API-First

API-First is a development approach where the API contract is designed and agreed upon before any code is written, treating the API as a first-class product instead of a byproduct of an application. It flips the usual order of operations: the interface leads, and the implementation follows the contract rather than the contract chasing the implementation.

• Design before implementation - The API definition — often an OpenAPI contract — is authored and reviewed first, so teams agree on the surface area before building.
• Contract as the source of truth - Mocks, tests, SDKs, and documentation all generate from the same definition, keeping them in sync.
• Parallel development - Once the contract exists, producer and consumer teams can work against it simultaneously instead of waiting on each other.
• Consistency across an estate - A shared design discipline yields interfaces that look and behave the same across many services.

In real API operations, API-First is the discipline that makes governance, discovery, and automation possible — you cannot lint, mock, or index a contract that does not exist yet. It underpins design-first workflows, feeds catalogs like APIs.json, and matters more than ever in the agentic turn, where an agent needs a stable, well-described contract to reason about an API before it ever makes a call.