What does API documentation need so a developer can integrate without asking you?

Good documentation answers the questions in the order a developer asks them: can I try it, how do I authenticate, and what does each call do?

Essentials:

• Quickstart. Make the first successful call in minutes, with a copy-paste curl command and a sandbox key.
• Authentication. How to get a key or token, the header to send, scopes and how to rotate credentials.
• Reference. Every endpoint with parameters, request and response schemas, and real example payloads. Generate it from the OpenAPI spec so it cannot drift.
• Concepts and guides. Pagination, filtering, idempotency, rate limits, webhooks and versioning, each explained once.
• Error catalog. Each error code, what causes it and how to fix it.
• Examples in several languages, ideally from the generated SDKs.
• Changelog and deprecation notices, with dates.

Also valuable: a try-it console, a status page and a contact path for support.

Run the examples in CI, so documentation that no longer works fails the build. Docs that are wrong are worse than no docs.