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Stop answering cancelled requests (#3188)

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Max 2026-07-27 23:26:00 +01:00 • committed by GitHub
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49
tests/server/test_cancel_handling.py
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@ -22,7 +22,6 @@ from mcp_types.version import LATEST_HANDSHAKE_VERSION
from mcp import Client
from mcp.server import Server, ServerRequestContext
from mcp.shared.exceptions import MCPError
from mcp.shared.message import SessionMessage
@ -33,6 +32,7 @@ async def test_server_remains_functional_after_cancel():
# Track tool calls
call_count = 0
ev_first_call = anyio.Event()
ev_first_call_cancelled = anyio.Event()
first_request_id = None
async def handle_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
@ -53,38 +53,45 @@ async def test_server_remains_functional_after_cancel():
if call_count == 1:
first_request_id = ctx.request_id
ev_first_call.set()
await anyio.sleep(5) # First call is slow
try:
await anyio.sleep_forever() # First call blocks until cancelled
except anyio.get_cancelled_exc_class():
ev_first_call_cancelled.set()
raise
return CallToolResult(content=[TextContent(type="text", text=f"Call number: {call_count}")])
raise ValueError(f"Unknown tool: {params.name}") # pragma: no cover
server = Server("test-server", on_list_tools=handle_list_tools, on_call_tool=handle_call_tool)
async with Client(server, mode="legacy") as client:
# First request (will be cancelled)
# First request (will be cancelled server-side, then abandoned here: a
# cancelled request is never answered, so nothing would wake this call)
async def first_request():
try:
await client.session.send_request(
CallToolRequest(params=CallToolRequestParams(name="test_tool", arguments={})),
CallToolResult,
)
pytest.fail("First request should have been cancelled") # pragma: no cover
except MCPError:
pass # Expected
await client.session.send_request(
CallToolRequest(params=CallToolRequestParams(name="test_tool", arguments={})),
CallToolResult,
)
raise NotImplementedError # unreachable: the task is cancelled before any answer
# Start first request
async with anyio.create_task_group() as tg:
tg.start_soon(first_request)
with anyio.fail_after(5):
async with anyio.create_task_group() as tg:
tg.start_soon(first_request)
# Wait for it to start
await ev_first_call.wait()
# Wait for it to start
await ev_first_call.wait()
# Cancel it
assert first_request_id is not None
await client.session.send_notification(
CancelledNotification(
params=CancelledNotificationParams(request_id=first_request_id, reason="Testing server recovery"),
# Cancel it
assert first_request_id is not None
await client.session.send_notification(
CancelledNotification(
params=CancelledNotificationParams(
request_id=first_request_id, reason="Testing server recovery"
),
)
)
)
await ev_first_call_cancelled.wait()
tg.cancel_scope.cancel() # abandon the parked call
# Second request (should work normally)
result = await client.call_tool("test_tool", {})