The API Contract
API-first means designing the API before writing implementation code. The contract becomes the single source of truth that frontend, backend, and testing teams share. Changes to the contract are deliberate, versioned, and communicated.
The traditional approach is code-first: backend engineers build the API, then document it, then frontend engineers consume it. The documentation is always out of date. The frontend team discovers breaking changes when their code breaks, not when the API changes.
API-first inverts this. The contract is designed collaboratively, reviewed as a document, and approved before implementation begins. Frontend and backend teams work from the same specification. Mock servers generated from the contract enable frontend development to proceed before the backend is complete. Contract tests validate both sides against the agreed specification. The result is parallel development, fewer integration bugs, and faster delivery.
OpenAPI as Specification
OpenAPI (Swagger) defines endpoints, request/response schemas, authentication, and error formats. It generates documentation, client SDKs, and server stubs. The specification lives in version control and is reviewed like code.
The OpenAPI specification is written in YAML or JSON and describes the entire API surface. For each endpoint, it defines the HTTP method, path parameters, query parameters, request body schema, response schemas, and error codes. The schema definitions use JSON Schema, which supports complex types: nested objects, arrays, enums, and validation rules.
The specification lives in version control, typically in a dedicated repository or a specific directory in the API repository. Changes are made through pull requests, reviewed by both frontend and backend teams, and approved by an API governance group.
Contract Testing
Contract tests verify that providers and consumers agree on the API specification. Tools like Pact record consumer expectations and verify provider compliance. This catches breaking changes before deployment, not in production.
We implement contract testing in CI for both consumer and provider builds. Consumer builds generate the contract and publish it to a Pact Broker. Provider builds verify the contract against the current implementation. The Pact Broker tracks contract versions, verifies compatibility, and provides visibility into which consumers depend on which provider versions.
Versioning and Breaking Changes
API versioning is inevitable. The question is how to manage it. We recommend URL versioning (/v1/, /v2/) over header versioning or content negotiation. URL versioning is explicit, cacheable, and easy to understand. Consumers know exactly which version they are calling.
Breaking changes should be rare and deliberate. A breaking change is anything that could cause a consumer to fail: removing a field, changing a field type, changing an endpoint path, or changing error response formats. Breaking changes require a major version bump. Non-breaking changes (adding optional fields, adding new endpoints) require only a minor version bump.
Our Recommendation
Design APIs with OpenAPI first. Generate code from the spec, not the other way around. Implement contract testing in CI. The upfront investment pays off in reduced integration bugs and faster parallel development.
API-first is not just a technical practice. It is an organisational practice that requires collaboration between frontend, backend, and testing teams. The investment is significant, but the return is faster delivery, fewer bugs, and more maintainable systems.