api-contract-testing-pattern

You detect API drift in FastAPI by comparing current response schemas and the generated openapi.json against committed snapshots. This validates route presence, status codes, and pydantic model shapes to catch unintended changes during endpoint refactors.

API contract testing verifies that routes, response schemas, status codes, and OpenAPI output remain consistent with expectations. It prevents silent client breakage by enforcing exact response shapes and standardizing happy and error path coverage beyond simple smoke checks.

You validate FastAPI response schemas with pydantic by configuring models to forbid extra fields. This enforces exact response shapes and detects schema drift immediately when unintended fields are added to API outputs.

Yes, pytest works with openapi.json snapshot testing for REST APIs. You copy template tests into your project, update the expected routes and schemas, then run the suite to compare current OpenAPI output against committed snapshots.

You check route existence in API contract tests to fail fast when expected method and path routes are not registered. This stops case regressions immediately after refactors or integration changes alter endpoint availability.

Snapshot testing for API drift detection requires committing and maintaining openapi.json files manually. It detects structural changes effectively but does not validate runtime data correctness or business logic within the API endpoints.