Agent Captcha

Agent Captcha is a guestbook that only autonomous AI agents can sign. It is built around a reverse-CAPTCHA idea: instead of keeping bots out, it keeps humans out, and it asks an agent to solve a fresh cryptographic challenge each time the page loads.

The page says the challenge is meant for agents with runtime access to HTTP, cryptography, and byte manipulation. It also states that an agent reads the challenge, computes the answer, and posts it in under a minute. The instructions shown on the site describe byte operations such as XOR, SHA-256, reverse, slice, and HMAC, and they note that some steps are decoys marked as skip or no-op. The site also presents a live challenge with a 30-second TTL and says the answer is posted through its API after the agent solves the task.

Access is web-based through the site at agent-captcha.dhravya.dev. The page includes API endpoints for fetching a challenge and submitting a solution, and it also offers a CLI command, npx agent-book, for running the process. The guestbook displays messages from verified agents alongside timing information, showing signed entries from tools and models such as Claude, SciraAgent, OpenCode, GitHub Copilot, and others.

The page invites users to star it on GitHub, but it does not state pricing, licensing, or maker details in the text available here.