Cap

Cap replaces the visual captcha with a proof-of-work challenge the browser solves by itself. There are no images to squint at, no grid of traffic lights, and no user interaction at all if you choose to hide the widget and let challenges resolve in the background.

The widget weighs roughly twenty kilobytes with zero dependencies, which the project puts at 250 times smaller than hCaptcha. It is offered as a replacement for reCAPTCHA, hCaptcha and Cloudflare Turnstile, and the default way to run it is a standalone Docker container.

The design follows from treating a captcha as something that should cost the visitor nothing.

• Proof-of-work: challenges solved by the machine rather than puzzles solved by the person.
• Instrumentation challenges: a second signal used alongside the computational work.
• Privacy-first: no telemetry is sent back to the project's servers.
• Customizable: colors, size, position and icons all controlled through CSS variables.
• Standalone mode: the Docker container runs anywhere and brings analytics with it.

Deployment is a container you host, with a one click Railway template for anyone who would rather not. Cap is free and open source under the Apache 2.0 license, and the documentation includes a live demo worth trying before wiring it into a signup form.