fix: Fix mcp timeout issue (#922)

Author: Unshure

src/strands/tools/mcp/mcp_client.py

@@ -18,6 +18,7 @@
 from types import TracebackType
 from typing import Any, Callable, Coroutine, Dict, Optional, TypeVar, Union, cast
 
+import anyio
 from mcp import ClientSession, ListToolsResult
 from mcp.types import CallToolResult as MCPCallToolResult
 from mcp.types import GetPromptResult, ListPromptsResult
@@ -378,6 +379,13 @@ def _handle_tool_result(self, tool_use_id: str, call_tool_result: MCPCallToolRes
 
         return result
 
+    # Raise an exception if the underlying client raises an exception in a message
+    # This happens when the underlying client has an http timeout error
+    async def _handle_error_message(self, message: Exception | Any) -> None:
+        if isinstance(message, Exception):
+            raise message
+        await anyio.lowlevel.checkpoint()
+
     async def _async_background_thread(self) -> None:
         """Asynchronous method that runs in the background thread to manage the MCP connection.
 
@@ -388,7 +396,9 @@ async def _async_background_thread(self) -> None:
         try:
             async with self._transport_callable() as (read_stream, write_stream, *_):
                 self._log_debug_with_thread("transport connection established")
-                async with ClientSession(read_stream, write_stream) as session:
+                async with ClientSession(
+                    read_stream, write_stream, message_handler=self._handle_error_message
+                ) as session:
                     self._log_debug_with_thread("initializing MCP session")
                     await session.initialize()
 

tests_integ/mcp/test_mcp_client.py

@@ -31,6 +31,11 @@ def start_comprehensive_mcp_server(transport: Literal["sse", "streamable-http"],
 
     mcp = FastMCP("Comprehensive MCP Server", port=port)
 
+    @mcp.tool(description="Tool that will timeout")
+    def timeout_tool() -> str:
+        time.sleep(10)
+        return "This tool has timed out"
+
     @mcp.tool(description="Calculator tool which performs calculations")
     def calculator(x: int, y: int) -> int:
         return x + y
@@ -297,3 +302,27 @@ def slow_transport():
     with client:
         tools = client.list_tools_sync()
         assert len(tools) >= 0  # Should work now
+
+
+@pytest.mark.skipif(
+    condition=os.environ.get("GITHUB_ACTIONS") == "true",
+    reason="streamable transport is failing in GitHub actions, debugging if linux compatibility issue",
+)
+@pytest.mark.asyncio
+async def test_streamable_http_mcp_client_times_out_before_tool():
+    """Test an mcp server that timesout before the tool is able to respond."""
+    server_thread = threading.Thread(
+        target=start_comprehensive_mcp_server, kwargs={"transport": "streamable-http", "port": 8001}, daemon=True
+    )
+    server_thread.start()
+    time.sleep(2)  # wait for server to startup completely
+
+    def transport_callback() -> MCPTransport:
+        return streamablehttp_client(sse_read_timeout=2, url="http://127.0.0.1:8001/mcp")
+
+    streamable_http_client = MCPClient(transport_callback)
+    with streamable_http_client:
+        # Test tools
+        result = await streamable_http_client.call_tool_async(tool_use_id="123", name="timeout_tool")
+        assert result["status"] == "error"
+        assert result["content"][0]["text"] == "Tool execution failed: Connection closed"