How should you document API authentication for an agent client?

An agent can read your API reference and still have no usable way to call it. A security scheme may be declared but never applied to operations. A sample may omit the required header. Or the only credential instructions may sit behind a login page the developer has not reached yet.

Start with one harmless operation and document the complete route to a successful request. Describe how a user gets authorized, what the credential permits and what the API returns when permission is missing.

Make the public reference useful before sign-in

A prospective integrator should be able to learn the API's purpose without seeing account data. Publish a small example, the base URL and a clear statement of which operations need authentication. Explain whether credentials belong to a user, service account or workspace in your product's terms.

Keep secret values out of public examples. Use unmistakable placeholders, and explain where the application should supply its real credential. If setup requires an administrator, state that before the reader starts implementing a request.

Choose an example that answers a concrete question, such as listing the caller's projects. A complex write introduces approval and recovery questions before the reader has proved the connection works.

Declare the scheme and apply it

In OpenAPI 3.1.1, defining a scheme under components.securitySchemes and referring to it through security are separate steps. A list of security requirement objects expresses alternatives; multiple schemes within one requirement object must all be satisfied. An empty requirement object permits anonymous access. These distinctions matter when a client generates requests from the description. OpenAPI 3.1.1 security requirements.

This deliberately small document illustrates HTTP bearer authentication:

openapi: 3.1.1
info:
  title: Example projects API
  version: 1.0.0
servers:
  - url: https://api.example.com
security:
  - bearerAuth: []
paths:
  /projects:
    get:
      operationId: listProjects
      summary: List projects visible to the caller
      responses:
        '200':
          description: The caller's visible projects
components:
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer

This is an illustration using a pinned specification version, not a complete production contract or a claim that 3.1.1 is the newest release. Expand the response schema and documented errors for your application. The scheme describes how a caller sends a credential; the server still has to check it.

Explain permissions beside the operation

A statement such as "requires an API key" does not explain which records a key can read. For the example above, define what "visible" means in the product. Can a workspace member list every project, only assigned projects or a subset selected by an administrator?

Use a permissions table if roles differ. Make it clear when a caller needs a different role rather than another retry. If you use OAuth scopes, describe the task each scope enables and the account boundaries that still apply. A client should not have to ask for broad write access to demonstrate a read operation.

This explanation is product-specific. Copying a vendor's scope names into your docs would create an inaccurate contract even if the OpenAPI document remains syntactically valid.

Prove the docs match the deployment

Give the public document to a developer who did not write the endpoint. Ask them to authenticate and list their own test projects using only the linked instructions. Record each question they cannot answer from the reference.

Repeat with two test users and deliberately different permissions. Attempt a request for a record the second user cannot access. The expected response should follow your published authorization policy and avoid exposing private records. A valid description cannot prove that enforcement is correct.

Also test an absent credential and an expired or revoked one. Inspect what arrives over HTTP, including gateway responses. If a login redirect sends the client to an HTML page, document the supported API authentication route rather than asking it to parse that page as JSON.

Keep browser access a separate decision

A successful command-line request does not establish whether browser JavaScript on another origin can read the API. That question belongs to CORS. Changing CORS also does not grant account permissions. MDN explains CORS as a browser mechanism governing cross-origin resource sharing. MDN CORS guide.

Assign an owner to the example and rerun its read request when authentication changes. Keep the permission checks with that review. Publish the reference only when its example matches the deployed endpoint and its permissions match the product.

Sources

• OpenAPI Specification 3.1.1, OpenAPI Initiative.
• Cross-Origin Resource Sharing, MDN.