Fix unawaited coroutine warning and treat as test error

[jlowin]

When _await_with_session_monitoring detects that the session task is already done, it raises immediately without closing the passed coroutine. This causes "coroutine was never awaited" warnings during test teardown.

Fix: Call coro.close() before raising to properly clean up the coroutine.

Prevention: Add pytest filterwarnings to treat unawaited coroutine warnings as errors so future occurrences will fail tests rather than emit warnings.

# pyproject.toml
filterwarnings = [
    "error:coroutine .* was never awaited:RuntimeWarning",
    "error:Exception ignored in.*coroutine:pytest.PytestUnraisableExceptionWarning",
]

[coderabbitai]

Walkthrough

This PR modifies the _await_with_session_monitoring method in src/fastmcp/client/client.py to add a coro.close() call when a session task has already completed. This ensures coroutines are properly closed to suppress "was never awaited" warnings that could occur when a session task finishes before its coroutine is awaited.

Possibly related PRs

• Fix client hanging on HTTP 4xx/5xx errors #2803: Also modifies the _await_with_session_monitoring implementation in the same file to handle coroutine closure and warning suppression.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: fixing an unawaited coroutine warning and configuring pytest to treat it as a test error.
Description check	✅ Passed	The description provides a clear problem statement, the fix applied, and prevention measures with specific code examples. However, it is missing the required Contributors and Review Checklist sections from the template.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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• src/fastmcp/client/client.py

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src/fastmcp/**/*.py

📄 CodeRabbit inference engine (AGENTS.md)

src/fastmcp/**/*.py: Python ≥ 3.10 with full type annotations required
Prioritize readable, understandable code - clarity over cleverness. Avoid obfuscated or confusing patterns even if shorter
Follow existing patterns and maintain consistency in code implementation
Be intentional about re-exports - don't blindly re-export everything to parent namespaces. Core types defining a module's purpose should be exported. Specialized features can live in submodules. Only re-export to fastmcp.* for most fundamental types
Never use bare except - be specific with exception types

Files:

• src/fastmcp/client/client.py

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• GitHub Check: Analyze (python)
• GitHub Check: Run tests: Python 3.10 on windows-latest
• GitHub Check: Run tests: Python 3.10 on ubuntu-latest
• GitHub Check: Run tests: Python 3.13 on ubuntu-latest
• GitHub Check: Run tests with lowest-direct dependencies

🔇 Additional comments (1)

src/fastmcp/client/client.py (1)

689-690: LGTM! Proper coroutine cleanup prevents runtime warnings.

The fix correctly calls coro.close() to clean up the coroutine when the session task has already completed. This prevents the "coroutine was never awaited" warning since the coroutine cannot be awaited after the session task is done. The change is minimal, well-commented, and follows Python's coroutine lifecycle best practices.

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