diff --git a/nanobot/channels/matrix.py b/nanobot/channels/matrix.py
index 481b2b86..8a2fc9d8 100644
--- a/nanobot/channels/matrix.py
+++ b/nanobot/channels/matrix.py
@@ -192,6 +192,10 @@ def _build_matrix_text_content(
     return content
 
 
+def _matrix_stream_key(chat_id: str, stream_id: str | None) -> str:
+    return chat_id if stream_id is None else f"{chat_id}\0{stream_id}"
+
+
 class MatrixConfig(Base):
     """Matrix (Element) channel configuration."""
 
@@ -543,7 +547,8 @@ class MatrixChannel(BaseChannel):
         relates_to = self._build_thread_relates_to(metadata)
 
         if stream_end:
-            buf = self._stream_bufs.pop(chat_id, None)
+            stream_key = _matrix_stream_key(chat_id, stream_id)
+            buf = self._stream_bufs.pop(stream_key, None)
             if not buf or not buf.event_id or not buf.text:
                 return
 
@@ -557,10 +562,11 @@ class MatrixChannel(BaseChannel):
             await self._send_room_content(chat_id, content)
             return
 
-        buf = self._stream_bufs.get(chat_id)
+        stream_key = _matrix_stream_key(chat_id, stream_id)
+        buf = self._stream_bufs.get(stream_key)
         if buf is None:
             buf = _StreamBuf()
-            self._stream_bufs[chat_id] = buf
+            self._stream_bufs[stream_key] = buf
         buf.text += delta
 
         if not buf.text.strip():
diff --git a/tests/channels/test_matrix_channel.py b/tests/channels/test_matrix_channel.py
index f1e20dfd..da82ba4b 100644
--- a/tests/channels/test_matrix_channel.py
+++ b/tests/channels/test_matrix_channel.py
@@ -1913,6 +1913,36 @@ async def test_send_delta_stream_end_replaces_existing_message() -> None:
     }
 
 
+@pytest.mark.asyncio
+async def test_send_delta_keeps_same_room_stream_ids_independent(monkeypatch) -> None:
+    channel = MatrixChannel(_make_config(), MessageBus())
+    client = _FakeAsyncClient("", "", "", None)
+    channel.client = client
+
+    event_ids = ["event-a", "event-b"]
+
+    async def _send_room_content(room_id, content):
+        client.room_send_calls.append({"room_id": room_id, "content": content})
+        return SimpleNamespace(event_id=event_ids.pop(0) if event_ids else "event-final")
+
+    monkeypatch.setattr(channel, "_send_room_content", _send_room_content)
+
+    await channel.send_delta("!room:matrix.org", "A", stream_id="stream-a")
+    await channel.send_delta("!room:matrix.org", "B", stream_id="stream-b")
+    await channel.send_delta("!room:matrix.org", "1", stream_id="stream-a")
+    await channel.send_delta("!room:matrix.org", "2", stream_id="stream-b")
+
+    await channel.send_delta("!room:matrix.org", "", stream_id="stream-a", stream_end=True)
+    await channel.send_delta("!room:matrix.org", "", stream_id="stream-b", stream_end=True)
+
+    final_a = client.room_send_calls[-2]["content"]
+    final_b = client.room_send_calls[-1]["content"]
+    assert final_a["body"] == "A1"
+    assert final_a["m.relates_to"]["event_id"] == "event-a"
+    assert final_b["body"] == "B2"
+    assert final_b["m.relates_to"]["event_id"] == "event-b"
+
+
 @pytest.mark.asyncio
 async def test_send_delta_starts_threaded_stream_inside_thread() -> None:
     channel = MatrixChannel(_make_config(), MessageBus())