OpenAPI vs GraphQL Schema: Which Contract?
OpenAPI describes REST endpoints in a separate document; a GraphQL schema is the built-in, mandatory type system that defines a GraphQL API.
OpenAPI is an optional, external description of a REST API - paths, methods, parameters, and response shapes - that drives docs, codegen, and validation. A GraphQL schema is intrinsic to the API itself: it defines every type, query, and mutation and is introspectable at runtime. OpenAPI documents an otherwise convention-based REST API; the GraphQL schema is the API contract by construction.
| OpenAPI | GraphQL Schema | |
|---|---|---|
| Relationship | External description | Built into API |
| Optional? | Yes (REST works without) | No (required) |
| Introspection | Via the doc | Runtime introspection |
| Drives | Docs, codegen, mocks | Types, validation, codegen |
| Best for | Describing REST | Defining GraphQL |
How each defines the contract
With REST you choose to write OpenAPI to formalize an otherwise loosely typed API, and you must keep it in sync with the implementation. With GraphQL the schema is the implementation contract and clients can introspect it directly. Both enable strong typing and codegen, but one is bolted on while the other is inherent.
In CI
OpenAPI pipelines lint the spec and check it matches the implementation; GraphQL pipelines validate the schema and guard against breaking changes. Both run on managed runners, where faster runners shorten validation and codegen steps.
The verdict
Documenting and generating tooling for a REST API: OpenAPI. Defining a strongly typed, introspectable API contract by construction: a GraphQL schema. The deeper point is that GraphQL bakes the contract in, while REST needs OpenAPI maintained alongside it.