fix(stdio): drain pending responses before closing read stream on EOF (#2678)#2821
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fix(stdio): drain pending responses before closing read stream on EOF (#2678)#2821friendlygeorge wants to merge 2 commits into
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Closes modelcontextprotocol#2678. When stdin hits EOF, the previous code closed read_stream_writer inside the `async with` block, which cascaded to close write_stream_reader before the server's pending responses could drain through stdout_writer. The fix removes the `async with read_stream_writer` wrapper from stdin_reader and instead calls `aclose()` in the `finally` block. This ensures: 1. All stdin lines are read and forwarded to the server 2. The read stream is closed promptly on EOF (signaling the server) 3. Buffered responses in write_stream_reader drain through stdout_writer before the task group exits All existing tests pass. Added regression test verifying responses are not dropped when stdin closes immediately after a request.
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Both close() and __del__ are no-op overrides that prevent TextIOWrapper from closing real stdio handles. They are not directly tested in this PR (the test is on modelcontextprotocol#2821) and __del__ relies on GC timing which is unreliable in tests.
- Collapse multi-line io.BytesIO() to single line (ruff format) - Add blank line after module docstring (ruff format) - Remove unused imports: time, anyio (ruff check --fix)
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Summary
Fixes #2678 — stdio transport drops in-flight server responses when the server process exits.
Problem
When a stdio MCP server exits, the client's closes on EOF before the server's pending responses are fully drained. This causes the client to lose responses that were already sent but not yet consumed.
Root Cause
calls on the stream immediately when EOF is detected, which signals the read loop to stop — even if there are buffered responses still in flight.
Fix
Replace the context manager with explicit in a block, allowing all pending responses to drain before the stream is closed.
All 10 existing stdio tests pass. Added test confirming pending responses are delivered before EOF.