[anth-volk]

Fixes #35

Summary

ObservabilityRuntime.end_request assigned its error argument to the request state unconditionally, which replaced an exception attached by record_exception with None. The Flask adapter ends each request from its teardown_request callback, and Flask passes None to that callback once a registered error handler has handled the exception. A Flask error handler that called record_exception(error, handled=False, status_code=500) and returned a 500 response therefore produced a request.completed record with outcome="error" but no error.type, error.message or error.stack, and the server span was ended without the exception event. The Public API (PolicyEngine/policyengine-api#3984) found this when its catch-all Flask error handler started recording exceptions.

Change

end_request still computes the outcome from its error argument and the resolved status code, exactly as before. When that outcome is "error" and no error argument was passed, it now uses the exception attached by record_exception for the request.completed error fields and for ending the span. The change is in policyengine_observability/runtime.py; the Flask adapter is unchanged.

The same applies to a FastAPI exception handler registered for a specific exception type that records the exception and returns a 500 response. Starlette runs those handlers inside the observability middleware, so the adapter only receives the response in that case and previously dropped the recorded exception in the same way.

Behavior that does not change

• The outcome calculation is unchanged. The attached exception is never used to compute it.
• A request that records a handled exception (record_exception(..., handled=True)) and returns a 2xx or 4xx response keeps outcome="success" or outcome="client_error", INFO severity and no error.* fields, and adds no request point to policyengine.error.count. Using the attached exception unconditionally would have turned these requests into errors.
• An error argument passed to end_request still takes precedence over an attached exception.
• The FastAPI adapter still passes an exception that propagates out of the application to end_request directly, and that exception is reported.
• record_exception still emits its own exception.recorded record and error metric point.

Tests

New tests, written before the fix:

• test_flask_error_handler_exception_is_kept_on_request_completion: a Flask error handler records the exception with handled=False, status_code=500 and returns a 500 response. Exactly one request.completed record is emitted, with outcome="error", ERROR severity and the exception's error.type, error.message and error.stack; the error metric receives one unhandled point and one request point; the span has error status and one exception event.
• test_flask_handled_exception_does_not_mark_request_failed, for a 200 and a 404 response: outcome="success" or "client_error", INFO severity, no error.* fields, only the handled error metric point, and a span with unset status and no events.
• test_server_error_completion_uses_recorded_exception: the runtime-level case, end_request(status_code=None) after record_exception(..., status_code=500).
• test_explicit_request_error_takes_precedence_over_recorded_exception and test_fastapi_raised_exception_takes_precedence_over_recorded_error: an exception passed to end_request is reported instead of an earlier recorded one.
• test_fastapi_handled_exception_does_not_mark_request_failed: a FastAPI route that records a handled exception and returns 200 keeps outcome="success".
• test_fastapi_exception_handler_error_is_kept_on_request_completion: a FastAPI exception handler that records the exception and returns 500.

Before the fix, the three tests that check for error fields on a 5xx completion failed with KeyError: 'error.type', and the other five passed. With a version of the fix that used the attached exception unconditionally, all three handled-exception tests failed with outcome == "error".

Verification

• uv run --extra dev ruff format --check .: 32 files already formatted
• uv run --extra dev ruff check .: all checks passed
• uv run --extra dev --extra all coverage run -m pytest: 148 passed
• uv run --extra dev --extra all coverage report: 91% total, above the 90% minimum
• uv run --extra dev --extra all pyright: 0 errors
• towncrier check --compare-with origin/main: finds changelog.d/35.fixed.md

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