Cap Captcha

Cap is a lightweight, modern open-source CAPTCHA alternative designed using SHA-256 PoW.

It's:

• 250x smaller than hCaptcha @cap.js/widget is extremely small, only 12kb minified and brotli'd.

• Private Cap's usage of proof-of-work eliminates the need for any tracking, fingerprinting or data collection.

• Fully customizable Cap's self-hostable so you can customize both the backend & frontend — or you can just use CSS variables

• Proof-of-work Cap uses proof-of-work instead of complex puzzles, making it easier for humans and harder for bots

• Standalone mode Cap offers a standalone mode with Docker, allowing you to use it with languages other than JS.

• Invisible mode Cap can run invisibly in the background using a simple JS API.

• Floating mode Cap's floating mode keeps your CAPTCHA hidden until it's needed.

• Fully open-source Completely open source under the Apache license 2.0 license.

Ideal for protecting APIs from bots, preventing spam on forms, blocking automated login attempts, and securing free-tier abuse. It's a modern alternative for reCAPTCHA, hCaptcha and Cloudflare Turnstile, but unlike them, Cap is computation-bound, not tracking-bound. Cap is built with JavaScript, runs on any JS runtime (Bun, Node.js, Deno), and has no dependencies. If you're not using any JS runtime, you can also use the standalone mode with Docker, which relies entirely on a simple REST API to create and validate challenges.