
ClickUp is a robust, cloud-based project management tool for boosting productivity.
ClickUp API is a free Documents & Productivity API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
Details from clickup.com.

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The ClickUp API is provided by ClickUp (clickup.com).
Yes. The ClickUp API is listed in the Documents & Productivity 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 ClickUp API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the ClickUp API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The ClickUp API is served over HTTPS, so it's safe to use in production.