Aspen

Free and with no login required, Aspen is a macOS app for testing REST APIs. You download it, open it and start sending requests, with no account to create first.

As an API testing tool, Aspen also uses AI to help with the integration work that usually follows a successful test call: it can generate data models, OpenAPI Specs and integration code from what you are working with.

Data handling is the other selling point.

Aspen runs all operations locally on your Mac, and nothing is stored externally.

That matters if you are testing against internal or client services and would rather not send request data through someone else's servers.

The scope is narrow, and that is mostly a good thing.

Aspen is built around REST APIs, so if your work leans on other protocols, you should look elsewhere.

It is also macOS only.

For Mac developers who want a no-cost way to test endpoints and get AI-drafted code and specs without handing over an account or their data, Aspen covers the basics cleanly.