Handle Abrupt Server Disconnect in Middle of Tool Call

src/mcp/client/streamable_http.py

@@ -337,7 +337,13 @@ async def _handle_sse_response(
                     break
         except Exception as e:
             logger.exception("Error reading SSE stream:")
-            await ctx.read_stream_writer.send(e)
+            # Forward the transport exception to the session read stream so it can
+            # fail any in-flight requests, then close the stream to signal that
+            # no further messages will arrive.
+            try:
+                await ctx.read_stream_writer.send(e)
+            finally:
+                await ctx.read_stream_writer.aclose()
 
     async def _handle_unexpected_content_type(
         self,

src/mcp/shared/session.py

@@ -336,7 +336,21 @@ async def _receive_loop(self) -> None:
             try:
                 async for message in self._read_stream:
                     if isinstance(message, Exception):
+                        # Transport-level exception. Forward it to the incoming
+                        # handler for logging/observation, then fail all
+                        # in-flight requests so callers don't hang forever.
                         await self._handle_incoming(message)
+                        error = ErrorData(code=CONNECTION_CLOSED, message=str(message))
+                        # Send error to any pending request response streams immediately
+                        for id, stream in list(self._response_streams.items()):
+                            try:
+                                await stream.send(JSONRPCError(jsonrpc="2.0", id=id, error=error))
+                                await stream.aclose()
+                            except Exception:
+                                pass
+                        self._response_streams.clear()
+                        # Break out of the receive loop; connection is no longer usable.
+                        break
                     elif isinstance(message.message.root, JSONRPCRequest):
                         try:
                             validated_request = self._receive_request_type.model_validate(

tests/shared/test_streamable_http.py

@@ -603,6 +603,69 @@ def test_streamable_http_transport_init_validation():
         StreamableHTTPServerTransport(mcp_session_id="test\n")
 
 
+@pytest.mark.anyio
+async def test_streamable_http_mid_call_disconnect(basic_server_port: int):
+    """A long-running tool call should error quickly if the server dies mid-call.
+
+    We use the existing server implementation's `wait_for_lock_with_notification` tool which
+    blocks waiting on an internal lock. We kill the server process after the tool starts and
+    assert the client surfaces a CONNECTION_CLOSED McpError instead of hanging.
+    """
+
+    # Launch a dedicated server process (don't reuse basic_server fixture so we can kill it)
+    proc = multiprocessing.Process(target=run_server, kwargs={"port": basic_server_port}, daemon=True)
+    proc.start()
+
+    # Wait for server readiness (reuse pattern from fixtures)
+    start_time = time.time()
+    while time.time() - start_time < 10:
+        try:
+            with socket.create_connection(("127.0.0.1", basic_server_port), timeout=0.2):
+                break
+        except OSError:
+            time.sleep(0.1)
+    else:  # pragma: no cover
+        proc.kill()
+        proc.join(timeout=2)
+        pytest.fail("Server failed to start in time")
+
+    server_url = f"http://127.0.0.1:{basic_server_port}/mcp"
+
+    try:
+        async with streamablehttp_client(server_url) as (read_stream, write_stream, _):
+            async with ClientSession(read_stream, write_stream) as session:
+                await session.initialize()
+                await session.list_tools()
+
+                result: dict[str, object] = {}
+
+                async def invoke():
+                    try:
+                        await session.call_tool("wait_for_lock_with_notification", {})  # pragma: no cover
+                        result["ok"] = True
+                    except McpError as e:
+                        result["err"] = e
+
+                async with anyio.create_task_group() as tg:
+                    tg.start_soon(invoke)
+                    # Give the request a moment to reach the server & tool to start (it sends a log notification)
+                    await anyio.sleep(0.6)
+                    proc.kill()
+                    proc.join(timeout=2)
+                    # Wait for propagated disconnect
+                    with anyio.fail_after(10):
+                        while "err" not in result:
+                            await anyio.sleep(0.2)
+
+                err = result.get("err")
+                assert isinstance(err, McpError), "Expected McpError from mid-call disconnect"
+                assert err.error.code == types.CONNECTION_CLOSED
+    finally:
+        if proc.is_alive():  # Safety cleanup
+            proc.kill()
+            proc.join(timeout=2)
+
+
 def test_session_termination(basic_server: None, basic_server_url: str):
     """Test session termination via DELETE and subsequent request handling."""
     response = requests.post(