python-tests

---
name: python-tests
description: Write regression tests for FastMCP changes using its pytest fixtures, in-memory clients, and protocol boundaries.
---

# FastMCP tests

Read nearby tests and the repository's validation requirements first. Extend the owning test module when it remains readable; use a focused module when the existing one is already too large.

For a bug fix, run the regression against unchanged code and confirm it fails for the reported reason, then run it with the fix. An import error or broken fixture is not a reproduced bug. Include supported adjacent behavior that the same branch could change, especially omitted defaults and explicit overrides.

- Async tests need no marker: `asyncio_mode = "auto"` is configured globally.
- Use `Client(server)` for real MCP dispatch without a network server. Use HTTP transport only when network behavior is the subject.
- Mock external boundaries, not the FastMCP functions under test. For OpenAPI requests, capture the outgoing request with `httpx2.MockTransport` and assert its bytes, headers, or decoded parameters.
- Keep imports at module scope. Prefer function-scoped fixtures and `tmp_path` for files.
- Parameterize variations of one contract; separate unrelated behavior. For JSON, equality alone does not distinguish `True` from `1`: check the resulting type when type preservation matters.
- Use existing `inline_snapshot` conventions for complex schema assertions. Inspect generated snapshots rather than accepting them blindly.
- The default timeout is five seconds. Use the repository's integration conventions for tests that legitimately need more time.

Run focused tests during development, then the full commands required by AGENTS.md before committing. If a failure appears unrelated, reproduce it on the unchanged base; do not call it pre-existing from the test name alone.

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• PrefectHQ/fastmcp28k