For a REST API developer portal, I’d choose tools in 5 layers: API spec, docs/portal, testing, auth/sandbox, and analytics/community.

1) API specification

Use a single source of truth for your API contract.

• OpenAPI (Swagger) — the standard choice for REST APIs
• Postman Collections — useful for sharing examples and tests
• JSON Schema — if you want stricter request/response modeling

Recommendation: Start with OpenAPI 3.1.

2) Documentation / developer portal

These tools turn your spec into a polished portal.

Popular options

• Redoc / Redocly
  • Great-looking API reference
  • Strong OpenAPI support
  • Good for self-hosted or managed docs
• Swagger UI
  • Simple, interactive, widely used
  • Best for lightweight API reference
• Stoplight
  • Good design-first workflows
  • Nice editor + docs + mocking
• Mintlify
  • Great modern docs experience
  • Easy to make polished docs quickly
• ReadMe
  • Full developer portal: docs, changelog, API explorer, analytics, onboarding
  • Strong choice if you want a SaaS portal fast
• Backstage
  • Better if you want an internal platform portal, not just public docs

Recommendation:

• If you want fast + polished SaaS: ReadMe
• If you want self-hosted + OpenAPI-first: Redocly
• If you want simple and free: Swagger UI + a docs site

3) API testing and mocking

These help developers try the API before integrating.

• Postman — collections, testing, environments, examples
• Stoplight Mock Server — mock endpoints from OpenAPI
• Prism — OpenAPI mocking and validation
• WireMock — flexible mocking for more advanced scenarios
• Hoppscotch — lightweight API client alternative to Postman

Recommendation: Use Postman for public examples and Prism or WireMock for mocks.

4) Authentication, keys, and sandbox

Your portal should make onboarding easy.

• OAuth2 / OpenID Connect — for user authentication
• API keys — for simple developer access
• JWTs — common for service auth
• Sandbox environment — separate test environment with fake or isolated data
• Rate limiting + quotas — protect the API and make limits clear

If your portal includes sign-up and key management, consider:

• Auth0
• Clerk
• Firebase Auth
• AWS Cognito

5) Portal extras that matter

These features improve adoption.

• Changelog / release notes
• SDK generation
  • OpenAPI generators for:
    • TypeScript
    • Python
    • Go
    • Java
• Code samples
• Search
• Usage analytics
• Error troubleshooting pages
• Status page
• Support/contact links

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Good stack combinations

Option A: Fastest to launch

• OpenAPI 3.1
• ReadMe
• Postman
• Auth0
• Prism for mocks

Best if you want a professional portal quickly with minimal engineering overhead.

Option B: Open-source / self-hosted

• OpenAPI 3.1
• Redocly or Redoc
• Swagger UI
• Prism
• Backstage if internal platform needs exist

Best if you want control and lower SaaS dependency.

Option C: Design-first product team workflow

• OpenAPI
• Stoplight
• Mock servers
• Postman
• Redocly for publishing

Best if product, design, and engineering collaborate closely.

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My practical recommendation

If you’re starting from scratch, use:

• OpenAPI 3.1 as the contract
• Redocly or ReadMe for the portal
• Postman for examples and testing
• Prism for mocks
• Auth0/Clerk for developer login and API keys
• OpenAPI generator for SDKs

That gives you a solid, scalable setup without overcomplicating things.

If you want, I can also suggest a stack based on your constraints, like:

• open-source only
• lowest cost
• best for startup MVP
• enterprise-grade portal
• self-hosted on AWS/GCP/Azure