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fix(stdio): handle BrokenResourceError in stdout_reader race (#1960) by ddullah · Pull Request #2450 · modelcontextprotocol/python-sdk · GitHub

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fix(stdio): handle BrokenResourceError in stdout_reader race (#1960) #2450

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fix(stdio): handle BrokenResourceError in stdout_reader race (#1960) #2450

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18 changes: 15 additions & 3 deletions src/mcp/client/stdio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -153,12 +153,24 @@ async def stdout_reader():
message = types.jsonrpc_message_adapter.validate_json(line, by_name=False)
except Exception as exc: # pragma: no cover
logger.exception("Failed to parse JSONRPC message from server")
await read_stream_writer.send(exc)
try:
await read_stream_writer.send(exc)
except (anyio.ClosedResourceError, anyio.BrokenResourceError):
return
Comment on lines +156 to +159

maxisbey Apr 16, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality
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try:
await read_stream_writer.send(exc)
except (anyio.ClosedResourceError, anyio.BrokenResourceError):
return
await read_stream_writer.send(exc)

continue

session_message = SessionMessage(message)
await read_stream_writer.send(session_message)
except anyio.ClosedResourceError: # pragma: lax no cover
try:
await read_stream_writer.send(session_message)
except (anyio.ClosedResourceError, anyio.BrokenResourceError):
# The caller exited the stdio_client context while the
# subprocess was still writing; the reader stream has been
# closed (ClosedResourceError) or the closure raced our
# in-flight send (BrokenResourceError). Either way there
# is nowhere to deliver this message, so shut down
# cleanly. Fixes #1960.
return
except (anyio.ClosedResourceError, anyio.BrokenResourceError): # pragma: lax no cover
Comment on lines +163 to +173

maxisbey Apr 16, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality
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try:
await read_stream_writer.send(session_message)
except (anyio.ClosedResourceError, anyio.BrokenResourceError):
# The caller exited the stdio_client context while the
# subprocess was still writing; the reader stream has been
# closed (ClosedResourceError) or the closure raced our
# in-flight send (BrokenResourceError). Either way there
# is nowhere to deliver this message, so shut down
# cleanly. Fixes #1960.
return
except (anyio.ClosedResourceError, anyio.BrokenResourceError): # pragma: lax no cover
await read_stream_writer.send(session_message)
except (anyio.ClosedResourceError, anyio.BrokenResourceError):
# BrokenResourceError: read_stream (receiver) was closed during cleanup while
# send() was blocked waiting for a consumer.

await anyio.lowlevel.checkpoint()

async def stdin_writer():
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39 changes: 39 additions & 0 deletions tests/client/test_stdio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,45 @@ async def test_stdio_context_manager_exiting():
pass


@pytest.mark.anyio
async def test_stdio_client_exits_cleanly_while_server_still_writing():
"""Regression test for #1960.

Exiting the ``stdio_client`` context while the subprocess is still writing to
stdout used to surface ``anyio.BrokenResourceError`` through the task group
(as an ``ExceptionGroup``). The ``finally`` block closes
``read_stream_writer`` while the background ``stdout_reader`` task is
mid-``send``.

The fix makes ``stdout_reader`` catch both ``ClosedResourceError`` and
``BrokenResourceError`` and return cleanly, so exiting the context is a
no-op no matter what the subprocess is doing.
"""
# A server that emits a large burst of valid JSON-RPC notifications without
# ever reading stdin. When we exit the context below, the subprocess is
# still in the middle of that burst, which is the exact shape of the race.
noisy_script = textwrap.dedent(
"""
import sys
for i in range(1000):
sys.stdout.write(
'{"jsonrpc":"2.0","method":"notifications/message",'
'"params":{"level":"info","data":"line ' + str(i) + '"}}\\n'
)
sys.stdout.flush()
"""
)

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

# The ``async with`` must complete without an ``ExceptionGroup`` /
# ``BrokenResourceError`` propagating. ``anyio.fail_after`` prevents a
# regression from hanging CI.
with anyio.fail_after(5.0):
async with stdio_client(server_params) as (_, _):
pass
Comment on lines +41 to +76

maxisbey Apr 16, 2026

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async def test_stdio_client_exits_cleanly_while_server_still_writing():
"""Regression test for #1960.
Exiting the ``stdio_client`` context while the subprocess is still writing to
stdout used to surface ``anyio.BrokenResourceError`` through the task group
(as an ``ExceptionGroup``). The ``finally`` block closes
``read_stream_writer`` while the background ``stdout_reader`` task is
mid-``send``.
The fix makes ``stdout_reader`` catch both ``ClosedResourceError`` and
``BrokenResourceError`` and return cleanly, so exiting the context is a
no-op no matter what the subprocess is doing.
"""
# A server that emits a large burst of valid JSON-RPC notifications without
# ever reading stdin. When we exit the context below, the subprocess is
# still in the middle of that burst, which is the exact shape of the race.
noisy_script = textwrap.dedent(
"""
import sys
for i in range(1000):
sys.stdout.write(
'{"jsonrpc":"2.0","method":"notifications/message",'
'"params":{"level":"info","data":"line ' + str(i) + '"}}\\n'
)
sys.stdout.flush()
"""
)
server_params = StdioServerParameters(command=sys.executable, args=["-c", noisy_script])
# The ``async with`` must complete without an ``ExceptionGroup`` /
# ``BrokenResourceError`` propagating. ``anyio.fail_after`` prevents a
# regression from hanging CI.
with anyio.fail_after(5.0):
async with stdio_client(server_params) as (_, _):
pass
async def test_stdio_client_exits_cleanly_with_unread_server_output():
"""Exiting the stdio_client context while stdout_reader is blocked in send() must
not raise. Cleanup closes read_stream (the receiver) first, which wakes the
blocked send() with BrokenResourceError; stdout_reader should swallow it.
"""
script = textwrap.dedent(
"""
import sys
sys.stdout.write('{"jsonrpc":"2.0","method":"notifications/message","params":{}}\\n')
sys.stdout.flush()
sys.stdin.read()
"""
)
server_params = StdioServerParameters(command=sys.executable, args=["-c", script])
with anyio.fail_after(5.0):
async with stdio_client(server_params) as (read_stream, _):
# Wait until stdout_reader is parked inside send() on the zero-buffer
# stream. statistics() makes this a positive check rather than a sleep.
while read_stream.statistics().tasks_waiting_send == 0:
await anyio.sleep(0)



@pytest.mark.anyio
@pytest.mark.skipif(tee is None, reason="could not find tee command")
async def test_stdio_client():
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