api-contract-testing-pattern

What problem does it solve?

This Skill prevents FastAPI/REST APIs from silently breaking clients by validating that routes, response schemas, status codes, and OpenAPI output remain consistent with expectations.

Core Features & Use Cases

• Pydantic response schema validation: Enforces exact response shapes (including forbidding extra fields) to detect schema drift and unintended field changes.
• OpenAPI snapshot drift detection: Detects unintended additions/removals and method changes by comparing current /openapi.json against a committed snapshot.
• Contract-style happy/error path coverage: Standardizes verification of correct status codes for auth, missing resources, and invalid credentials, beyond simple 200 OK smoke checks.
• Route existence checks to stop “case 20” regressions: Fails fast when expected method/path routes are not registered after refactors or integration changes.

Quick Start

Copy the template tests into your project, update APP_IMPORT/EXPECTED_ROUTES/ResponseSchema to match your API, then run your contract test suite with pytest.