server: skip duplicate response on CancelledError (#1153)

Co-authored-by: ihrpr <[email protected]>

Author: lukacf

src/mcp/server/lowlevel/server.py

@@ -647,6 +647,12 @@ async def _handle_request(
                 response = await handler(req)
             except McpError as err:
                 response = err.error
+            except anyio.get_cancelled_exc_class():
+                logger.info(
+                    "Request %s cancelled - duplicate response suppressed",
+                    message.request_id,
+                )
+                return
             except Exception as err:
                 if raise_exceptions:
                     raise err

tests/server/test_cancel_handling.py

@@ -0,0 +1,111 @@
+"""Test that cancelled requests don't cause double responses."""
+
+import anyio
+import pytest
+
+import mcp.types as types
+from mcp.server.lowlevel.server import Server
+from mcp.shared.exceptions import McpError
+from mcp.shared.memory import create_connected_server_and_client_session
+from mcp.types import (
+    CallToolRequest,
+    CallToolRequestParams,
+    CallToolResult,
+    CancelledNotification,
+    CancelledNotificationParams,
+    ClientNotification,
+    ClientRequest,
+    Tool,
+)
+
+
+@pytest.mark.anyio
+async def test_server_remains_functional_after_cancel():
+    """Verify server can handle new requests after a cancellation."""
+
+    server = Server("test-server")
+
+    # Track tool calls
+    call_count = 0
+    ev_first_call = anyio.Event()
+    first_request_id = None
+
+    @server.list_tools()
+    async def handle_list_tools() -> list[Tool]:
+        return [
+            Tool(
+                name="test_tool",
+                description="Tool for testing",
+                inputSchema={},
+            )
+        ]
+
+    @server.call_tool()
+    async def handle_call_tool(name: str, arguments: dict | None) -> list:
+        nonlocal call_count, first_request_id
+        if name == "test_tool":
+            call_count += 1
+            if call_count == 1:
+                first_request_id = server.request_context.request_id
+                ev_first_call.set()
+                await anyio.sleep(5)  # First call is slow
+            return [types.TextContent(type="text", text=f"Call number: {call_count}")]
+        raise ValueError(f"Unknown tool: {name}")
+
+    async with create_connected_server_and_client_session(server) as client:
+        # First request (will be cancelled)
+        async def first_request():
+            try:
+                await client.send_request(
+                    ClientRequest(
+                        CallToolRequest(
+                            method="tools/call",
+                            params=CallToolRequestParams(name="test_tool", arguments={}),
+                        )
+                    ),
+                    CallToolResult,
+                )
+                pytest.fail("First request should have been cancelled")
+            except McpError:
+                pass  # Expected
+
+        # Start first request
+        async with anyio.create_task_group() as tg:
+            tg.start_soon(first_request)
+
+            # Wait for it to start
+            await ev_first_call.wait()
+
+            # Cancel it
+            assert first_request_id is not None
+            await client.send_notification(
+                ClientNotification(
+                    CancelledNotification(
+                        method="notifications/cancelled",
+                        params=CancelledNotificationParams(
+                            requestId=first_request_id,
+                            reason="Testing server recovery",
+                        ),
+                    )
+                )
+            )
+
+        # Second request (should work normally)
+        result = await client.send_request(
+            ClientRequest(
+                CallToolRequest(
+                    method="tools/call",
+                    params=CallToolRequestParams(name="test_tool", arguments={}),
+                )
+            ),
+            CallToolResult,
+        )
+
+        # Verify second request completed successfully
+        assert len(result.content) == 1
+        # Type narrowing for pyright
+        content = result.content[0]
+        assert content.type == "text"
+        assert isinstance(content, types.TextContent)
+        assert content.text == "Call number: 2"
+        assert call_count == 2