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More control over stderr in stdio client #1806
Description
Description
Currently, a TextIO object (defaulting to sys.stderr) must be set for errlog when creating a new stdio client session.
python-sdk/src/mcp/client/stdio/__init__.py
Line 106 in 2aa1ad2
However, the underlying calls to Python's native
Popen and anyio.create_process both support subprocess.DEVNULL: int = -3 for stderr, which allows the stream to be ignored.By updating the type hint, we can easily support
int for errlog. This change would benefit downstream projects and resolve issues for CLI users. Furthermore, I believe ignoring errlog is a better default than piping them into stderr, as the current behaviour can create unexpected side-effects that are difficult to trace.
References
Related Issues:
#156
langchain-ai/langchain-mcp-adapters#72
Documentation:
https://docs.python.org/3/library/subprocess.html#subprocess.Popen
https://anyio.readthedocs.io/en/stable/api.html#anyio.open_process
Activity
[-]More Control over stderr in stdio client[/-][+]More control over stderr in stdio client[/+]SamSi0322 commentedon Apr 10, 2026
I'll take this. The fix is straightforward — updating the type hints on
stdio_client()and_create_platform_compatible_process()to acceptint(forsubprocess.DEVNULL,subprocess.PIPE, etc.) in addition toTextIO. Will have a PR up shortly.