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Title: fix(client/stdio): default encoding_error_handler to 'replace' for resilient UTF-8 decoding by sarvesh1327 · Pull Request #2907 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: fix(client/stdio): default encoding_error_handler to 'replace' for resilient UTF-8 decoding by sarvesh1327 · Pull Request #2907 · modelcontextprotocol/python-sdk

X Title: fix(client/stdio): default encoding_error_handler to 'replace' for resilient UTF-8 decoding by sarvesh1327 · Pull Request #2907 · modelcontextprotocol/python-sdk

Description: Summary Changes the default encoding_error_handler in StdioServerParameters from "strict" to "replace". Malformed UTF-8 bytes from a child server stdout previously crashed the client transport with UnicodeDecodeError. With "replace", invalid bytes become U+FFFD and the line fails JSON-RPC validation, which is surfaced as an in-stream Exception that the session can handle. This mirrors the server-side stdio hardening from PR #2302. Test Plan Reproduced the crash with a child process emitting \xff\xfe\n followed by valid JSON-RPC. Verified the fix: first item is a ValidationError, second item is the valid SessionMessage. Added regression test test_invalid_utf8_from_the_server_surfaces_as_an_in_stream_exception. Full tests/client/test_stdio.py suite passes (34 passed, 1 skipped). Notes Fixes #2454 Scope intentionally small.

Open Graph Description: Summary Changes the default encoding_error_handler in StdioServerParameters from "strict" to "replace". Malformed UTF-8 bytes from a child server stdout previously crashed the ...

X Description: Summary Changes the default encoding_error_handler in StdioServerParameters from "strict" to "replace". Malformed UTF-8 bytes from a child server stdout previou...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2907

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5b5011b fix(client/stdio): default encoding_error_handler to 'replace' for re… sarvesh1327 Jun 18, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2907/commits/5b5011bc3743adfea2a02d4b59556d1174d0857d
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