# Cap – Open-source, self-hosted CAPTCHA alternative to reCAPTCHA > Cap is a lightweight, modern open-source CAPTCHA alternative using proof-of-work, time-lock and instrumentation challenges ## Table of Contents ### Compare Cap - [Quickstart](/guide.md): Set up Cap, the open-source self-hosted CAPTCHA, in about five minutes. Run the server with Docker, drop in the widget, verify tokens. No Google, no telemetry, no visual puzzles. - [CAPTCHA Comparison: Cap vs reCAPTCHA, Turnstile & More](/guide/alternatives.md): Compare Cap against reCAPTCHA, hCaptcha, Turnstile, Altcha and more. See how the open-source, self-hosted, proof-of-work CAPTCHA stacks up on privacy and cost. - [Migrating from reCAPTCHA to Cap](/guide/alternatives/migrate-from-recaptcha.md): Google is moving reCAPTCHA into Google Cloud and auto-migrating Classic keys. Migrate to Cap instead: a faster, private, self-hosted, open-source CAPTCHA with a reCAPTCHA-compatible siteverify API and Turnstile-tier bot detection. ### vs - [Cap vs reCAPTCHA](/guide/alternatives/recaptcha.md): Cap vs Google reCAPTCHA v2 and v3: a privacy-first, self-hosted, open-source alternative. No Google tracking or visual puzzles, just proof-of-work challenges. - [Cap vs Cloudflare Turnstile](/guide/alternatives/turnstile.md): Cap vs Cloudflare Turnstile: a self-hosted, customizable, open-source CAPTCHA you fully control. No vendor lock-in, with proof-of-work and instrumentation. - [Cap vs hCaptcha](/guide/alternatives/hcaptcha.md): Cap vs hCaptcha: a self-hosted, puzzle-free, open-source CAPTCHA alternative. No image grids, no data sold to advertisers, free at scale via proof-of-work. - [Cap vs Altcha](/guide/alternatives/altcha.md): Cap vs Altcha: two open-source, self-hosted proof-of-work CAPTCHAs compared. See how Cap adds instrumentation challenges, a widget, and a managed dashboard. - [Cap vs FriendlyCaptcha](/guide/alternatives/friendlycaptcha.md): Cap vs Friendly Captcha: a free, self-hosted, open-source proof-of-work CAPTCHA versus a paid SaaS. Compare privacy, pricing, and where your visitor data lives. - [Cap vs SilentShield](/guide/alternatives/silentshield.md): Cap vs SilentShield: open-source, self-hosted proof-of-work CAPTCHA versus a hosted behavioral bot-detection service. Compare privacy, pricing and control. - [Cap vs Anubis](/guide/alternatives/anubis.md): Cap vs Anubis: Anubis deters scrapers site-wide, Cap is a self-hosted, open-source CAPTCHA for forms and logins using proof-of-work. When to pick each one. ### Guides - [Best CAPTCHA Alternatives for 2026](/guide/best-captcha-alternatives.md): The best CAPTCHA alternatives for 2026, ranked: Cap, Turnstile, ALTCHA, FriendlyCaptcha, hCaptcha, reCAPTCHA. Compare privacy, UX and cost to pick yours. - [CAPTCHA Conversion Rate: The Cost of Puzzles](/guide/captcha-conversion-rate.md): How CAPTCHA affects conversion rates on signup, login and checkout forms, and how low-friction alternatives like Cap protect forms without losing users. - [Best Open-Source CAPTCHA Options in 2026](/guide/open-source-captcha.md): The best open-source, self-hosted CAPTCHA options for developers compared: Cap, ALTCHA, mCAPTCHA, and Anubis, with licensing, architecture, and how to choose. - [CAPTCHA for Mobile Forms Without the Puzzles](/guide/mobile-form-bot-protection.md): How to stop bots on mobile forms without image puzzles: what a mobile-friendly CAPTCHA needs and how Cap's one-tap proof-of-work fits. Try the live demo. ### Compare Cap - [Quickstart](/guide.md): Set up Cap, the open-source self-hosted CAPTCHA, in about five minutes. Run the server with Docker, drop in the widget, verify tokens. No Google, no telemetry, no visual puzzles. - [CAPTCHA Comparison: Cap vs reCAPTCHA, Turnstile & More](/guide/alternatives.md): Compare Cap against reCAPTCHA, hCaptcha, Turnstile, Altcha and more. See how the open-source, self-hosted, proof-of-work CAPTCHA stacks up on privacy and cost. - [Migrating from reCAPTCHA to Cap](/guide/alternatives/migrate-from-recaptcha.md): Google is moving reCAPTCHA into Google Cloud and auto-migrating Classic keys. Migrate to Cap instead: a faster, private, self-hosted, open-source CAPTCHA with a reCAPTCHA-compatible siteverify API and Turnstile-tier bot detection. ### vs - [Cap vs reCAPTCHA](/guide/alternatives/recaptcha.md): Cap vs Google reCAPTCHA v2 and v3: a privacy-first, self-hosted, open-source alternative. No Google tracking or visual puzzles, just proof-of-work challenges. - [Cap vs Cloudflare Turnstile](/guide/alternatives/turnstile.md): Cap vs Cloudflare Turnstile: a self-hosted, customizable, open-source CAPTCHA you fully control. No vendor lock-in, with proof-of-work and instrumentation. - [Cap vs hCaptcha](/guide/alternatives/hcaptcha.md): Cap vs hCaptcha: a self-hosted, puzzle-free, open-source CAPTCHA alternative. No image grids, no data sold to advertisers, free at scale via proof-of-work. - [Cap vs Altcha](/guide/alternatives/altcha.md): Cap vs Altcha: two open-source, self-hosted proof-of-work CAPTCHAs compared. See how Cap adds instrumentation challenges, a widget, and a managed dashboard. - [Cap vs FriendlyCaptcha](/guide/alternatives/friendlycaptcha.md): Cap vs Friendly Captcha: a free, self-hosted, open-source proof-of-work CAPTCHA versus a paid SaaS. Compare privacy, pricing, and where your visitor data lives. - [Cap vs SilentShield](/guide/alternatives/silentshield.md): Cap vs SilentShield: open-source, self-hosted proof-of-work CAPTCHA versus a hosted behavioral bot-detection service. Compare privacy, pricing and control. - [Cap vs Anubis](/guide/alternatives/anubis.md): Cap vs Anubis: Anubis deters scrapers site-wide, Cap is a self-hosted, open-source CAPTCHA for forms and logins using proof-of-work. When to pick each one. ### Guides - [Best CAPTCHA Alternatives for 2026](/guide/best-captcha-alternatives.md): The best CAPTCHA alternatives for 2026, ranked: Cap, Turnstile, ALTCHA, FriendlyCaptcha, hCaptcha, reCAPTCHA. Compare privacy, UX and cost to pick yours. - [CAPTCHA Conversion Rate: The Cost of Puzzles](/guide/captcha-conversion-rate.md): How CAPTCHA affects conversion rates on signup, login and checkout forms, and how low-friction alternatives like Cap protect forms without losing users. - [Best Open-Source CAPTCHA Options in 2026](/guide/open-source-captcha.md): The best open-source, self-hosted CAPTCHA options for developers compared: Cap, ALTCHA, mCAPTCHA, and Anubis, with licensing, architecture, and how to choose. - [CAPTCHA for Mobile Forms Without the Puzzles](/guide/mobile-form-bot-protection.md): How to stop bots on mobile forms without image puzzles: what a mobile-friendly CAPTCHA needs and how Cap's one-tap proof-of-work fits. Try the live demo. ### Compare Cap - [Quickstart](/guide.md): Set up Cap, the open-source self-hosted CAPTCHA, in about five minutes. Run the server with Docker, drop in the widget, verify tokens. No Google, no telemetry, no visual puzzles. - [CAPTCHA Comparison: Cap vs reCAPTCHA, Turnstile & More](/guide/alternatives.md): Compare Cap against reCAPTCHA, hCaptcha, Turnstile, Altcha and more. See how the open-source, self-hosted, proof-of-work CAPTCHA stacks up on privacy and cost. - [Migrating from reCAPTCHA to Cap](/guide/alternatives/migrate-from-recaptcha.md): Google is moving reCAPTCHA into Google Cloud and auto-migrating Classic keys. Migrate to Cap instead: a faster, private, self-hosted, open-source CAPTCHA with a reCAPTCHA-compatible siteverify API and Turnstile-tier bot detection. ### vs - [Cap vs reCAPTCHA](/guide/alternatives/recaptcha.md): Cap vs Google reCAPTCHA v2 and v3: a privacy-first, self-hosted, open-source alternative. No Google tracking or visual puzzles, just proof-of-work challenges. - [Cap vs Cloudflare Turnstile](/guide/alternatives/turnstile.md): Cap vs Cloudflare Turnstile: a self-hosted, customizable, open-source CAPTCHA you fully control. No vendor lock-in, with proof-of-work and instrumentation. - [Cap vs hCaptcha](/guide/alternatives/hcaptcha.md): Cap vs hCaptcha: a self-hosted, puzzle-free, open-source CAPTCHA alternative. No image grids, no data sold to advertisers, free at scale via proof-of-work. - [Cap vs Altcha](/guide/alternatives/altcha.md): Cap vs Altcha: two open-source, self-hosted proof-of-work CAPTCHAs compared. See how Cap adds instrumentation challenges, a widget, and a managed dashboard. - [Cap vs FriendlyCaptcha](/guide/alternatives/friendlycaptcha.md): Cap vs Friendly Captcha: a free, self-hosted, open-source proof-of-work CAPTCHA versus a paid SaaS. Compare privacy, pricing, and where your visitor data lives. - [Cap vs SilentShield](/guide/alternatives/silentshield.md): Cap vs SilentShield: open-source, self-hosted proof-of-work CAPTCHA versus a hosted behavioral bot-detection service. Compare privacy, pricing and control. - [Cap vs Anubis](/guide/alternatives/anubis.md): Cap vs Anubis: Anubis deters scrapers site-wide, Cap is a self-hosted, open-source CAPTCHA for forms and logins using proof-of-work. When to pick each one. ### Guides - [Best CAPTCHA Alternatives for 2026](/guide/best-captcha-alternatives.md): The best CAPTCHA alternatives for 2026, ranked: Cap, Turnstile, ALTCHA, FriendlyCaptcha, hCaptcha, reCAPTCHA. Compare privacy, UX and cost to pick yours. - [CAPTCHA Conversion Rate: The Cost of Puzzles](/guide/captcha-conversion-rate.md): How CAPTCHA affects conversion rates on signup, login and checkout forms, and how low-friction alternatives like Cap protect forms without losing users. - [Best Open-Source CAPTCHA Options in 2026](/guide/open-source-captcha.md): The best open-source, self-hosted CAPTCHA options for developers compared: Cap, ALTCHA, mCAPTCHA, and Anubis, with licensing, architecture, and how to choose. - [CAPTCHA for Mobile Forms Without the Puzzles](/guide/mobile-form-bot-protection.md): How to stop bots on mobile forms without image puzzles: what a mobile-friendly CAPTCHA needs and how Cap's one-tap proof-of-work fits. Try the live demo. ### Compare Cap - [Quickstart](/guide.md): Set up Cap, the open-source self-hosted CAPTCHA, in about five minutes. Run the server with Docker, drop in the widget, verify tokens. No Google, no telemetry, no visual puzzles. - [CAPTCHA Comparison: Cap vs reCAPTCHA, Turnstile & More](/guide/alternatives.md): Compare Cap against reCAPTCHA, hCaptcha, Turnstile, Altcha and more. See how the open-source, self-hosted, proof-of-work CAPTCHA stacks up on privacy and cost. - [Migrating from reCAPTCHA to Cap](/guide/alternatives/migrate-from-recaptcha.md): Google is moving reCAPTCHA into Google Cloud and auto-migrating Classic keys. Migrate to Cap instead: a faster, private, self-hosted, open-source CAPTCHA with a reCAPTCHA-compatible siteverify API and Turnstile-tier bot detection. ### vs - [Cap vs reCAPTCHA](/guide/alternatives/recaptcha.md): Cap vs Google reCAPTCHA v2 and v3: a privacy-first, self-hosted, open-source alternative. No Google tracking or visual puzzles, just proof-of-work challenges. - [Cap vs Cloudflare Turnstile](/guide/alternatives/turnstile.md): Cap vs Cloudflare Turnstile: a self-hosted, customizable, open-source CAPTCHA you fully control. No vendor lock-in, with proof-of-work and instrumentation. - [Cap vs hCaptcha](/guide/alternatives/hcaptcha.md): Cap vs hCaptcha: a self-hosted, puzzle-free, open-source CAPTCHA alternative. No image grids, no data sold to advertisers, free at scale via proof-of-work. - [Cap vs Altcha](/guide/alternatives/altcha.md): Cap vs Altcha: two open-source, self-hosted proof-of-work CAPTCHAs compared. See how Cap adds instrumentation challenges, a widget, and a managed dashboard. - [Cap vs FriendlyCaptcha](/guide/alternatives/friendlycaptcha.md): Cap vs Friendly Captcha: a free, self-hosted, open-source proof-of-work CAPTCHA versus a paid SaaS. Compare privacy, pricing, and where your visitor data lives. - [Cap vs SilentShield](/guide/alternatives/silentshield.md): Cap vs SilentShield: open-source, self-hosted proof-of-work CAPTCHA versus a hosted behavioral bot-detection service. Compare privacy, pricing and control. - [Cap vs Anubis](/guide/alternatives/anubis.md): Cap vs Anubis: Anubis deters scrapers site-wide, Cap is a self-hosted, open-source CAPTCHA for forms and logins using proof-of-work. When to pick each one. ### Guides - [Best CAPTCHA Alternatives for 2026](/guide/best-captcha-alternatives.md): The best CAPTCHA alternatives for 2026, ranked: Cap, Turnstile, ALTCHA, FriendlyCaptcha, hCaptcha, reCAPTCHA. Compare privacy, UX and cost to pick yours. - [CAPTCHA Conversion Rate: The Cost of Puzzles](/guide/captcha-conversion-rate.md): How CAPTCHA affects conversion rates on signup, login and checkout forms, and how low-friction alternatives like Cap protect forms without losing users. - [Best Open-Source CAPTCHA Options in 2026](/guide/open-source-captcha.md): The best open-source, self-hosted CAPTCHA options for developers compared: Cap, ALTCHA, mCAPTCHA, and Anubis, with licensing, architecture, and how to choose. - [CAPTCHA for Mobile Forms Without the Puzzles](/guide/mobile-form-bot-protection.md): How to stop bots on mobile forms without image puzzles: what a mobile-friendly CAPTCHA needs and how Cap's one-tap proof-of-work fits. Try the live demo. ### Compare Cap - [Quickstart](/guide.md): Set up Cap, the open-source self-hosted CAPTCHA, in about five minutes. Run the server with Docker, drop in the widget, verify tokens. No Google, no telemetry, no visual puzzles. - [CAPTCHA Comparison: Cap vs reCAPTCHA, Turnstile & More](/guide/alternatives.md): Compare Cap against reCAPTCHA, hCaptcha, Turnstile, Altcha and more. See how the open-source, self-hosted, proof-of-work CAPTCHA stacks up on privacy and cost. - [Migrating from reCAPTCHA to Cap](/guide/alternatives/migrate-from-recaptcha.md): Google is moving reCAPTCHA into Google Cloud and auto-migrating Classic keys. Migrate to Cap instead: a faster, private, self-hosted, open-source CAPTCHA with a reCAPTCHA-compatible siteverify API and Turnstile-tier bot detection. ### vs - [Cap vs reCAPTCHA](/guide/alternatives/recaptcha.md): Cap vs Google reCAPTCHA v2 and v3: a privacy-first, self-hosted, open-source alternative. No Google tracking or visual puzzles, just proof-of-work challenges. - [Cap vs Cloudflare Turnstile](/guide/alternatives/turnstile.md): Cap vs Cloudflare Turnstile: a self-hosted, customizable, open-source CAPTCHA you fully control. No vendor lock-in, with proof-of-work and instrumentation. - [Cap vs hCaptcha](/guide/alternatives/hcaptcha.md): Cap vs hCaptcha: a self-hosted, puzzle-free, open-source CAPTCHA alternative. No image grids, no data sold to advertisers, free at scale via proof-of-work. - [Cap vs Altcha](/guide/alternatives/altcha.md): Cap vs Altcha: two open-source, self-hosted proof-of-work CAPTCHAs compared. See how Cap adds instrumentation challenges, a widget, and a managed dashboard. - [Cap vs FriendlyCaptcha](/guide/alternatives/friendlycaptcha.md): Cap vs Friendly Captcha: a free, self-hosted, open-source proof-of-work CAPTCHA versus a paid SaaS. Compare privacy, pricing, and where your visitor data lives. - [Cap vs SilentShield](/guide/alternatives/silentshield.md): Cap vs SilentShield: open-source, self-hosted proof-of-work CAPTCHA versus a hosted behavioral bot-detection service. Compare privacy, pricing and control. - [Cap vs Anubis](/guide/alternatives/anubis.md): Cap vs Anubis: Anubis deters scrapers site-wide, Cap is a self-hosted, open-source CAPTCHA for forms and logins using proof-of-work. When to pick each one. ### Guides - [Best CAPTCHA Alternatives for 2026](/guide/best-captcha-alternatives.md): The best CAPTCHA alternatives for 2026, ranked: Cap, Turnstile, ALTCHA, FriendlyCaptcha, hCaptcha, reCAPTCHA. Compare privacy, UX and cost to pick yours. - [CAPTCHA Conversion Rate: The Cost of Puzzles](/guide/captcha-conversion-rate.md): How CAPTCHA affects conversion rates on signup, login and checkout forms, and how low-friction alternatives like Cap protect forms without losing users. - [Best Open-Source CAPTCHA Options in 2026](/guide/open-source-captcha.md): The best open-source, self-hosted CAPTCHA options for developers compared: Cap, ALTCHA, mCAPTCHA, and Anubis, with licensing, architecture, and how to choose. - [CAPTCHA for Mobile Forms Without the Puzzles](/guide/mobile-form-bot-protection.md): How to stop bots on mobile forms without image puzzles: what a mobile-friendly CAPTCHA needs and how Cap's one-tap proof-of-work fits. Try the live demo. ### Standalone - [Cap Standalone](/guide/standalone.md): Cap Standalone is the easiest way to self-host the open-source CAPTCHA backend: Docker, instrumentation, a reCAPTCHA-compatible API, and a web dashboard. - [API](/guide/standalone/api.md): REST API reference for Cap Standalone: create and manage site keys and sessions for the self-hosted, open-source CAPTCHA using bearer-authenticated requests. - [Options](/guide/standalone/options.md): Configuration options and environment variables for Cap Standalone, the self-hosted open-source CAPTCHA: CORS, asset server, widget and WASM versions, and more. ### Widget - [Widget](/guide/widget.md): Cap's client widget requests, solves and displays proof-of-work challenges with a native web component and WASM. Client-side docs for the open-source CAPTCHA. - [Programmatic mode](/guide/programmatic.md): Programmatic mode for Cap: call new Cap() and solve() to run the open-source CAPTCHA from your own JavaScript, with proof-of-work and no visible widget. - [Floating mode](/guide/floating.md): Floating mode for Cap's widget hides the open-source CAPTCHA until a button is pressed, then runs proof-of-work on demand. Set it up with one data attribute. ### Libraries - [Core](/guide/capjs-core.md): capjs-core is Cap's stateless server library for generating and verifying JWT proof-of-work challenges. Self-host the open-source CAPTCHA on Workers or edge. - [Community libraries](/guide/community.md): Community-maintained libraries for using Cap, the self-hosted open-source CAPTCHA, in more languages and frameworks. Verify proof-of-work tokens outside SDKs. ### Details - [Effectiveness](/guide/effectiveness.md): How effective Cap is against bots: the open-source CAPTCHA pairs proof-of-work with instrumentation to make abuse expensive while staying invisible to users. - [Instrumentation challenges](/guide/instrumentation.md): Cap's instrumentation challenges run server-generated JS to verify a real browser, working alongside proof-of-work in the self-hosted, open-source CAPTCHA. - [HashWX proof of work](/guide/hashwx.md): HashWX is Cap's GPU-resistant proof of work. Each challenge generates a fresh hash function, so a GPU gains about 2x over a CPU instead of 150x. - [How does Cap work?](/guide/workings.md): How Cap works under the hood: the self-hosted CAPTCHA generates seeded proof-of-work challenges, solves them in WASM, then redeems a signed token on the server. ### Other - [@cap.js/server](/guide/server.md) - [About Cap](/about.md): Who builds Cap, the open-source, self-hosted CAPTCHA alternative: its maintainer, history, license, funding, and how to get in touch. - [About checkpoints](/guide/middleware.md): Cap Checkpoints add a browser-check interstitial, like Cloudflare's, to block bots before they reach your site. Self-hosted, open-source proof-of-work CAPTCHA. - [Benchmark](/guide/benchmark.md): Cap CAPTCHA benchmark: proof-of-work solve times across low-end to high-end devices. See how fast the self-hosted, open-source widget runs in real browsers. - [Compliance](/guide/compliance.md): How Cap meets GDPR, CCPA and WCAG 2.2. The self-hosted, open-source CAPTCHA keeps user data on your servers, with no cookies, tracking, or third-party calls. - [Demo](/guide/demo.md): Try Cap live: a working demo of the open-source, self-hosted CAPTCHA widget solving proof-of-work challenges in your browser, with no visual puzzles to click. - [Elysia checkpoint](/guide/middleware/elysia.md): Add a Cap Checkpoint to an Elysia app with the official middleware. Gate routes behind the self-hosted, open-source proof-of-work CAPTCHA in just a few lines. - [Express checkpoint](/guide/middleware/express.md): Add a Cap Checkpoint to Express with @cap.js/checkpoint-express. Protect routes with the self-hosted, open-source proof-of-work CAPTCHA and a browser check. - [Hono checkpoint](/guide/middleware/hono.md): Add a Cap Checkpoint to a Hono app with @cap.js/checkpoint-hono. Protect routes with the self-hosted, open-source proof-of-work CAPTCHA and browser check. - [Installation](/guide/standalone/installation.md) - [Installation](/guide/standalone/usage.md) - [M2M](/guide/solver.md): @cap.js/solver solves Cap proof-of-work challenges server-side on Bun for machine-to-machine flows. A tiny, dependency-free piece of the open-source CAPTCHA. - [Troubleshooting](/guide/troubleshooting/instrumentation.md): Troubleshooting Cap verification failures: fixes for the open-source, self-hosted CAPTCHA when proof-of-work or instrumentation checks fail in your browser.