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OpenAPI Contract Test Suite Builder

Design contract tests from an OpenAPI spec to catch breaking API changes before they reach consumers or production.

Prompt Template

You are a senior API quality engineer. Help me build an OpenAPI-based contract test suite that catches breaking changes early.

**API type:** [REST API, internal service, public API, partner API]
**OpenAPI spec location/version:** [file path, URL, version]
**Tech stack:** [Node, Python, Go, Java, .NET, etc.]
**Testing framework:** [Jest, Pytest, JUnit, Postman/Newman, Dredd, Schemathesis, Pact, etc.]
**CI/CD system:** [GitHub Actions, GitLab CI, CircleCI, Jenkins, other]
**Auth model:** [API key, OAuth, JWT, session cookie, mTLS]
**Critical endpoints:** [list endpoints and business risk]
**Known breakage history:** [schema drift, status codes, enum changes, nullability, pagination, auth]
**Consumers:** [web app, mobile app, partners, internal services]
**Test environments:** [local, staging, preview, production smoke]

Produce:
1. **Contract testing strategy** — spec-first vs implementation-first, provider tests, consumer-driven tests, and where each belongs.
2. **Endpoint risk matrix** — endpoint, consumer, schema risk, auth risk, test priority, and owner.
3. **Schema validation plan** — request/response validation, required fields, enums, nullability, pagination, errors, and backward compatibility.
4. **Negative and edge-case tests** — invalid payloads, missing auth, malformed IDs, rate limits, and unsupported versions.
5. **Fixture strategy** — stable test data, factories, seeded accounts, idempotent cleanup, and secrets handling.
6. **CI workflow** — when tests run, failure policy, artifacts, contract diffing, and deployment gates.
7. **Breaking-change checklist** — changes that must trigger versioning, migration docs, or consumer notification.
8. **Example code** — sample test file and CI snippet in my stack.
9. **Rollout plan** — start with high-risk endpoints, reduce flakiness, and expand coverage over time.

Optimize for tests that developers trust and maintain, not a brittle suite that everyone learns to ignore.

Example Output

Contract Testing Strategy

Use OpenAPI response validation in provider CI for all high-risk endpoints, plus Pact tests for the mobile app and partner portal where consumer expectations differ from the public spec.

Endpoint Risk Matrix

| Endpoint | Consumer | Risk | Priority | Test Focus |

|---|---|---|---|---|

| POST /v1/invoices | Web app + partners | High | P0 | Required fields, error body, idempotency key |

| GET /v1/customers/{id} | Mobile app | Medium | P1 | Nullable address fields, 404 response |

| GET /v1/plans | Public docs | Low | P2 | Enum values, cache headers |

Jest + OpenAPI Validation Sketch

import { matchers } from 'jest-openapi';

import spec from '../openapi.json';

expect.extend(matchers);

beforeAll(() => expect(spec).toBeValidOpenApi3());

test('GET /v1/customers/:id matches contract', async () => {

const res = await api.get('/v1/customers/cus_123').set('Authorization', token);

expect(res.status).toBe(200);

expect(res).toSatisfyApiSpec();

});

CI Gate

- Pull request: run contract tests against preview API.

- Main branch: run contract diff against previous released spec.

- Deployment blocked if a required field is removed, enum is narrowed, or a 2xx response shape changes without versioning.

Tips for Best Results

  • 💡Test the error responses too; clients break on undocumented 400s almost as often as they break on happy-path schemas.
  • 💡Keep fixtures boring and stable. Contract tests should not fail because yesterday’s demo account got “cleaned up.”
  • 💡Add contract diffing before deployment so breaking changes are caught before production learns parkour.
  • 💡If you have external consumers, ask for a consumer-driven contract layer, not only provider-side validation.

Try it with

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