Gender probability for a first name, optionally per country; documentation in German.
Vornamencheck API is a free Development API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
API für Vornamen: Zu jedem Vornamen die Wahrscheinlichkeit, dass er männlich oder weiblich ist, bezogen auf ein Land. Einfacher HTTP-GET-Aufruf mit JSON-Antwort.
Details from vornamencheck.de.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Turn a URL into Markdown, screenshots, metadata or Lighthouse audits, rendered in real Chrome
Extract SEO metadata, contact emails, social links, and tech stack (<200ms)
The Vornamencheck API is provided by Vornamencheck (vornamencheck.de). API für Vornamen: Zu jedem Vornamen die Wahrscheinlichkeit, dass er männlich oder weiblich ist, bezogen auf ein Land. Einfacher HTTP-GET-Aufruf mit JSON-Antwort.
Yes. The Vornamencheck 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.
No. The Vornamencheck API requires no authentication, so you can start making requests without signing up.
Yes. The Vornamencheck API supports CORS, so front-end JavaScript can call it directly.
Yes. The Vornamencheck API is served over HTTPS, so it's safe to use in production.