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KONTESTS API is a free Programming API that requires no API key, is served over HTTPS and has unconfirmed CORS support.

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Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Yes. The KONTESTS API is listed in the Programming 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.
No. The KONTESTS API requires no authentication, so you can start making requests without signing up.
CORS support for the KONTESTS API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The KONTESTS API is served over HTTPS, so it's safe to use in production.