OpenAPI describes an API's endpoints, its request and response shapes, and its status codes as a schema document that is independent of any particular implementation.

Contract-first development writes that schema before the implementation. Even while developers are still writing the actual code, the client-side and server-side teams can work against the shape they agreed on. This is the opposite of building the implementation first and writing the schema afterward to describe what it does.

From that one schema, tools can generate interactive documentation, client SDKs in many languages, and request and response validation. Because all of them are generated from the same schema, they stay consistent with each other, and the documentation and validation logic can't drift away from the actual behavior over time.