The Art of CTO API Contract Designer is a free visual endpoint builder that generates OpenAPI 3.0 specifications and Markdown documentation, and reports the breaking changes between two saved contracts — removed endpoints, removed or newly required parameters, changed types, changed response schemas and changed security.
What should this API look like before we build it?
API Contract DesignerAbout 30 min · Canvas · ProAbout this toolWhy it matters, common mistakes, FAQ
What Should This API Look Like Before Anyone Builds It?
APIs are the hardest thing to change once someone depends on them, and the second consumer arrives sooner than expected. Design decisions made in an afternoon become a compatibility obligation for years.
The contract is derived from the implementation — endpoints mirror internal tables, and the database schema leaks into a public surface. Designing the contract first is what keeps the internal model free to change.
Questions CTOs ask
- What is API contract-first design?
- Contract-first (or API-first) design means defining your API specification before writing implementation code. You create the OpenAPI/Swagger document specifying endpoints, request/response schemas, and error codes, then generate server stubs and client SDKs from that contract. This approach ensures frontend and backend teams can work in parallel, catches design issues early, and produces consistent, well-documented APIs. It is particularly effective for public APIs and microservices architectures.
- How do you prevent breaking API changes?
- Prevent breaking changes by versioning your API from day one (URL path or header-based), using automated contract testing in CI/CD that compares new schemas against the previous version, and following additive-only change policies (new fields are optional, existing fields never removed or type-changed). When breaking changes are unavoidable, use a deprecation cycle: announce the deprecation, support both versions for a migration period (typically 6-12 months), provide migration guides, and monitor usage of deprecated endpoints before removal.
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