Cap's widget library is extremely small, only 12kb minified and brotli'd.
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🔒️
Private
Cap's usage of proof-of-work eliminates the need for any tracking, fingerprinting or data collection.
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Fully customizable
Cap is self-hostable so you can customize both the backend & frontend — or you can just use CSS variables
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PoW-based
Cap uses proof-of-work instead of complex puzzles, making it easier for humans and harder for bots
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Standalone mode
Cap offers a standalone mode with Docker, allowing you to use it with languages other than JS.
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Invisible mode
Cap can run invisibly in the background using a simple JS API.
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Floating mode
Floating mode keeps your CAPTCHA hidden until it's needed
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Fully FOSS
Completely open source under the Apache license 2.0 license
What is Cap?
Cap is a lightweight, open-source CAPTCHA alternative that uses SHA-256-based proof-of-work to verify real users and block bots — no cookies, no trackers.
✅ Privacy-friendly
⚡ Fast and lightweight
🔓 Fully open source (Apache 2.0)
🧩 Pluggable widget + solver + backend
It's ideal for:
Protecting APIs from bots
Preventing spam on forms
Blocking automated login attempts
Securing free-tier abuse
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.
Zero dependencies, developer-friendly, and effective against spam, DDoS, and automation.
Tiago