What is Cap?
Cap is a modern, open‑source CAPTCHA that uses SHA‑256 proof‑of‑work. It consists of a tiny widget library (@cap.js/widget) for rendering and solving challenges in the browser and a zero‑dependency server library (@cap.js/server) for generating and validating those challenges.
Key Features
≈12 KB minified widget (~250× smaller than hCaptcha)
Privacy‑first: no telemetry or tracking
Fully customizable UI via CSS variables
Proof‑of‑work (SHA‑256) replaces visual puzzles
Standalone Docker container with REST API
Invisible/background solving mode
M2M server‑side solver for friendly robots
Zero external dependencies, works on any JS runtime
Open‑source under Apache 2.0
Who Will Use Cap?
Web developersSite owners needing spam protectionAPI providers wanting bot‑free endpointsSecurity engineers concerned with privacyDevelopers looking for a self‑hosted CAPTCHA solution
How to Use Cap
- 1Add the @cap.js/widget script to your frontend and configure it via CSS variables or the JS API.
- 2Use @cap.js/server on the backend (Node, Bun, Deno, etc.) to create and validate proof‑of‑work challenges.
- 3Run the provided Docker image in standalone mode for language‑agnostic integration via a simple REST API.
- 4Enable invisible mode by calling the widget’s solve API without displaying the UI.
- 5Optionally use the M2M server‑side solver for protecting public API endpoints.
Pros & Cons
Pros
- Extremely small footprint and fast loading
- No user tracking or data collection
- Highly customizable and self‑hostable
- Open‑source with permissive license
- Works on any JavaScript environment
- Offers invisible and floating modes
Cons
- Proof‑of‑work relies on client CPU, which may affect performance on low‑end devices
- Less widely recognized than reCAPTCHA or hCaptcha
- Requires JavaScript/WASM support in the browser
- Powerful hardware could solve challenges quickly, potentially reducing bot resistance
Pricing
Pricing:Free
Free plan available
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