メッセージを送信",
+      "attachImage": "画像を添付",
+      "encoding": "処理中…",
+      "remove": "添付を削除",
+      "normalizedSizeHint": "{{orig}} → {{current}}（自動圧縮）",
+      "imageRejected": {
+        "unsupported_type": "対応していないファイル形式です",
+        "too_many_images": "1 メッセージにつき最大 {{max}} 枚です",
+        "magic_mismatch": "画像ファイルではないようです",
+        "decode_failed": "この画像をデコードできません",
+        "too_large": "画像が大きすぎます。小さいものを選んでください",
+        "io": "このファイルを読み込めません"
+      }
     },
     "scrollToBottom": "一番下へスクロール"
   },
@@ -72,12 +84,29 @@
     "streaming": "生成中",
     "assistantTyping": "アシスタントが入力中",
     "toolSingle": "ツールを使用中",
-    "toolMany": "{{count}} 個のツールを使用"
+    "toolMany": "{{count}} 個のツールを使用",
+    "imageAttachment": "画像の添付"
+  },
+  "lightbox": {
+    "title": "画像プレビュー",
+    "open": "画像を表示",
+    "prev": "前の画像",
+    "next": "次の画像",
+    "close": "プレビューを閉じる"
   },
   "code": {
     "fallbackLanguage": "コード",
     "copyAria": "コードをコピー",
     "copy": "コピー",
     "copied": "コピーしました"
+  },
+  "common": {
+    "dismiss": "閉じる"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "メッセージが大きすぎます",
+      "body": "サイズ上限を超えたため、直前のメッセージはサーバーに拒否されました。画像を減らすか、より小さいファイルに差し替えて再送してください。"
+    }
   }
 }
diff --git a/webui/src/i18n/locales/ko/common.json b/webui/src/i18n/locales/ko/common.json
index a1e52aca..bb89af25 100644
--- a/webui/src/i18n/locales/ko/common.json
+++ b/webui/src/i18n/locales/ko/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "새 채팅을 여는 중…",
       "inputAria": "메시지 입력",
       "sendHint": "Enter로 전송 · Shift+Enter로 줄바꿈",
-      "send": "메시지 보내기"
+      "send": "메시지 보내기",
+      "attachImage": "이미지 첨부",
+      "encoding": "처리 중…",
+      "remove": "첨부 제거",
+      "normalizedSizeHint": "{{orig}} → {{current}} (자동 압축)",
+      "imageRejected": {
+        "unsupported_type": "지원하지 않는 파일 형식입니다",
+        "too_many_images": "메시지당 최대 {{max}}장까지 가능합니다",
+        "magic_mismatch": "이미지 파일이 아닌 것 같습니다",
+        "decode_failed": "이 이미지를 디코딩할 수 없습니다",
+        "too_large": "이미지가 너무 큽니다. 더 작은 걸로 시도해 주세요",
+        "io": "이 파일을 읽을 수 없습니다"
+      }
     },
     "scrollToBottom": "맨 아래로 스크롤"
   },
@@ -72,12 +84,29 @@
     "streaming": "생성 중",
     "assistantTyping": "도우미가 입력 중",
     "toolSingle": "도구 사용 중",
-    "toolMany": "도구 {{count}}개 사용됨"
+    "toolMany": "도구 {{count}}개 사용됨",
+    "imageAttachment": "이미지 첨부"
+  },
+  "lightbox": {
+    "title": "이미지 미리보기",
+    "open": "이미지 보기",
+    "prev": "이전 이미지",
+    "next": "다음 이미지",
+    "close": "미리보기 닫기"
   },
   "code": {
     "fallbackLanguage": "코드",
     "copyAria": "코드 복사",
     "copy": "복사",
     "copied": "복사됨"
+  },
+  "common": {
+    "dismiss": "닫기"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "메시지가 너무 큽니다",
+      "body": "마지막 메시지가 서버의 크기 제한을 초과하여 거부되었습니다. 이미지를 줄이거나 더 작은 파일로 바꿔서 다시 보내 주세요."
+    }
   }
 }
diff --git a/webui/src/i18n/locales/vi/common.json b/webui/src/i18n/locales/vi/common.json
index 33099878..f2b64e33 100644
--- a/webui/src/i18n/locales/vi/common.json
+++ b/webui/src/i18n/locales/vi/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "Đang mở cuộc trò chuyện mới…",
       "inputAria": "Ô nhập tin nhắn",
       "sendHint": "Enter để gửi · Shift+Enter để xuống dòng",
-      "send": "Gửi tin nhắn"
+      "send": "Gửi tin nhắn",
+      "attachImage": "Đính kèm ảnh",
+      "encoding": "Đang xử lý…",
+      "remove": "Xóa tệp đính kèm",
+      "normalizedSizeHint": "{{orig}} → {{current}} (tự động)",
+      "imageRejected": {
+        "unsupported_type": "Loại tệp không được hỗ trợ",
+        "too_many_images": "Tối đa {{max}} ảnh mỗi tin nhắn",
+        "magic_mismatch": "Tệp này không phải là một ảnh thực",
+        "decode_failed": "Không thể giải mã ảnh này",
+        "too_large": "Ảnh quá lớn — hãy thử ảnh nhỏ hơn",
+        "io": "Không thể đọc tệp này"
+      }
     },
     "scrollToBottom": "Cuộn xuống cuối"
   },
@@ -72,12 +84,29 @@
     "streaming": "đang truyền",
     "assistantTyping": "Trợ lý đang nhập",
     "toolSingle": "Đang dùng một công cụ",
-    "toolMany": "Đã dùng {{count}} công cụ"
+    "toolMany": "Đã dùng {{count}} công cụ",
+    "imageAttachment": "Tệp hình ảnh đính kèm"
+  },
+  "lightbox": {
+    "title": "Xem trước ảnh",
+    "open": "Xem ảnh",
+    "prev": "Ảnh trước",
+    "next": "Ảnh tiếp theo",
+    "close": "Đóng xem trước"
   },
   "code": {
     "fallbackLanguage": "mã",
     "copyAria": "Sao chép mã",
     "copy": "Sao chép",
     "copied": "Đã sao chép"
+  },
+  "common": {
+    "dismiss": "Đóng"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "Tin nhắn quá lớn",
+      "body": "Máy chủ đã từ chối tin nhắn trước vì vượt quá giới hạn kích thước. Hãy bớt ảnh hoặc chọn tệp nhỏ hơn rồi thử lại."
+    }
   }
 }
diff --git a/webui/src/i18n/locales/zh-CN/common.json b/webui/src/i18n/locales/zh-CN/common.json
index ae7815e9..349e2625 100644
--- a/webui/src/i18n/locales/zh-CN/common.json
+++ b/webui/src/i18n/locales/zh-CN/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "正在打开新对话…",
       "inputAria": "消息输入框",
       "sendHint": "Enter 发送 · Shift+Enter 换行",
-      "send": "发送消息"
+      "send": "发送消息",
+      "attachImage": "添加图片",
+      "encoding": "处理中…",
+      "remove": "移除附件",
+      "normalizedSizeHint": "{{orig}} → {{current}}（已自动压缩）",
+      "imageRejected": {
+        "unsupported_type": "不支持的文件类型",
+        "too_many_images": "每条消息最多 {{max}} 张图片",
+        "magic_mismatch": "文件看起来不像真实的图片",
+        "decode_failed": "无法解码这张图片",
+        "too_large": "图片太大，请换一张小一点的",
+        "io": "无法读取该文件"
+      }
     },
     "scrollToBottom": "滚动到底部"
   },
@@ -72,12 +84,29 @@
     "streaming": "流式输出中",
     "assistantTyping": "助手正在输入",
     "toolSingle": "正在使用工具",
-    "toolMany": "已使用 {{count}} 个工具"
+    "toolMany": "已使用 {{count}} 个工具",
+    "imageAttachment": "图片附件"
+  },
+  "lightbox": {
+    "title": "图片预览",
+    "open": "查看图片",
+    "prev": "上一张",
+    "next": "下一张",
+    "close": "关闭预览"
   },
   "code": {
     "fallbackLanguage": "代码",
     "copyAria": "复制代码",
     "copy": "复制",
     "copied": "已复制"
+  },
+  "common": {
+    "dismiss": "关闭"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "消息过大",
+      "body": "服务端因超过大小限制拒收了上一条消息。可移除部分图片或使用更小的图片后重试。"
+    }
   }
 }
diff --git a/webui/src/i18n/locales/zh-TW/common.json b/webui/src/i18n/locales/zh-TW/common.json
index f606a7f1..b8a1e83d 100644
--- a/webui/src/i18n/locales/zh-TW/common.json
+++ b/webui/src/i18n/locales/zh-TW/common.json
@@ -64,7 +64,19 @@
       "placeholderOpening": "正在開啟新對話…",
       "inputAria": "訊息輸入框",
       "sendHint": "Enter 送出 · Shift+Enter 換行",
-      "send": "送出訊息"
+      "send": "送出訊息",
+      "attachImage": "附加圖片",
+      "encoding": "處理中…",
+      "remove": "移除附件",
+      "normalizedSizeHint": "{{orig}} → {{current}}（已自動壓縮）",
+      "imageRejected": {
+        "unsupported_type": "不支援的檔案類型",
+        "too_many_images": "每則訊息最多 {{max}} 張圖片",
+        "magic_mismatch": "檔案看起來不像真正的圖片",
+        "decode_failed": "無法解碼這張圖片",
+        "too_large": "圖片太大，請換一張小一點的",
+        "io": "無法讀取這個檔案"
+      }
     },
     "scrollToBottom": "捲動到底部"
   },
@@ -72,12 +84,29 @@
     "streaming": "串流輸出中",
     "assistantTyping": "助理正在輸入",
     "toolSingle": "正在使用工具",
-    "toolMany": "已使用 {{count}} 個工具"
+    "toolMany": "已使用 {{count}} 個工具",
+    "imageAttachment": "圖片附件"
+  },
+  "lightbox": {
+    "title": "圖片預覽",
+    "open": "檢視圖片",
+    "prev": "上一張",
+    "next": "下一張",
+    "close": "關閉預覽"
   },
   "code": {
     "fallbackLanguage": "程式碼",
     "copyAria": "複製程式碼",
     "copy": "複製",
     "copied": "已複製"
+  },
+  "common": {
+    "dismiss": "關閉"
+  },
+  "errors": {
+    "messageTooBig": {
+      "title": "訊息過大",
+      "body": "伺服器因超過大小限制拒收了上一則訊息。可移除部分圖片或改用較小的圖片後再試。"
+    }
   }
 }
diff --git a/webui/src/lib/api.ts b/webui/src/lib/api.ts
index 5f3e5bed..9012ca0d 100644
--- a/webui/src/lib/api.ts
+++ b/webui/src/lib/api.ts
@@ -57,6 +57,16 @@ export async function listSessions(
   }));
 }
 
+/** Signed image URL attached to a historical user message. The server
+ * emits these in place of raw on-disk paths so the client can render
+ * previews without learning where media lives on disk. Each URL is a
+ * self-authenticating ``/api/media/...`` route (see backend
+ * ``_sign_media_path``) safe to drop into an ``<img src>`` attribute. */
+export interface SessionMediaUrl {
+  url: string;
+  name?: string;
+}
+
 export async function fetchSessionMessages(
   token: string,
   key: string,
@@ -72,6 +82,9 @@ export async function fetchSessionMessages(
     tool_calls?: unknown;
     tool_call_id?: string;
     name?: string;
+    /** Present on ``user`` turns that attached images. Paths have already
+     * been stripped server-side; only the signed fetch URLs survive. */
+    media_urls?: SessionMediaUrl[];
   }>;
 }> {
   return request(
diff --git a/webui/src/lib/imageEncode.ts b/webui/src/lib/imageEncode.ts
new file mode 100644
index 00000000..9606807f
--- /dev/null
+++ b/webui/src/lib/imageEncode.ts
@@ -0,0 +1,97 @@
+/**
+ * Main-thread client for the image encoder Worker.
+ *
+ * Lazily boots a single ``imageEncode.worker`` and multiplexes requests onto
+ * it by a random request id. Falls back to an inline call when the Worker
+ * can't be constructed (tests, ancient browsers) so the Composer always has a
+ * working path.
+ */
+import {
+  encodeImageInWorker,
+  type EncodeResponse,
+} from "@/workers/imageEncode.worker";
+
+export type { EncodeResponse, EncodeSuccess, EncodeFailure } from "@/workers/imageEncode.worker";
+export { TARGET_MAX_BYTES } from "@/workers/imageEncode.worker";
+
+type Pending = {
+  resolve: (r: EncodeResponse) => void;
+  reject: (err: Error) => void;
+};
+
+let worker: Worker | null = null;
+let bootAttempted = false;
+const pending = new Map<string, Pending>();
+
+function bootWorker(): Worker | null {
+  if (bootAttempted) return worker;
+  bootAttempted = true;
+  if (typeof Worker === "undefined") return null;
+  try {
+    worker = new Worker(
+      new URL("@/workers/imageEncode.worker.ts", import.meta.url),
+      { type: "module" },
+    );
+    worker.addEventListener("message", (ev: MessageEvent<EncodeResponse>) => {
+      const entry = pending.get(ev.data.id);
+      if (!entry) return;
+      pending.delete(ev.data.id);
+      entry.resolve(ev.data);
+    });
+    worker.addEventListener("error", (ev) => {
+      // Cancel every in-flight request on a Worker crash.
+      for (const [, entry] of pending) {
+        entry.reject(new Error(`image encoder worker error: ${ev.message}`));
+      }
+      pending.clear();
+      worker?.terminate();
+      worker = null;
+    });
+    return worker;
+  } catch {
+    worker = null;
+    return null;
+  }
+}
+
+function newId(): string {
+  // ``crypto.randomUUID`` is widely available; fall back to Math.random if not.
+  if (typeof crypto !== "undefined" && "randomUUID" in crypto) {
+    return (crypto as Crypto).randomUUID();
+  }
+  return `img-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+}
+
+/** Encode ``file`` off the main thread when possible. Always resolves — errors
+ * are returned as ``{ok: false, reason}`` — so callers can render inline
+ * validation without wrapping in try/catch. */
+export async function encodeImage(file: File): Promise<EncodeResponse> {
+  const id = newId();
+  const w = bootWorker();
+  if (!w) {
+    // Inline fallback: same logic, just on the main thread.
+    return encodeImageInWorker({ id, file });
+  }
+  return new Promise<EncodeResponse>((resolve, reject) => {
+    pending.set(id, { resolve, reject });
+    try {
+      w.postMessage({ id, file });
+    } catch (err) {
+      pending.delete(id);
+      reject(err instanceof Error ? err : new Error(String(err)));
+    }
+  });
+}
+
+/** Release the singleton Worker (tests / teardown). */
+export function disposeImageEncoder(): void {
+  if (worker) {
+    worker.terminate();
+    worker = null;
+  }
+  bootAttempted = false;
+  for (const [, entry] of pending) {
+    entry.reject(new Error("image encoder disposed"));
+  }
+  pending.clear();
+}
diff --git a/webui/src/lib/nanobot-client.ts b/webui/src/lib/nanobot-client.ts
index d504ef21..f5039f93 100644
--- a/webui/src/lib/nanobot-client.ts
+++ b/webui/src/lib/nanobot-client.ts
@@ -1,4 +1,9 @@
-import type { ConnectionStatus, InboundEvent, Outbound } from "./types";
+import type {
+  ConnectionStatus,
+  InboundEvent,
+  Outbound,
+  OutboundMedia,
+} from "./types";
 
 /** WebSocket readyState constants, referenced by value to stay portable
  * across runtimes that don't expose a global ``WebSocket`` (tests, SSR). */
@@ -9,6 +14,22 @@ type Unsubscribe = () => void;
 type EventHandler = (ev: InboundEvent) => void;
 type StatusHandler = (status: ConnectionStatus) => void;
 
+/** Structured connection-level errors surfaced to the UI.
+ *
+ * These are *not* InboundEvent errors from the server application layer —
+ * those arrive as ``{event: "error"}`` messages via ``onChat``. These are
+ * transport-level or protocol-level faults the UI should make visible so
+ * the user understands *why* their action failed (as opposed to silently
+ * reconnecting under the hood).
+ */
+export type StreamError =
+  /** Server rejected the inbound frame as too large (WS close code 1009).
+   * Typically means the user attached images whose base64 size exceeded
+   * ``maxMessageBytes`` on the server. */
+  | { kind: "message_too_big" };
+
+type ErrorHandler = (error: StreamError) => void;
+
 interface PendingNewChat {
   resolve: (chatId: string) => void;
   reject: (err: Error) => void;
@@ -36,6 +57,7 @@ export interface NanobotClientOptions {
 export class NanobotClient {
   private socket: WebSocket | null = null;
   private statusHandlers = new Set<StatusHandler>();
+  private errorHandlers = new Set<ErrorHandler>();
   // chat_id -> handlers listening on it
   private chatHandlers = new Map<string, Set<EventHandler>>();
   // chat_ids we've attached to since connect; re-attached after reconnects
@@ -84,6 +106,14 @@ export class NanobotClient {
     };
   }
 
+  /** Subscribe to transport-level faults (see :type:`StreamError`). */
+  onError(handler: ErrorHandler): Unsubscribe {
+    this.errorHandlers.add(handler);
+    return () => {
+      this.errorHandlers.delete(handler);
+    };
+  }
+
   /** Subscribe to events for a given chat_id. Auto-attaches on the next open. */
   onChat(chatId: string, handler: EventHandler): Unsubscribe {
     let handlers = this.chatHandlers.get(chatId);
@@ -110,7 +140,7 @@ export class NanobotClient {
     sock.onopen = () => this.handleOpen();
     sock.onmessage = (ev) => this.handleMessage(ev);
     sock.onerror = () => this.setStatus("error");
-    sock.onclose = () => this.handleClose();
+    sock.onclose = (ev) => this.handleClose(ev);
   }
 
   close(): void {
@@ -151,9 +181,13 @@ export class NanobotClient {
     }
   }
 
-  sendMessage(chatId: string, content: string): void {
+  sendMessage(chatId: string, content: string, media?: OutboundMedia[]): void {
     this.knownChats.add(chatId);
-    this.queueSend({ type: "message", chat_id: chatId, content });
+    const frame: Outbound =
+      media && media.length > 0
+        ? { type: "message", chat_id: chatId, content, media }
+        : { type: "message", chat_id: chatId, content };
+    this.queueSend(frame);
   }
 
   // -- internals ---------------------------------------------------------
@@ -211,13 +245,20 @@ export class NanobotClient {
     for (const h of handlers) h(ev);
   }
 
-  private handleClose(): void {
+  private handleClose(event?: { code?: number }): void {
     this.socket = null;
     if (this.pendingNewChat) {
       clearTimeout(this.pendingNewChat.timer);
       this.pendingNewChat.reject(new Error("socket closed"));
       this.pendingNewChat = null;
     }
+    // Surface structured reasons *before* reconnect logic so the UI can
+    // display the error even while the client transparently reconnects.
+    // Browsers populate ``CloseEvent.code`` with the wire-level close code;
+    // 1009 = Message Too Big (server's max frame guard).
+    if (event?.code === 1009) {
+      this.emitError({ kind: "message_too_big" });
+    }
     if (this.intentionallyClosed || !this.shouldReconnect) {
       this.setStatus("closed");
       return;
@@ -225,6 +266,20 @@ export class NanobotClient {
     this.scheduleReconnect();
   }
 
+  private emitError(error: StreamError): void {
+    // Isolate subscribers so a throwing handler cannot abort the surrounding
+    // ``handleClose`` flow (which still owes us a reconnect decision + status
+    // update). We deliberately swallow here: error reporting is best-effort
+    // and must never be allowed to compound the failure it's reporting.
+    for (const handler of this.errorHandlers) {
+      try {
+        handler(error);
+      } catch {
+        // best-effort: subscriber fault must not stall transport bookkeeping
+      }
+    }
+  }
+
   private scheduleReconnect(): void {
     this.setStatus("reconnecting");
     const attempt = this.reconnectAttempts++;
diff --git a/webui/src/lib/types.ts b/webui/src/lib/types.ts
index fcd80da2..6ffc75a9 100644
--- a/webui/src/lib/types.ts
+++ b/webui/src/lib/types.ts
@@ -4,6 +4,24 @@ export type Role = "user" | "assistant" | "tool" | "system";
  * progress pings) that should not be rendered as conversational replies. */
 export type MessageKind = "message" | "trace";
 
+/** One image attached to a UIMessage.
+ *
+ * ``url`` can arrive in three different shapes, which the bubble renders
+ * identically:
+ * - A ``data:image/...;base64,...`` URL generated by the Composer for the
+ *   optimistic preview of an in-flight user turn. Self-contained, no
+ *   lifecycle.
+ * - A signed ``/api/media/...`` URL attached to a historical user turn by
+ *   the backend on session replay. Safe to drop into an ``<img src>``.
+ * - Absent. The backend couldn't resolve a stored path (file moved,
+ *   deleted, or pre-media-persistence session). The bubble shows a
+ *   placeholder tile with ``name`` as the label.
+ */
+export interface UIImage {
+  url?: string;
+  name?: string;
+}
+
 export interface UIMessage {
   id: string;
   role: Role;
@@ -14,6 +32,8 @@ export interface UIMessage {
   /** For trace rows: each individual hint line, so consecutive hints can
    * render as a single collapsible group. */
   traces?: string[];
+  /** User turn: optimistic blob URLs for preview. Replay: placeholder chips. */
+  images?: UIImage[];
 }
 
 export interface ChatSummary {
@@ -68,7 +88,25 @@ export type InboundEvent =
     }
   | { event: "error"; chat_id?: string; detail?: string };
 
+/** Base64-encoded image attached to an outbound ``message`` envelope.
+ *
+ * ``data_url`` must be a ``data:image/<png|jpeg|webp|gif>;base64,...`` string
+ * — the server whitelists those MIME types and rejects everything else
+ * (including SVG, to avoid an XSS surface). ``name`` is advisory: it's
+ * preserved for the file on disk and surfaced as the placeholder label when
+ * the session is replayed.
+ */
+export interface OutboundMedia {
+  data_url: string;
+  name?: string;
+}
+
 export type Outbound =
   | { type: "new_chat" }
   | { type: "attach"; chat_id: string }
-  | { type: "message"; chat_id: string; content: string };
+  | {
+      type: "message";
+      chat_id: string;
+      content: string;
+      media?: OutboundMedia[];
+    };
diff --git a/webui/src/tests/app-layout.test.tsx b/webui/src/tests/app-layout.test.tsx
index fd9756d1..4c8485b7 100644
--- a/webui/src/tests/app-layout.test.tsx
+++ b/webui/src/tests/app-layout.test.tsx
@@ -53,6 +53,7 @@ vi.mock("@/lib/nanobot-client", () => {
     defaultChatId: string | null = null;
     connect = connectSpy;
     onStatus = () => () => {};
+    onError = () => () => {};
     onChat = () => () => {};
     sendMessage = vi.fn();
     newChat = vi.fn();
diff --git a/webui/src/tests/nanobot-client.test.ts b/webui/src/tests/nanobot-client.test.ts
index 3ce14271..b95ef680 100644
--- a/webui/src/tests/nanobot-client.test.ts
+++ b/webui/src/tests/nanobot-client.test.ts
@@ -20,7 +20,7 @@ class FakeSocket {
   onopen: (() => void) | null = null;
   onmessage: ((ev: MessageEvent) => void) | null = null;
   onerror: (() => void) | null = null;
-  onclose: (() => void) | null = null;
+  onclose: ((ev?: { code?: number }) => void) | null = null;
 
   constructor(url: string) {
     this.url = url;
@@ -36,6 +36,13 @@ class FakeSocket {
     this.onclose?.();
   }
 
+  /** Simulate a server-initiated drop with a specific wire-level close code
+   * (e.g. ``1009`` for Message Too Big). */
+  fakeCloseWithCode(code: number) {
+    this.readyState = FakeSocket.CLOSED;
+    this.onclose?.({ code });
+  }
+
   fakeOpen() {
     this.readyState = FakeSocket.OPEN;
     this.onopen?.();
@@ -172,6 +179,95 @@ describe("NanobotClient", () => {
     expect(seen.at(-1)).toBe("closed");
   });
 
+  it("passes media through into the message envelope", () => {
+    const client = new NanobotClient({
+      url: "ws://test",
+      reconnect: false,
+      socketFactory: (url) => new FakeSocket(url) as unknown as WebSocket,
+    });
+    client.connect();
+    lastSocket().fakeOpen();
+    client.sendMessage("chat-x", "look", [
+      { data_url: "data:image/png;base64,AAAA", name: "shot.png" },
+    ]);
+    const lastFrame = JSON.parse(lastSocket().sent.at(-1) as string);
+    expect(lastFrame).toEqual({
+      type: "message",
+      chat_id: "chat-x",
+      content: "look",
+      media: [{ data_url: "data:image/png;base64,AAAA", name: "shot.png" }],
+    });
+  });
+
+  it("omits media from the envelope when no images are attached", () => {
+    const client = new NanobotClient({
+      url: "ws://test",
+      reconnect: false,
+      socketFactory: (url) => new FakeSocket(url) as unknown as WebSocket,
+    });
+    client.connect();
+    lastSocket().fakeOpen();
+    client.sendMessage("chat-x", "hello");
+    const lastFrame = JSON.parse(lastSocket().sent.at(-1) as string);
+    expect(lastFrame).not.toHaveProperty("media");
+    expect(lastFrame).toEqual({
+      type: "message",
+      chat_id: "chat-x",
+      content: "hello",
+    });
+  });
+
+  it("emits a message_too_big error when the socket closes with code 1009", () => {
+    const client = new NanobotClient({
+      url: "ws://test",
+      reconnect: false,
+      socketFactory: (url) => new FakeSocket(url) as unknown as WebSocket,
+    });
+    const errors: Array<{ kind: string }> = [];
+    client.onError((e) => errors.push(e));
+    client.connect();
+    lastSocket().fakeOpen();
+    // Server rejected an outbound frame as too large.
+    lastSocket().fakeCloseWithCode(1009);
+    expect(errors).toEqual([{ kind: "message_too_big" }]);
+  });
+
+  it("isolates throwing error handlers so reconnect bookkeeping still runs", async () => {
+    const client = new NanobotClient({
+      url: "ws://test",
+      reconnect: true,
+      maxBackoffMs: 5,
+      socketFactory: (url) => new FakeSocket(url) as unknown as WebSocket,
+    });
+    // First handler explodes; subsequent reconnect state must be untouched.
+    client.onError(() => {
+      throw new Error("subscriber blew up");
+    });
+    const seenStatuses: string[] = [];
+    client.onStatus((s) => seenStatuses.push(s));
+    client.connect();
+    lastSocket().fakeOpen();
+    lastSocket().fakeCloseWithCode(1009);
+    // Despite the throwing handler, the client must still schedule a reconnect.
+    expect(seenStatuses).toContain("reconnecting");
+    await vi.advanceTimersByTimeAsync(20);
+    expect(FakeSocket.instances.length).toBeGreaterThan(1);
+  });
+
+  it("does not emit a stream error on a vanilla socket close", () => {
+    const client = new NanobotClient({
+      url: "ws://test",
+      reconnect: false,
+      socketFactory: (url) => new FakeSocket(url) as unknown as WebSocket,
+    });
+    const errors: Array<{ kind: string }> = [];
+    client.onError((e) => errors.push(e));
+    client.connect();
+    lastSocket().fakeOpen();
+    lastSocket().close();
+    expect(errors).toEqual([]);
+  });
+
   it("surfaces 'reconnecting' only on an unexpected drop", async () => {
     const client = new NanobotClient({
       url: "ws://test",
diff --git a/webui/src/tests/thread-composer-attach.test.tsx b/webui/src/tests/thread-composer-attach.test.tsx
new file mode 100644
index 00000000..2bcbb627
--- /dev/null
+++ b/webui/src/tests/thread-composer-attach.test.tsx
@@ -0,0 +1,172 @@
+import {
+  act,
+  fireEvent,
+  render,
+  screen,
+  waitFor,
+} from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import { ThreadComposer } from "@/components/thread/ThreadComposer";
+import type { EncodeResponse } from "@/lib/imageEncode";
+
+const encodeImage = vi.fn<[File], Promise<EncodeResponse>>();
+
+vi.mock("@/lib/imageEncode", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@/lib/imageEncode")>();
+  return {
+    ...actual,
+    encodeImage: (file: File) => encodeImage(file),
+  };
+});
+
+function pngFile(name = "a.png", size = 10) {
+  return new File([new Uint8Array(size)], name, { type: "image/png" });
+}
+
+function resolveReady(file: File): EncodeResponse {
+  return {
+    id: "stub",
+    ok: true,
+    dataUrl: `data:image/png;base64,${btoa(file.name)}`,
+    mimeType: "image/png",
+    bytes: file.size,
+    normalized: false,
+  } as EncodeResponse;
+}
+
+beforeEach(() => {
+  encodeImage.mockReset();
+  let id = 0;
+  // Tests never read the preview URL contents so a stable blob: stub is fine.
+  if (!(globalThis.URL as unknown as { createObjectURL?: unknown }).createObjectURL) {
+    (globalThis.URL as unknown as { createObjectURL: (b: Blob) => string }).createObjectURL =
+      () => `blob:mock/${++id}`;
+  }
+  if (!(globalThis.URL as unknown as { revokeObjectURL?: unknown }).revokeObjectURL) {
+    (globalThis.URL as unknown as { revokeObjectURL: (u: string) => void }).revokeObjectURL =
+      () => {};
+  }
+});
+
+describe("ThreadComposer — image attachments", () => {
+  it("attaches a picked image and includes its data url on send", async () => {
+    const file = pngFile("a.png");
+    encodeImage.mockResolvedValueOnce(resolveReady(file));
+    const onSend = vi.fn();
+
+    render(<ThreadComposer onSend={onSend} />);
+
+    const input = screen
+      .getByLabelText(/message input/i)
+      .closest("form")!
+      .querySelector('input[type="file"]') as HTMLInputElement;
+
+    await act(async () => {
+      fireEvent.change(input, { target: { files: [file] } });
+    });
+
+    await waitFor(() =>
+      expect(screen.getByTestId("composer-chip")).toBeInTheDocument(),
+    );
+
+    const textarea = screen.getByLabelText(/message input/i);
+    fireEvent.change(textarea, { target: { value: "hi" } });
+    fireEvent.keyDown(textarea, { key: "Enter" });
+
+    expect(onSend).toHaveBeenCalledTimes(1);
+    const [content, images] = onSend.mock.calls[0];
+    expect(content).toBe("hi");
+    expect(images).toHaveLength(1);
+    expect(images[0].media.data_url).toContain("data:image/png;base64,");
+    expect(images[0].media.name).toBe("a.png");
+  });
+
+  it("blocks send while an image is still encoding", async () => {
+    const file = pngFile("slow.png");
+    let resolveEncode: (r: EncodeResponse) => void = () => {};
+    encodeImage.mockReturnValueOnce(
+      new Promise((r) => {
+        resolveEncode = r;
+      }),
+    );
+    const onSend = vi.fn();
+
+    render(<ThreadComposer onSend={onSend} />);
+
+    const fileInput = screen
+      .getByLabelText(/message input/i)
+      .closest("form")!
+      .querySelector('input[type="file"]') as HTMLInputElement;
+
+    await act(async () => {
+      fireEvent.change(fileInput, { target: { files: [file] } });
+    });
+
+    const textarea = screen.getByLabelText(/message input/i);
+    fireEvent.change(textarea, { target: { value: "hello" } });
+    fireEvent.keyDown(textarea, { key: "Enter" });
+    expect(onSend).not.toHaveBeenCalled();
+
+    await act(async () => {
+      resolveEncode(resolveReady(file));
+      await Promise.resolve();
+    });
+    fireEvent.keyDown(textarea, { key: "Enter" });
+    expect(onSend).toHaveBeenCalledTimes(1);
+  });
+
+  it("rejects a non-image paste silently without adding a chip", async () => {
+    const onSend = vi.fn();
+    render(<ThreadComposer onSend={onSend} />);
+    const textarea = screen.getByLabelText(/message input/i);
+
+    fireEvent.paste(textarea, {
+      clipboardData: {
+        files: [],
+        items: [
+          {
+            kind: "string",
+            type: "text/plain",
+            getAsFile: () => null,
+          },
+        ],
+        types: ["text/plain"],
+        getData: () => "some pasted text",
+      },
+    });
+
+    expect(screen.queryByTestId("composer-chip")).toBeNull();
+    expect(encodeImage).not.toHaveBeenCalled();
+  });
+
+  it("surfaces an inline error when encoding fails", async () => {
+    const file = pngFile("bad.png");
+    encodeImage.mockResolvedValueOnce({
+      id: "stub",
+      ok: false,
+      reason: "decode_failed",
+    } as EncodeResponse);
+    const onSend = vi.fn();
+
+    render(<ThreadComposer onSend={onSend} />);
+    const fileInput = screen
+      .getByLabelText(/message input/i)
+      .closest("form")!
+      .querySelector('input[type="file"]') as HTMLInputElement;
+
+    await act(async () => {
+      fireEvent.change(fileInput, { target: { files: [file] } });
+    });
+
+    await waitFor(() => {
+      const chip = screen.getByTestId("composer-chip");
+      expect(chip.textContent ?? "").toMatch(/decode|image/i);
+    });
+
+    const textarea = screen.getByLabelText(/message input/i);
+    fireEvent.change(textarea, { target: { value: "hi" } });
+    fireEvent.keyDown(textarea, { key: "Enter" });
+    expect(onSend).not.toHaveBeenCalled();
+  });
+});
diff --git a/webui/src/tests/thread-shell.test.tsx b/webui/src/tests/thread-shell.test.tsx
index e42cc7bf..9688d640 100644
--- a/webui/src/tests/thread-shell.test.tsx
+++ b/webui/src/tests/thread-shell.test.tsx
@@ -6,11 +6,21 @@ import { ThreadShell } from "@/components/thread/ThreadShell";
 import { ClientProvider } from "@/providers/ClientProvider";
 
 function makeClient() {
+  const errorHandlers = new Set<(err: { kind: string }) => void>();
   return {
     status: "open" as const,
     defaultChatId: null as string | null,
     onStatus: () => () => {},
     onChat: () => () => {},
+    onError: (handler: (err: { kind: string }) => void) => {
+      errorHandlers.add(handler);
+      return () => {
+        errorHandlers.delete(handler);
+      };
+    },
+    _emitError(err: { kind: string }) {
+      for (const h of errorHandlers) h(err);
+    },
     sendMessage: vi.fn(),
     newChat: vi.fn(),
     attach: vi.fn(),
@@ -88,6 +98,7 @@ describe("ThreadShell", () => {
       expect(client.sendMessage).toHaveBeenCalledWith(
         "chat-a",
         "persist me across tabs",
+        undefined,
       ),
     );
     expect(screen.getByText("persist me across tabs")).toBeInTheDocument();
@@ -151,6 +162,7 @@ describe("ThreadShell", () => {
       expect(client.sendMessage).toHaveBeenCalledWith(
         "chat-a",
         "delete me cleanly",
+        undefined,
       ),
     );
     expect(screen.getByText("delete me cleanly")).toBeInTheDocument();
@@ -241,6 +253,84 @@ describe("ThreadShell", () => {
     expect(screen.queryByText("old answer")).not.toBeInTheDocument();
   });
 
+  it("surfaces a dismissible banner when the stream reports message_too_big", async () => {
+    const client = makeClient();
+    const onNewChat = vi.fn().mockResolvedValue("chat-a");
+
+    render(
+      wrap(
+        client,
+        <ThreadShell
+          session={session("chat-a")}
+          title="Chat chat-a"
+          onToggleSidebar={() => {}}
+          onGoHome={() => {}}
+          onNewChat={onNewChat}
+        />,
+      ),
+    );
+
+    // No banner yet: only appears once the client emits a matching error.
+    expect(screen.queryByRole("alert")).not.toBeInTheDocument();
+
+    await act(async () => {
+      client._emitError({ kind: "message_too_big" });
+    });
+
+    const banner = await screen.findByRole("alert");
+    expect(banner).toHaveTextContent("Message too large");
+
+    fireEvent.click(screen.getByRole("button", { name: "Dismiss" }));
+
+    await waitFor(() => {
+      expect(screen.queryByRole("alert")).not.toBeInTheDocument();
+    });
+  });
+
+  it("clears the stream error banner when the user switches to another chat", async () => {
+    const client = makeClient();
+    const onNewChat = vi.fn().mockResolvedValue("chat-a");
+
+    const { rerender } = render(
+      wrap(
+        client,
+        <ThreadShell
+          session={session("chat-a")}
+          title="Chat chat-a"
+          onToggleSidebar={() => {}}
+          onGoHome={() => {}}
+          onNewChat={onNewChat}
+        />,
+      ),
+    );
+
+    await act(async () => {
+      client._emitError({ kind: "message_too_big" });
+    });
+    expect(await screen.findByRole("alert")).toBeInTheDocument();
+
+    // Switch to a different chat. The banner was about the *previous* send
+    // in chat-a; it must not leak into chat-b's view.
+    await act(async () => {
+      rerender(
+        wrap(
+          client,
+          <ThreadShell
+            session={session("chat-b")}
+            title="Chat chat-b"
+            onToggleSidebar={() => {}}
+            onGoHome={() => {}}
+            onNewChat={onNewChat}
+          />,
+        ),
+      );
+    });
+
+    await waitFor(() => {
+      expect(screen.queryByRole("alert")).not.toBeInTheDocument();
+    });
+  });
+
   it("clears the previous thread immediately while the next session loads", async () => {
     const client = makeClient();
     const onNewChat = vi.fn().mockResolvedValue("chat-b");
diff --git a/webui/src/tests/useNanobotStream.test.tsx b/webui/src/tests/useNanobotStream.test.tsx
index 2b51a16f..91b3036c 100644
--- a/webui/src/tests/useNanobotStream.test.tsx
+++ b/webui/src/tests/useNanobotStream.test.tsx
@@ -13,6 +13,7 @@ function fakeClient() {
       status: "open" as const,
       defaultChatId: null as string | null,
       onStatus: () => () => {},
+      onError: () => () => {},
       onChat(chatId: string, h: (ev: InboundEvent) => void) {
         let set = handlers.get(chatId);
         if (!set) {
diff --git a/webui/src/tests/useSessions.test.tsx b/webui/src/tests/useSessions.test.tsx
index 7ac8ce9e..aaabe3d8 100644
--- a/webui/src/tests/useSessions.test.tsx
+++ b/webui/src/tests/useSessions.test.tsx
@@ -2,7 +2,7 @@ import { act, renderHook, waitFor } from "@testing-library/react";
 import type { ReactNode } from "react";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
-import { useSessions } from "@/hooks/useSessions";
+import { useSessionHistory, useSessions } from "@/hooks/useSessions";
 import * as api from "@/lib/api";
 import { ClientProvider } from "@/providers/ClientProvider";
 
@@ -21,6 +21,7 @@ function fakeClient() {
     status: "open" as const,
     defaultChatId: null as string | null,
     onStatus: () => () => {},
+    onError: () => () => {},
     onChat: () => () => {},
     sendMessage: vi.fn(),
     newChat: vi.fn(),
@@ -86,6 +87,55 @@ describe("useSessions", () => {
     expect(result.current.sessions.map((s) => s.key)).toEqual(["websocket:chat-b"]);
   });
 
+  it("hydrates media_urls from historical user turns into UIMessage.images", async () => {
+    // Round-trip check for the signed-media replay: the backend emits
+    // ``media_urls`` on a historical user row and the hook must surface them
+    // as ``images`` so the bubble can render the preview. Assistant turns
+    // carry no media_urls and should not sprout an ``images`` field.
+    vi.mocked(api.fetchSessionMessages).mockResolvedValue({
+      key: "websocket:chat-media",
+      created_at: "2026-04-20T10:00:00Z",
+      updated_at: "2026-04-20T10:05:00Z",
+      messages: [
+        {
+          role: "user",
+          content: "what's this?",
+          timestamp: "2026-04-20T10:00:00Z",
+          media_urls: [
+            { url: "/api/media/sig-1/payload-1", name: "snap.png" },
+            { url: "/api/media/sig-2/payload-2", name: "diag.jpg" },
+          ],
+        },
+        {
+          role: "assistant",
+          content: "it's a cat",
+          timestamp: "2026-04-20T10:00:01Z",
+        },
+        {
+          role: "user",
+          content: "follow-up without images",
+          timestamp: "2026-04-20T10:01:00Z",
+        },
+      ],
+    });
+
+    const { result } = renderHook(() => useSessionHistory("websocket:chat-media"), {
+      wrapper: wrap(fakeClient()),
+    });
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+    const [first, second, third] = result.current.messages;
+    expect(first.role).toBe("user");
+    expect(first.images).toEqual([
+      { url: "/api/media/sig-1/payload-1", name: "snap.png" },
+      { url: "/api/media/sig-2/payload-2", name: "diag.jpg" },
+    ]);
+    expect(second.role).toBe("assistant");
+    expect(second.images).toBeUndefined();
+    expect(third.role).toBe("user");
+    expect(third.images).toBeUndefined();
+  });
+
   it("keeps the session in the list when delete fails", async () => {
     vi.mocked(api.listSessions).mockResolvedValue([
       {
diff --git a/webui/src/workers/imageEncode.worker.ts b/webui/src/workers/imageEncode.worker.ts
new file mode 100644
index 00000000..e23f89f2
--- /dev/null
+++ b/webui/src/workers/imageEncode.worker.ts
@@ -0,0 +1,264 @@
+/**
+ * Off-main-thread image encoder.
+ *
+ * Accepts a ``File``, validates it via magic bytes (ignoring the extension to
+ * defeat rename-based spoofs), and either passes through or *normalizes* the
+ * bytes so the resulting base64 data URL stays ≤ ``TARGET_MAX_BYTES``. The
+ * normalization path uses ``createImageBitmap`` + ``OffscreenCanvas`` so the
+ * full decode/resize/re-encode cycle never blocks the UI thread.
+ *
+ * Output contract:
+ *   ``{ok: true, dataUrl, mime, bytes, origBytes, normalized}`` on success, or
+ *   ``{ok: false, reason}`` for every recoverable failure — magic-bytes
+ *   mismatch, unsupported MIME, decode error, or a post-normalization payload
+ *   that *still* exceeds the budget (extreme aspect ratios).
+ */
+
+/// <reference lib="webworker" />
+
+// --- Types -------------------------------------------------------------------
+
+export type EncodeInput = {
+  id: string;
+  file: File;
+};
+
+export type EncodeSuccess = {
+  id: string;
+  ok: true;
+  dataUrl: string;
+  mime: string;
+  bytes: number;
+  origBytes: number;
+  /** True iff the Worker re-encoded the image to hit the size budget. */
+  normalized: boolean;
+};
+
+export type EncodeFailure = {
+  id: string;
+  ok: false;
+  reason:
+    | "invalid_mime"
+    | "magic_mismatch"
+    | "too_large_after_normalize"
+    | "decode_failed"
+    | "io";
+};
+
+export type EncodeResponse = EncodeSuccess | EncodeFailure;
+
+// --- Budgets -----------------------------------------------------------------
+
+/** Upper bound for the final base64-decoded payload. Matches the server-side
+ * safeguard (8 MB) minus safety margin; anything this function yields should
+ * safely pass ``_MAX_IMAGE_BYTES`` on the server. */
+export const TARGET_MAX_BYTES = 6 * 1024 * 1024;
+
+/** Long-edge pixel cap when we resize a large image. 2048 keeps retina UIs
+ * crisp while bounding decode cost and matching most LLM vision tiers'
+ * internal downscale target. */
+const NORMALIZE_MAX_EDGE = 2048;
+
+/** JPEG/WebP quality during normalization. 0.85 is the sweet spot — visually
+ * lossless for content photography, ~30% smaller than libjpeg default. */
+const WEBP_QUALITY = 0.85;
+
+/** PNG / GIF kept as PNG after normalization so crisp UI screenshots stay
+ * lossless. JPEG / WebP re-encode as WebP for better compression. */
+const NORMALIZE_LOSSY_MIMES = new Set(["image/jpeg", "image/webp"]);
+
+const SUPPORTED_MIMES = new Set([
+  "image/png",
+  "image/jpeg",
+  "image/webp",
+  "image/gif",
+]);
+
+// --- Magic bytes -------------------------------------------------------------
+
+/** Sniff the first 12 bytes; returns the canonical MIME or ``null``.
+ *
+ * Covers PNG, JPEG, WebP, GIF — the same whitelist honoured by the server.
+ */
+export function sniffImageMime(bytes: Uint8Array): string | null {
+  if (bytes.length >= 8) {
+    if (
+      bytes[0] === 0x89 &&
+      bytes[1] === 0x50 &&
+      bytes[2] === 0x4e &&
+      bytes[3] === 0x47 &&
+      bytes[4] === 0x0d &&
+      bytes[5] === 0x0a &&
+      bytes[6] === 0x1a &&
+      bytes[7] === 0x0a
+    ) {
+      return "image/png";
+    }
+  }
+  if (bytes.length >= 3) {
+    if (bytes[0] === 0xff && bytes[1] === 0xd8 && bytes[2] === 0xff) {
+      return "image/jpeg";
+    }
+  }
+  if (bytes.length >= 6) {
+    const g1 =
+      bytes[0] === 0x47 && bytes[1] === 0x49 && bytes[2] === 0x46 &&
+      bytes[3] === 0x38 && bytes[5] === 0x61;
+    if (g1 && (bytes[4] === 0x37 || bytes[4] === 0x39)) {
+      return "image/gif";
+    }
+  }
+  if (bytes.length >= 12) {
+    const riff =
+      bytes[0] === 0x52 && bytes[1] === 0x49 && bytes[2] === 0x46 && bytes[3] === 0x46;
+    const webp =
+      bytes[8] === 0x57 && bytes[9] === 0x45 && bytes[10] === 0x42 && bytes[11] === 0x50;
+    if (riff && webp) return "image/webp";
+  }
+  return null;
+}
+
+// --- Encoder -----------------------------------------------------------------
+
+function bufferToBase64(buf: ArrayBuffer): string {
+  // ``btoa`` can't take large strings — chunk through 32 KB windows.
+  const bytes = new Uint8Array(buf);
+  let binary = "";
+  const CHUNK = 0x8000;
+  for (let i = 0; i < bytes.length; i += CHUNK) {
+    binary += String.fromCharCode.apply(
+      null,
+      bytes.subarray(i, i + CHUNK) as unknown as number[],
+    );
+  }
+  return self.btoa(binary);
+}
+
+function computeScaledDims(
+  srcW: number,
+  srcH: number,
+  maxEdge: number,
+): { w: number; h: number } {
+  const longest = Math.max(srcW, srcH);
+  if (longest <= maxEdge) return { w: srcW, h: srcH };
+  const scale = maxEdge / longest;
+  return {
+    w: Math.max(1, Math.round(srcW * scale)),
+    h: Math.max(1, Math.round(srcH * scale)),
+  };
+}
+
+async function normalize(
+  file: File,
+  sourceMime: string,
+): Promise<{ dataUrl: string; mime: string; bytes: number } | { error: EncodeFailure["reason"] }> {
+  // Re-encode paths: JPEG/WebP → WebP q=0.85; PNG/GIF → PNG (keep crisp).
+  const targetMime = NORMALIZE_LOSSY_MIMES.has(sourceMime)
+    ? "image/webp"
+    : "image/png";
+  let bitmap: ImageBitmap;
+  try {
+    bitmap = await createImageBitmap(file);
+  } catch {
+    return { error: "decode_failed" };
+  }
+  const { w, h } = computeScaledDims(bitmap.width, bitmap.height, NORMALIZE_MAX_EDGE);
+  try {
+    const canvas = new OffscreenCanvas(w, h);
+    const ctx = canvas.getContext("2d", { alpha: true });
+    if (!ctx) {
+      bitmap.close();
+      return { error: "decode_failed" };
+    }
+    ctx.imageSmoothingQuality = "high";
+    ctx.drawImage(bitmap, 0, 0, w, h);
+    bitmap.close();
+    const options: ImageEncodeOptions = { type: targetMime };
+    if (targetMime === "image/webp") options.quality = WEBP_QUALITY;
+    const blob = await canvas.convertToBlob(options);
+    if (blob.size > TARGET_MAX_BYTES) {
+      return { error: "too_large_after_normalize" };
+    }
+    const buf = await blob.arrayBuffer();
+    const dataUrl = `data:${targetMime};base64,${bufferToBase64(buf)}`;
+    return { dataUrl, mime: targetMime, bytes: blob.size };
+  } catch {
+    try {
+      bitmap.close();
+    } catch {
+      // bitmap already closed
+    }
+    return { error: "decode_failed" };
+  }
+}
+
+export async function encodeImageInWorker(
+  input: EncodeInput,
+): Promise<EncodeResponse> {
+  const { id, file } = input;
+  const origBytes = file.size;
+
+  let buffer: ArrayBuffer;
+  try {
+    buffer = await file.arrayBuffer();
+  } catch {
+    return { id, ok: false, reason: "io" };
+  }
+
+  const head = new Uint8Array(buffer.slice(0, 12));
+  const sniffed = sniffImageMime(head);
+  if (!sniffed) return { id, ok: false, reason: "magic_mismatch" };
+  if (!SUPPORTED_MIMES.has(sniffed)) {
+    return { id, ok: false, reason: "invalid_mime" };
+  }
+  // Defend against MIME spoofing: the declared ``file.type`` can lie.
+  if (file.type && SUPPORTED_MIMES.has(file.type) && file.type !== sniffed) {
+    // Trust the magic bytes; proceed with the sniffed MIME.
+  }
+
+  if (origBytes <= TARGET_MAX_BYTES) {
+    const dataUrl = `data:${sniffed};base64,${bufferToBase64(buffer)}`;
+    return {
+      id,
+      ok: true,
+      dataUrl,
+      mime: sniffed,
+      bytes: origBytes,
+      origBytes,
+      normalized: false,
+    };
+  }
+
+  const result = await normalize(file, sniffed);
+  if ("error" in result) {
+    return { id, ok: false, reason: result.error };
+  }
+  return {
+    id,
+    ok: true,
+    dataUrl: result.dataUrl,
+    mime: result.mime,
+    bytes: result.bytes,
+    origBytes,
+    normalized: true,
+  };
+}
+
+// --- Worker boot -------------------------------------------------------------
+// Only attach the message listener when running *inside* a Worker so the same
+// module can be imported by tests (and by the thin ``imageEncode.ts`` wrapper
+// in the main thread, which also calls ``encodeImageInWorker`` as a
+// fall-through path when the Worker isn't available).
+
+declare const self: DedicatedWorkerGlobalScope;
+
+if (
+  typeof self !== "undefined" &&
+  typeof (self as unknown as { importScripts?: unknown }).importScripts ===
+    "function"
+) {
+  self.addEventListener("message", async (event: MessageEvent<EncodeInput>) => {
+    const response = await encodeImageInWorker(event.data);
+    self.postMessage(response);
+  });
+}