Aspen.io

Aspen is a free native macOS app for testing REST APIs, with a built in AI assistant that speeds up integration work. Developers enter the parameters of a request, run it locally and see the response in under a second, then ask the assistant, called Alfred, to generate data models, OpenAPI specifications and integration code from the results. It is aimed at developers who design or integrate APIs and want a lightweight alternative to Postman or Insomnia. Aspen is built around a zero trust approach: no login is required, it does not save request data, and operations run on the user's own machine. It focuses specifically on REST APIs rather than trying to cover every protocol. The Collections feature lets users organise, import, export and share API requests, including importing collections from Postman, which makes it usable for small team collaboration. Aspen is completely free, with no paid plan and no account required. Aspen is developed by Treblle, an API lifecycle platform whose own products cover API observability, analytics, governance, generated documentation and an API assistant, with a free tier available. Aspen works as a standalone app, but teams that want monitoring of production APIs would need to look at Treblle's separate platform.