docs: document OpenAPI spec auth-gating and path-based multi-API URLs

[devin-ai-integration]

Summary

Updates the Download OpenAPI spec page to cover three behaviors that were missing or inaccurate. All confirmed against fern-platform source (packages/fern-docs/bundle/src/app/[domain]/(main)/api/fern-docs/openapi/route.ts).

• Path-based multi-API selection: documents /openapi/<apiId>.json (also .yaml/.yml) as the recommended, CDN-friendly way to select a specific API alongside the existing ?api=<id> query param. Notes that the <apiId> segment resolves by slug, title, or UUID, and briefly mentions the ?title=<api title> param.
• Auth-gating: the spec endpoints inherit the site's password/SSO auth and return 401 to unauthenticated requests; there is no separate public toggle.
• Hidden endpoints: corrected the "includes all endpoints" wording — endpoints and API references marked hidden: true are excluded from the served spec.

No changelog entry: this is a documentation-accuracy fix for existing behavior, not a new feature.

Source: Slack thread https://buildwithfern.slack.com/archives/C052DFWVCG6/p1784157608928309

Checks

• pre-commit (vale) on the changed file: passed.
• fern check --warnings: 0 errors (warnings are preexisting and unrelated).

Link to Devin session: https://app.devin.ai/sessions/e593e9c759f544a2a50462a6b464d59b

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1784560230-openapi-spec-auth-paths.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/developer-tools/openapi-spec