diff --git a/tests/tools/test_send_message_tool.py b/tests/tools/test_send_message_tool.py
index b9d99e2b3d..9a277d5980 100644
--- a/tests/tools/test_send_message_tool.py
+++ b/tests/tools/test_send_message_tool.py
@@ -886,3 +886,39 @@ class TestSendToPlatformDiscordThread:
         send_mock.assert_awaited_once()
         _, call_kwargs = send_mock.await_args
         assert call_kwargs["thread_id"] is None
+
+
+class TestSendMatrixUrlEncoding:
+    """_send_matrix URL-encodes Matrix room IDs in the API path."""
+
+    def test_room_id_is_percent_encoded_in_url(self):
+        """Matrix room IDs with ! and : are percent-encoded in the PUT URL."""
+        import aiohttp
+
+        mock_resp = MagicMock()
+        mock_resp.status = 200
+        mock_resp.json = AsyncMock(return_value={"event_id": "$evt123"})
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=None)
+
+        mock_session = MagicMock()
+        mock_session.put = MagicMock(return_value=mock_resp)
+        mock_session.__aenter__ = AsyncMock(return_value=mock_session)
+        mock_session.__aexit__ = AsyncMock(return_value=None)
+
+        with patch("aiohttp.ClientSession", return_value=mock_session):
+            from tools.send_message_tool import _send_matrix
+            result = asyncio.get_event_loop().run_until_complete(
+                _send_matrix(
+                    "test_token",
+                    {"homeserver": "https://matrix.example.org"},
+                    "!HLOQwxYGgFPMPJUSNR:matrix.org",
+                    "hello",
+                )
+            )
+
+        assert result["success"] is True
+        # Verify the URL was called with percent-encoded room ID
+        put_url = mock_session.put.call_args[0][0]
+        assert "%21HLOQwxYGgFPMPJUSNR%3Amatrix.org" in put_url
+        assert "!HLOQwxYGgFPMPJUSNR:matrix.org" not in put_url
diff --git a/tools/send_message_tool.py b/tools/send_message_tool.py
index f99bcdaf46..6c9632b2c3 100644
--- a/tools/send_message_tool.py
+++ b/tools/send_message_tool.py
@@ -68,7 +68,7 @@ SEND_MESSAGE_SCHEMA = {
             },
             "target": {
                 "type": "string",
-                "description": "Delivery target. Format: 'platform' (uses home channel), 'platform:#channel-name', 'platform:chat_id', or 'platform:chat_id:thread_id' for Telegram topics and Discord threads. Examples: 'telegram', 'telegram:-1001234567890:17585', 'discord:999888777:555444333', 'discord:#bot-home', 'slack:#engineering', 'signal:+155****4567'"
+                "description": "Delivery target. Format: 'platform' (uses home channel), 'platform:#channel-name', 'platform:chat_id', or 'platform:chat_id:thread_id' for Telegram topics and Discord threads. Examples: 'telegram', 'telegram:-1001234567890:17585', 'discord:999888777:555444333', 'discord:#bot-home', 'slack:#engineering', 'signal:+155****4567', 'matrix:!roomid:server.org', 'matrix:@user:server.org'"
             },
             "message": {
                 "type": "string",
@@ -819,7 +819,9 @@ async def _send_matrix(token, extra, chat_id, message):
         if not homeserver or not token:
             return {"error": "Matrix not configured (MATRIX_HOMESERVER, MATRIX_ACCESS_TOKEN required)"}
         txn_id = f"hermes_{int(time.time() * 1000)}_{os.urandom(4).hex()}"
-        url = f"{homeserver}/_matrix/client/v3/rooms/{chat_id}/send/m.room.message/{txn_id}"
+        from urllib.parse import quote
+        encoded_room = quote(chat_id, safe="")
+        url = f"{homeserver}/_matrix/client/v3/rooms/{encoded_room}/send/m.room.message/{txn_id}"
         headers = {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}
 
         # Build message payload with optional HTML formatted_body.