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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"stdio_client crashes on malformed UTF-8 from child stdout instead of surfacing parse error","articleBody":"## Summary\n\n`mcp.client.stdio.stdio_client()` crashes when the spawned child process writes invalid UTF-8 bytes to stdout.\n\nThe transport currently decodes child stdout with `encoding_error_handler=\"strict\"`, so malformed bytes raise during `TextReceiveStream(...)` iteration. That exception escapes the decoding loop and brings down the transport task group instead of surfacing the bad line as an in-stream parse error.\n\n## Why this looks like a bug\n\nThe SDK already hardened the server side for the analogous case in PR #2302 (`fix: handle non-UTF-8 bytes in stdio server stdin`). That change explicitly preferred:\n\n- replace invalid bytes with U+FFFD,\n- let JSON validation fail on the malformed line, and\n- keep the transport alive so subsequent valid messages can still be processed.\n\nThe client side still behaves asymmetrically today. A buggy or non-compliant child server can kill the Python client transport with a single malformed line even if the next line is valid JSON-RPC.\n\nThat seems inconsistent with the current stdio robustness direction.\n\n## Reproduction\n\nA minimal child process that writes one malformed line and then one valid JSON-RPC line:\n\n```python\nimport sys\nimport time\n\nsys.stdout.buffer.write(b\"\\xff\\xfe\\n\")\nsys.stdout.buffer.write(b'{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"ping\"}\\n')\nsys.stdout.buffer.flush()\ntime.sleep(0.2)\n```\n\nWith current `stdio_client(...)` defaults, the transport raises `ExceptionGroup` instead of continuing.\n\n## Expected behavior\n\nThe malformed line should be surfaced as an in-stream parse / validation error, and the next valid JSON-RPC line should still be received.\n\n## Observed behavior\n\nThe transport task group fails before the valid follow-up message is delivered.\n\n## Proposed fix\n\nMatch the server-side approach from PR #2302:\n\n1. default `StdioServerParameters.encoding_error_handler` to `\"replace\"`\n2. continue treating malformed decoded lines as JSON validation failures\n3. keep the background stdio tasks resilient during early subprocess shutdown / abrupt close\n\n## Validation\n\nI reproduced this locally against current `main` and verified that a minimal patch plus a regression test fixes it.\n\nA focused regression test in `tests/client/test_stdio.py` can assert:\n\n- first item from the read stream is an `Exception`\n- second item is the valid `SessionMessage`\n","author":{"url":"https://github.com/shaun0927","@type":"Person","name":"shaun0927"},"datePublished":"2026-04-16T15:37:25.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/2454/python-sdk/issues/2454"}