Fix stdio_server from closing process stdio handles

[aframedelta]

Summary

• prevent stdio_server() from closing underlying process stdio handles
• add a regression test that verifies stdio remains usable after server shutdown

Why

Closes #1933.

stdio_server() wraps sys.stdin.buffer / sys.stdout.buffer in a TextIOWrapper. When the async wrapper is closed, the underlying process stdio can be closed too, causing subsequent print() calls to fail with ValueError: I/O operation on closed file.

Changes

• introduce _NonClosingTextIOWrapper in mcp.server.stdio
• use it for default process stdio wrapping
• add test_stdio_server_does_not_close_process_stdio regression test

Validation

• . .venv/bin/activate && pytest tests/server/test_stdio.py -q
  • 2 passed

[maxisbey]

Thanks for working on this — the _NonClosingTextIOWrapper approach does work and appreciate the test being in tests/server/test_stdio.py.

We're going to consolidate on #2040 (opened Feb 12) which uses os.dup(). Two reasons:

1. Simpler — 1 line of substantive change vs a new subclass + helper.
2. Avoids shared-buffer state — this approach still wraps the same sys.stdin.buffer object, so two TextIOWrappers share one BufferedReader. Bytes pulled into the MCP wrapper's decode buffer become invisible to sys.stdin after the server exits. os.dup() gives a fully independent fd chain.

Closing in favor of #2040.

_{AI Disclaimer}