Automate the software development process using continuous integration and continuous delivery.
CircleCI API is a free Continuous Integration API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Introduction to the CircleCI API versions and their capabilities including a how to choose the right API version for your use case.
Details from circleci.com.

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The CircleCI API is provided by CircleCI Docs (circleci.com). Introduction to the CircleCI API versions and their capabilities including a how to choose the right API version for your use case.
Yes. The CircleCI API is listed in the Continuous Integration category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The CircleCI API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the CircleCI API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The CircleCI API is served over HTTPS, so it's safe to use in production.