Self-serve customer engagement solution for Push Notifications, Email, SMS & In-App.
OneSignal API is a free Development API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
The world's leader for mobile push notifications, web push, SMS, email and in-app messaging. Trusted by 1.8 million+ businesses to send 10 billion+ messages per day.
Details from developers.onesignal.com.

Unified API suite · now live
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The OneSignal API is provided by OneSignal (developers.onesignal.com). The world's leader for mobile push notifications, web push, SMS, email and in-app messaging. Trusted by 1.8 million+ businesses to send 10 billion+ messages per day.
Yes. The OneSignal API is listed in the Development 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 OneSignal API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the OneSignal API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The OneSignal API is served over HTTPS, so it's safe to use in production.