API Design

API Design

Use this for API contracts that other code, users, or services depend on.

Project Fit Check

Before changing an API:

1. Read existing routes, schemas, OpenAPI/GraphQL docs, clients, tests, and auth rules.
2. Identify consumers: UI, external users, workers, webhooks, integrations, or internal packages.
3. Preserve existing naming, error shape, pagination style, and versioning policy unless the task is to change them.
4. Treat compatibility as a product constraint.
5. If documentation is missing, add the smallest useful contract near the existing source of truth.

Contract Rules

• Read every module the contract touches before drafting it, not after the first draft. A draft written from an assumed call graph is wrong about the guards, limits, and auth it has to preserve, and the correction costs more than the reading would have.
• Validate input at the boundary.
• Make response shape stable and documented.
• Use machine-readable error codes plus human-readable messages.
• Keep outcomes distinct when their causes differ. A request the system chose not to attempt is not a request that failed. Recording them as one outcome invents a failure, and downstream logic that suppresses retries on failure then suppresses the retry that would have succeeded.
• Keep auth and ownership checks at the strongest available boundary.
• Make pagination, filtering, sorting, and limits explicit.
• Keep idempotency clear for create/update/retry paths.
• Avoid leaking provider errors or internal stack details.

REST Rules

• Use resources and actions consistently.
• Choose status codes deliberately.
• Keep destructive operations explicit.
• Support partial failure only when the client can act on it.
• Use versioning when breaking changes cannot be coordinated.

GraphQL Rules

• Design schema around consumer needs, not database tables.
• Keep nullability meaningful.
• Avoid resolver waterfalls; batch where needed.
• Make authorization explicit per object or field where sensitivity differs.
• Deprecate before removing fields.

Verification

• schema/typecheck for contracts
• unit tests for validation and errors
• integration tests for auth, pagination, and persistence
• client or e2e tests for user-visible flows

Red Flags

• silent response shape change
• unbounded list endpoint
• inconsistent error format
• auth only in the frontend
• endpoint mirrors database internals
• breaking change without migration or version plan