What should I use for internal API documentation?

For internal API documentation, a good default choice is usually one of these:

Best general-purpose options

• OpenAPI/Swagger
  Best if you want a standard, machine-readable API spec, interactive docs, and easy integration with tooling. Great for REST APIs.

• Postman
  Good for teams that want to document, test, and share APIs in one place. Very practical for internal use, especially early on.

• Redoc
  Great if you already have an OpenAPI spec and want clean, readable documentation for developers.

If you want lightweight internal docs

• Markdown in a repo + examples
  Best for small teams or internal services where you want docs close to the code and easy to update.
• Confluence / Notion
  Good for process-heavy teams, but less ideal if you need API docs that stay tightly synced with implementation.

My recommendation

• If your API is REST and you want a scalable long-term setup: OpenAPI + Swagger UI or Redoc
• If you want quick collaboration and testing: Postman
• If docs are small and mostly for engineers: Markdown in Git

If you want, I can recommend the best tool based on your stack, team size, and whether your API is REST, GraphQL, or gRPC.