About Capcat

Capcat (capcat.ai) is a customized distribution based on the open-source project Cap.

Cap is a lightweight, modern open-source CAPTCHA alternative developed by tiago. It uses a two-layer verification mechanism combining proof of work (SHA-256 PoW / RSW time-lock puzzles) with browser instrumentation, and is released under the Apache 2.0 license.

Relationship to upstream

• Built on the upstream engine, kept in sync: challenge generation, solving, and verification follow upstream, and upstream updates are merged continuously;
• Capcat adds its own hardening on top: strengthened automated-browser (headless) detection, adaptive challenge difficulty, and opt-in time-lock (RSW) challenges, plus the hosted service layer — console, key management, quotas, and billing. See Protection Stack for the full picture. These enhancements are maintained in the same repo, with modifications noted in the version history;
• Site and deployment content are kept separate: this site, deployment configuration, and other Capcat-specific content live in their own directories in the repo;
• This document is a simplified version. For full technical details (how it works, effectiveness evaluation, comparisons with other approaches, etc.), see the upstream official documentation.

Let AI read this documentation

The documentation is available in a machine-friendly Markdown format (following the llms.txt spec) — just share the link with your AI assistant to let it read the full content:

• Index: capcat.ai/llms.txt — a table of contents and summaries, so an AI can fetch individual pages as needed;
• Full bundle: capcat.ai/llms-full.txt — all documentation merged into a single file, readable in one fetch;
• Every documentation page also has a plain-text .md version at the same path, e.g. capcat.ai/guide/siteverify.md.

License notes

Capcat complies with the requirements of the Apache 2.0 license:

• The upstream project's LICENSE and copyright notices are retained;
• Modifications to upstream code are noted in the version history;
• The "Cap" name and mark belong to the upstream project. Capcat does not represent the upstream project officially and is not endorsed by it.

Thanks to tiago and the Cap community for their excellent work.