API Documentation

API documentation is reference material describing how to use a software interface: what operations exist, how to authenticate, what inputs each operation takes, what it returns, and how it fails. Its job is to let a developer who has never seen the API integrate with it correctly without reading the implementation source.

Key components

• Overview / getting started — what the API does, the base URL or package name, and the smallest possible working example, so a reader can make one successful call before learning everything else.
• Authentication — exactly how to obtain and present credentials (API key header, OAuth flow, signed request), including what happens on missing or invalid credentials.
• Endpoints or methods — each operation's name, HTTP verb and path (or method signature), every parameter with its type and whether it's required, and the shape of the return value.
• Request/response examples — real, complete examples for each operation: an actual request (with realistic values) paired with the actual response it produces, not a schema alone.
• Error codes — every error or status code the API can return, what condition triggers it, and what the caller should do about it.
• Versioning and change references — which API version the docs describe, and a pointer to the Changelog or Release Notes for what changed between versions.

Why runnable examples beat prose alone

A paragraph describing what a parameter "should" do leaves the reader to guess at edge cases; a real request and the real response it produces removes the guesswork. Runnable examples also double as a correctness check on the documentation itself — an example that's actually been executed against the API can't silently drift into describing behavior the API no longer has. Prose is still useful for explaining why an endpoint exists or how operations relate to each other, but for how to call something, a working example is the more reliable teacher.

Common pitfalls

• Documenting the API as designed, not as it behaves — written from the spec or the original ticket instead of the running system, so it describes intended behavior that the implementation quietly diverged from.
• Missing error-case documentation — only the happy path is described, so callers discover failure modes, rate limits, and edge cases by hitting them in production instead of reading about them first.
• Examples that don't actually run — a request/response pair typed by hand and never executed against the real API, which drifts wrong the moment a field is renamed or a default changes.
• No indication of required vs. optional parameters — forces every caller to trial-and-error which fields actually matter.
• Undocumented limits — pagination behavior, rate limits, payload size caps, and timeout values left out until a caller hits them and has to reverse-engineer the constraint.
• Docs updated separately from code, and lagging it — a breaking parameter change ships in the API before the docs catch up, so the documented example itself starts failing.

Learn more

• README Writing for orienting a newcomer to the project as a whole, as opposed to documenting one interface's operations in depth.
• Changelog for the cumulative record of what changed across API versions that documentation should stay in sync with.
• Release Notes for the announcement of what changed in one specific API version, including breaking changes callers must react to.
• Architecture Decision Record for recording why an API was designed a particular way, as distinct from documenting how to call it.