What is the purpose of an API-design document?

What is the purpose of an API-design document?

An API-design document describes the contract between callers and the application operations they use. It gives developers and AI assistants a shared reference for implementing features without guessing how requests, responses, and permissions should behave.

Describe each operation clearly

For every endpoint, document its purpose, HTTP method, path, accepted input, successful response, and meaningful failure responses. Include required authentication and the resource-level permission rule.

For example, creating a wedding event might use POST /api/weddings/{weddingId}/events. Specify required fields, how dates and time zones are represented, and whether the response contains the newly created event. The wedding identifier in the path describes the target; the server still determines whether the caller can use it.

Cover behavior beyond the happy path

Define how validation errors identify problematic fields and how callers distinguish a missing session from insufficient permission. Decide how collection endpoints handle pagination, filtering, and sorting.

Also describe operations that may be retried. If submitting an invitation request twice could send duplicate messages, the contract should explain the intended behavior and any duplicate-prevention mechanism. Do not leave the interface to infer these rules from accidental implementation details.

Use examples to align implementation

Include representative requests and responses with synthetic data. Frontend developers can build against fixtures, backend developers can implement the same contract, and reviewers can compare both sides with the approved behavior.

Ask AI to check for inconsistent names, missing permissions, and requirements without an endpoint. Keep the document beside the code and revise it when behavior changes. A useful API design is concise enough to consult during development and precise enough to expose disagreement before features are joined together.