diff --git a/bridge/src/whatsapp.ts b/bridge/src/whatsapp.ts
index a98f3a88..55d3a85b 100644
--- a/bridge/src/whatsapp.ts
+++ b/bridge/src/whatsapp.ts
@@ -17,7 +17,7 @@ import { Boom } from '@hapi/boom';
 import qrcode from 'qrcode-terminal';
 import pino from 'pino';
 import { readFile, writeFile, mkdir } from 'fs/promises';
-import { join, basename } from 'path';
+import { join, basename, resolve, sep } from 'path';
 import { randomBytes } from 'crypto';
 
 const VERSION = '0.1.0';
@@ -196,17 +196,18 @@ export class WhatsAppClient {
 
       let outFilename: string;
       if (fileName) {
-        // Documents have a filename — use it with a unique prefix to avoid collisions
-        const prefix = `wa_${Date.now()}_${randomBytes(4).toString('hex')}_`;
-        outFilename = prefix + fileName;
+        const safeName = basename(fileName).replace(/[^a-zA-Z0-9._-]/g, '_');
+        outFilename = `wa_${Date.now()}_${randomBytes(4).toString('hex')}_${safeName}`;
       } else {
         const mime = mimetype || 'application/octet-stream';
-        // Derive extension from mimetype subtype (e.g. "image/png" → ".png", "application/pdf" → ".pdf")
         const ext = '.' + (mime.split('/').pop()?.split(';')[0] || 'bin');
         outFilename = `wa_${Date.now()}_${randomBytes(4).toString('hex')}${ext}`;
       }
 
-      const filepath = join(mediaDir, outFilename);
+      const filepath = resolve(mediaDir, outFilename);
+      if (!filepath.startsWith(resolve(mediaDir) + sep)) {
+        throw new Error(`Path traversal blocked: ${outFilename}`);
+      }
       await writeFile(filepath, buffer);
 
       return filepath;
diff --git a/nanobot/channels/whatsapp.py b/nanobot/channels/whatsapp.py
index a7fd8265..e2485da7 100644
--- a/nanobot/channels/whatsapp.py
+++ b/nanobot/channels/whatsapp.py
@@ -1,6 +1,7 @@
 """WhatsApp channel implementation using Node.js bridge."""
 
 import asyncio
+import hashlib
 import json
 import mimetypes
 import os
@@ -316,13 +317,7 @@ def _ensure_bridge_setup() -> Path:
     from nanobot.config.paths import get_bridge_install_dir
 
     user_bridge = get_bridge_install_dir()
-
-    if (user_bridge / "dist" / "index.js").exists():
-        return user_bridge
-
-    npm_path = shutil.which("npm")
-    if not npm_path:
-        raise RuntimeError("npm not found. Please install Node.js >= 18.")
+    stamp_file = user_bridge / ".nanobot-bridge-source-hash"
 
     # Find source bridge
     current_file = Path(__file__)
@@ -341,6 +336,33 @@ def _ensure_bridge_setup() -> Path:
             "Try reinstalling: pip install --force-reinstall nanobot"
         )
 
+    def source_hash(root: Path) -> str:
+        digest = hashlib.sha256()
+        for path in sorted(root.rglob("*")):
+            if not path.is_file():
+                continue
+            rel = path.relative_to(root)
+            if rel.parts and rel.parts[0] in {"node_modules", "dist"}:
+                continue
+            digest.update(rel.as_posix().encode("utf-8"))
+            digest.update(b"\0")
+            digest.update(path.read_bytes())
+            digest.update(b"\0")
+        return digest.hexdigest()
+
+    expected_hash = source_hash(source)
+    current_hash = stamp_file.read_text().strip() if stamp_file.exists() else None
+
+    if (user_bridge / "dist" / "index.js").exists() and current_hash == expected_hash:
+        return user_bridge
+
+    if (user_bridge / "dist" / "index.js").exists() and current_hash != expected_hash:
+        logger.info("WhatsApp bridge source changed; rebuilding bridge...")
+
+    npm_path = shutil.which("npm")
+    if not npm_path:
+        raise RuntimeError("npm not found. Please install Node.js >= 18.")
+
     logger.info("Setting up WhatsApp bridge...")
     user_bridge.parent.mkdir(parents=True, exist_ok=True)
     if user_bridge.exists():
@@ -352,6 +374,7 @@ def _ensure_bridge_setup() -> Path:
 
     logger.info("  Building...")
     subprocess.run([npm_path, "run", "build"], cwd=user_bridge, check=True, capture_output=True)
+    stamp_file.write_text(expected_hash + "\n")
 
     logger.info("Bridge ready")
     return user_bridge
diff --git a/nanobot/cli/commands.py b/nanobot/cli/commands.py
index 2fe39746..903555b4 100644
--- a/nanobot/cli/commands.py
+++ b/nanobot/cli/commands.py
@@ -1271,6 +1271,7 @@ def channels_status(
 
 def _get_bridge_dir() -> Path:
     """Get the bridge directory, setting it up if needed."""
+    import hashlib
     import shutil
     import subprocess
 
@@ -1278,16 +1279,7 @@ def _get_bridge_dir() -> Path:
     from nanobot.config.paths import get_bridge_install_dir
 
     user_bridge = get_bridge_install_dir()
-
-    # Check if already built
-    if (user_bridge / "dist" / "index.js").exists():
-        return user_bridge
-
-    # Check for npm
-    npm_path = shutil.which("npm")
-    if not npm_path:
-        console.print("[red]npm not found. Please install Node.js >= 18.[/red]")
-        raise typer.Exit(1)
+    stamp_file = user_bridge / ".nanobot-bridge-source-hash"
 
     # Find source bridge: first check package data, then source dir
     pkg_bridge = Path(__file__).parent.parent / "bridge"  # nanobot/bridge (installed)
@@ -1304,6 +1296,36 @@ def _get_bridge_dir() -> Path:
         console.print("Try reinstalling: pip install --force-reinstall nanobot")
         raise typer.Exit(1)
 
+    def source_hash(root: Path) -> str:
+        digest = hashlib.sha256()
+        for path in sorted(root.rglob("*")):
+            if not path.is_file():
+                continue
+            rel = path.relative_to(root)
+            if rel.parts and rel.parts[0] in {"node_modules", "dist"}:
+                continue
+            digest.update(rel.as_posix().encode("utf-8"))
+            digest.update(b"\0")
+            digest.update(path.read_bytes())
+            digest.update(b"\0")
+        return digest.hexdigest()
+
+    expected_hash = source_hash(source)
+    current_hash = stamp_file.read_text().strip() if stamp_file.exists() else None
+
+    # Reuse only a bridge built from the currently installed source.
+    if (user_bridge / "dist" / "index.js").exists() and current_hash == expected_hash:
+        return user_bridge
+
+    if (user_bridge / "dist" / "index.js").exists() and current_hash != expected_hash:
+        console.print(f"{__logo__} WhatsApp bridge source changed; rebuilding bridge...")
+
+    # Check for npm
+    npm_path = shutil.which("npm")
+    if not npm_path:
+        console.print("[red]npm not found. Please install Node.js >= 18.[/red]")
+        raise typer.Exit(1)
+
     console.print(f"{__logo__} Setting up bridge...")
 
     # Copy to user directory
@@ -1319,6 +1341,7 @@ def _get_bridge_dir() -> Path:
 
         console.print("  Building...")
         subprocess.run([npm_path, "run", "build"], cwd=user_bridge, check=True, capture_output=True)
+        stamp_file.write_text(expected_hash + "\n")
 
         console.print("[green]✓[/green] Bridge ready\n")
     except subprocess.CalledProcessError as e: