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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Fix flaky tests in TestChildProcessCleanup (test_stdio.py)","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\nThree tests in `TestChildProcessCleanup` are flaky. They fail intermittently on macOS (darwin) with the same root cause: the child process hasn't started writing to the marker file before the assertion checks.\n\n**Affected tests:**\n- `TestChildProcessCleanup.test_basic_child_process_cleanup`\n- `TestChildProcessCleanup.test_nested_process_tree`\n- `TestChildProcessCleanup.test_early_parent_exit`\n\n**CI evidence:** Tests observed failing on 2025-11-12 and continue to be flaky.\n\n**Output:**\n\n```python\n________ TestChildProcessCleanup.test_basic_child_process_cleanup ________\n[gw9] darwin -- Python 3.11.14\n\n    async def test_basic_child_process_cleanup(self):\n        ...\n        # Verify child is writing\n        if os.path.exists(marker_file):  # pragma: no branch\n            initial_size = os.path.getsize(marker_file)\n            await anyio.sleep(0.3)\n            size_after_wait = os.path.getsize(marker_file)\n\u003e           assert size_after_wait \u003e initial_size, \"Child process should be writing\"\nE           AssertionError: Child process should be writing\nE           assert 0 \u003e 0\n\ntests/client/test_stdio.py:310: AssertionError\n```\n\n```python\n________ TestChildProcessCleanup.test_nested_process_tree ________\n[gw9] darwin -- Python 3.11.14\n\n\u003e           assert new_size \u003e initial_size, f\"{name} process should be writing\"\nE           AssertionError: parent process should be writing\nE           assert 0 \u003e 0\n\ntests/client/test_stdio.py:413: AssertionError\n```\n\n```python\n________ TestChildProcessCleanup.test_early_parent_exit ________\n[gw9] darwin -- Python 3.11.14\n\n\u003e           assert size2 \u003e size1, \"Child should be writing\"\nE           AssertionError: Child should be writing\nE           assert 0 \u003e 0\n\ntests/client/test_stdio.py:494: AssertionError\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\nPython 3.11.14, macOS (darwin)\nLatest SDK version\n```\n\n\u003csub\u003e[AI Disclaimer](https://gist.github.com/maxisbey/6123d132484e4c533eab519a2800693d)\u003c/sub\u003e","author":{"url":"https://github.com/maxisbey","@type":"Person","name":"maxisbey"},"datePublished":"2025-12-11T14:19:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/1775/python-sdk/issues/1775"}