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Title: fix: handle non-UTF-8 bytes in stdio server stdin by maxisbey · Pull Request #2302 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: fix: handle non-UTF-8 bytes in stdio server stdin by maxisbey · Pull Request #2302 · modelcontextprotocol/python-sdk

X Title: fix: handle non-UTF-8 bytes in stdio server stdin by maxisbey · Pull Request #2302 · modelcontextprotocol/python-sdk

Description: TextIOWrapper(sys.stdin.buffer, encoding="utf-8") defaults to errors="strict", which raises UnicodeDecodeError when stdin contains bytes that are not valid UTF-8. The exception occurs during async for line in stdin: iteration — outside the JSON-parsing try/except — so it propagates through the task group and terminates the server process. With errors="replace", invalid bytes become U+FFFD. The resulting line then fails JSON validation, which is already caught and sent into the read stream as an exception for the session layer to handle — the same path as any other malformed input. Motivation and Context The MCP spec requires clients to send valid UTF-8 JSON-RPC, and in stdio transport the client spawns the server as its own subprocess — so in practice this only triggers when a client sends protocol-violating input to its own child. But the server process dying on a single bad byte is worse than surfacing a parse error, and the other MCP SDKs (TypeScript, C#, Go, Rust) all handle this gracefully. How Has This Been Tested? Regression test test_stdio_server_invalid_utf8 monkeypatches sys.stdin with a binary buffer containing \xff\xfe\n followed by a valid JSON-RPC message. It verifies the invalid line surfaces as an in-stream exception and the valid message still comes through afterward. Fails on main with ExceptionGroup(UnicodeDecodeError), passes with this fix. Breaking Changes None. Types of changes Bug fix (non-breaking change which fixes an issue) New feature (non-breaking change which adds functionality) Breaking change (fix or feature that would cause existing functionality to change) Documentation update Checklist I have read the MCP Documentation My code follows the repository's style guidelines New and existing tests pass locally I have added appropriate error handling I have added or updated documentation as needed Additional context Reported by @manthanghasadiya. src/mcp/client/stdio.py has a related issue — StdioServerParameters.encoding_error_handler defaults to "strict", so a buggy server can crash the client the same way. It's configurable so lower priority; worth a follow-up to change the default.

Open Graph Description: TextIOWrapper(sys.stdin.buffer, encoding="utf-8") defaults to errors="strict", which raises UnicodeDecodeError when stdin contains bytes that are not valid UTF-8. The exception ...

X Description: TextIOWrapper(sys.stdin.buffer, encoding="utf-8") defaults to errors="strict", which raises UnicodeDecodeError when stdin contains bytes that are not valid UTF-8...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2302

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