Surface streamable HTTP connection errors

Author: FU-max-boop

src/mcp/client/streamable_http.py

@@ -467,17 +467,26 @@ async def _handle_message(session_message: SessionMessage) -> None:
                         read_stream_writer=read_stream_writer,
                     )
 
-                    async def handle_request_async():
+                    async def send_message() -> None:
                         if is_resumption:
                             await self._handle_resumption_request(ctx)
                         else:
                             await self._handle_post_request(ctx)
 
+                    async def handle_request_async(request: JSONRPCRequest) -> None:
+                        try:
+                            await send_message()
+                        except httpx.TransportError as exc:
+                            logger.debug("Error handling request", exc_info=True)
+                            error_data = ErrorData(code=INTERNAL_ERROR, message=f"Transport error: {exc}")
+                            error_msg = SessionMessage(JSONRPCError(jsonrpc="2.0", id=request.id, error=error_data))
+                            await ctx.read_stream_writer.send(error_msg)
+
                     # If this is a request, start a new task to handle it
                     if isinstance(message, JSONRPCRequest):
-                        tg.start_soon(handle_request_async)
+                        tg.start_soon(handle_request_async, message)
                     else:
-                        await handle_request_async()
+                        await send_message()
 
                 async for session_message in write_stream_reader:
                     sender_ctx = write_stream_reader.last_context

tests/client/test_session_group.py

@@ -278,6 +278,25 @@ async def test_client_session_group_disconnect_non_existent_server():
         await group.disconnect_from_server(session)
 
 
+@pytest.mark.anyio
+async def test_client_session_group_streamable_http_connection_error_surfaces() -> None:
+    async def fail_request(request: httpx.Request) -> httpx.Response:
+        raise httpx.ConnectError("offline", request=request)
+
+    http_client = httpx.AsyncClient(transport=httpx.MockTransport(fail_request))
+
+    with mock.patch("mcp.client.session_group.create_mcp_http_client", return_value=http_client):
+        async with ClientSessionGroup() as group:
+            with pytest.raises(MCPError) as excinfo:
+                await group.connect_to_server(
+                    StreamableHttpParameters(url="http://example.test/mcp"),
+                    ClientSessionParameters(read_timeout_seconds=2),
+                )
+
+    assert excinfo.value.error.code == types.INTERNAL_ERROR
+    assert excinfo.value.error.message == "Transport error: offline"
+
+
 # TODO(Marcelo): This is horrible. We should drop this test.
 @pytest.mark.anyio
 @pytest.mark.parametrize(