ALTCHA is a CAPTCHA alternative that blocks bots and spam without puzzles. Instead of asking visitors to solve an image, the browser silently completes a proof-of-work challenge that the server then verifies. The core widget is open source under the MIT licence, free to use without sign-up or usage limits, and can be added to any website and self-hosted. For organisations that need more, ALTCHA also offers a managed cloud service, a self-hosted platform called Sentinel and a WordPress plugin. In the cloud and Sentinel variants, risk signals decide whether a visitor must also pass an accessible interactive check.
ALTCHA is operated by BAU Software s.r.o. in Czechia, and the ALTCHA trademark is registered in the EU. Because the code is open and auditable, organisations can run the whole stack on their own infrastructure and keep all data under their own control. According to the vendor, the product sets no cookies, does not fingerprint visitors, collects no personal data and shares nothing with third parties, and the endpoints of the managed cloud service are EU-only.