Do I need a tool for internal vs external API docs?

Not necessarily. It depends on what you mean by “need.”

If your API is small or early-stage

You may not need a special tool yet. A single docs source can be enough if you can cleanly separate:

• Public/external docs: what customers or third-party developers should see
• Internal docs: implementation details, admin endpoints, debugging notes, support workflows

You can keep them in:

• separate Markdown files
• separate sections in the same docs site
• separate portals with access control

When a tool becomes useful

A dedicated documentation tool helps if you need:

• different audiences with different permissions
• versioning for API releases
• interactive API examples / try-it-out
• sync with OpenAPI/Swagger
• approval workflows
• search, changelogs, and generated references

Common approach

A lot of teams use:

• OpenAPI/Swagger as the source of truth for endpoint specs
• a docs platform for the external portal
• an internal knowledge base/wiki for engineering-only notes

Rule of thumb

• Use no special tool if the docs are simple and you can manage separation manually.
• Use a docs tool if you have multiple consumers, frequent changes, or need access control/versioning.

If you want, I can recommend a setup based on your stack and whether you mean internal vs external audiences, REST vs GraphQL, or public vs private endpoints.