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Fix stdio_client BrokenResourceError race condition during shutdown#2268

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Fix stdio_client BrokenResourceError race condition during shutdown #2268

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4 changes: 2 additions & 2 deletions src/mcp/client/stdio.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -158,7 +158,7 @@ async def stdout_reader():

session_message = SessionMessage(message)
await read_stream_writer.send(session_message)
except anyio.ClosedResourceError: # pragma: lax no cover
except (anyio.ClosedResourceError, anyio.BrokenResourceError): # pragma: lax no cover

@BortlesboatBortlesboatMar 16, 2026

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Noting for future readers: the SSE client (sse.py) avoids this class of race by calling tg.cancel_scope.cancel() in its finally block, which cancels tasks before closing their streams. The stdio client takes a different approach — letting stream closure propagate to tasks and catching the resulting exceptions. Both are valid; this PR correctly handles the full set of exceptions that anyio can raise in this scenario.

await anyio.lowlevel.checkpoint()

async def stdin_writer():
Expand All@@ -174,7 +174,7 @@ async def stdin_writer():
errors=server.encoding_error_handler,
)
)
except anyio.ClosedResourceError: # pragma: no cover
except (anyio.ClosedResourceError, anyio.BrokenResourceError): # pragma: no cover
await anyio.lowlevel.checkpoint()

async with anyio.create_task_group() as tg, process:
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38 changes: 38 additions & 0 deletions tests/client/test_stdio.py
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Expand Up@@ -508,6 +508,44 @@ def handle_term(sig, frame):
pass


@pytest.mark.anyio
@pytest.mark.filterwarnings("ignore::ResourceWarning")
async def test_stdio_client_no_broken_resource_error_on_shutdown():
"""Test that exiting stdio_client without consuming the read stream does not
raise BrokenResourceError.

Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/1960.
The race condition occurs when stdout_reader is blocked on send() into a
zero-buffer memory stream and the finally block closes the receiving end,
causing BrokenResourceError instead of ClosedResourceError.
"""
# Server sends a JSON-RPC message and then sleeps, keeping stdout open.
# The client exits without consuming the read stream, triggering the race.
server_script = textwrap.dedent(
"""
import sys
import time

sys.stdout.write('{"jsonrpc":"2.0","id":1,"result":{}}\\n')
sys.stdout.flush()
time.sleep(5.0)
"""
)

server_params = StdioServerParameters(
command=sys.executable,
args=["-c", server_script],
)

# This should exit cleanly without raising an ExceptionGroup
# containing BrokenResourceError.
with anyio.fail_after(10.0):
async with stdio_client(server_params) as (_read_stream, _write_stream):
# Give stdout_reader time to read the message and block on send()
await anyio.sleep(0.3)
# Exit without consuming read_stream - this triggers the race


@pytest.mark.anyio
async def test_stdio_client_graceful_stdin_exit():
"""Test that a process exits gracefully when stdin is closed,
Expand Down