Captcha MCP

Name: Captcha MCP
Author: zekebuilds-lab

by zekebuilds-labversion 0.1.1i★ –iRuns on your computer

Registry nameio.github.zekebuilds-lab/captcha-mcp

Install

Overview

Description from the maker (original English)

Charge AI agents per-call without accounts. PoW solve = free tier, Lightning = paid tier.

PopNx MCP’s three pillars

These three are never merged into one score: opinions, measurements and popularity are different things.

User voiceiNo reviews yetReviews open together with accounts in a later step
Our basic checksi1 of 1 passedChecked from public information; code not scanned yet
UsageiNot measured yet★ –

What you can do with it

We have not written a plain-language summary for this one yet. Below are the facts we read from the registry.

Connection: Runs on your computer
Transport: stdio
Category: Other

Install

Pick the AI app you use, then copy the settings.

Before you install

Basic checks: 1 of 1 passed
Runs on your computer with your user’s permissions (files, network, apps).
You fill in yourself: CAPTCHA_URL

You need

npmNode.js installed (for the npx command)

Which app do you use?

ClaudeClaude CodeCursorVS CodeChatGPT

Pick your app above. We remember it, so next time the button says “Install in …”.

Open Claude Desktop → Settings → Developer → Edit Config, and paste this into claude_desktop_config.json

{
  "mcpServers": {
    "captcha": {
      "command": "npx",
      "args": [
        "-y",
        "@powforge/captcha-mcp"
      ],
      "env": {
        "CAPTCHA_URL": "your-value"
      }
    }
  }
}

Run this in your terminal

claude mcp add captcha --env CAPTCHA_URL=your-value -- npx -y @powforge/captcha-mcp

Fill in the empty values before running it.

Open in Cursor

Or copy the settings yourself

Paste into .cursor/mcp.json in your project, or ~/.cursor/mcp.json

{
  "mcpServers": {
    "captcha": {
      "command": "npx",
      "args": [
        "-y",
        "@powforge/captcha-mcp"
      ],
      "env": {
        "CAPTCHA_URL": "your-value"
      }
    }
  }
}

Your browser asks to open the app, then the app shows the settings and asks you to confirm. Key values are left empty for you to fill in.

Open in VS Code

Or copy the settings yourself

Paste into .vscode/mcp.json in your project

{
  "servers": {
    "captcha": {
      "type": "stdio",
      "command": "npx",
      "args": [
        "-y",
        "@powforge/captcha-mcp"
      ],
      "env": {
        "CAPTCHA_URL": "your-value"
      }
    }
  }
}

Your browser asks to open the app, then the app shows the settings and asks you to confirm. Key values are left empty for you to fill in.

This one runs on your computer, and ChatGPT can only connect to servers on the internet. Pick another app.

Your choice stays on this device in a cookie; no account needed.

Values you fill in

CAPTCHA_URL env
PowForge captcha backend URL. Defaults to public production endpoint when running via npx; override for self-hosted or local dev.

These settings are generated from the maker’s registry entry. If the README says otherwise, follow the README.

What it can do

Runs on your computer; the tool list appears in your AI app after you install it.

Basic checksi

We check information anyone can see; we have not read the code. Checks that do not apply are left out of the count. · Checked Nov 8, 2026

✓ Pass
Maker matches the repo
The registry name was verified with the GitHub account zekebuilds-lab, which owns the repo
… Not checked yet
Still maintained
We have not read this repo from GitHub yet
… Not checked yet
Has a licence
We have not read this repo from GitHub yet
– Not applicable
Endpoint responds
This one runs on your computer; there is no internet endpoint
– Not applicable
Sign-in with OAuth
Runs on your computer and uses local settings instead
… Not checked yet
No hidden instructions in tool descriptions
Runs on your computer; we can only read tool lists from internet endpoints

Compared with others in Otheri

More stars than 0% of the other 18428 servers in this category
Published to the registry more recently than 7% of the other 18428 servers in this category
Basic checks equal to or better than 100% of the other 18428 servers in this category

Compared with every active server in this category in the registry. Stars count only when the publisher owns the repo.

Common questions

Is Captcha MCP safe?

It passed 1 of 1 applicable basic checks. We have not read the code. Before adding a key or token, see what access the server asks for and use a key with only the permissions you need.

What keys or codes does it need?

The registry does not list a required key, though some servers ask you to sign in on first use.

Does it run on my computer or in the cloud?

It runs on your computer. Your AI app starts it when needed, and data does not pass through the maker’s servers (unless it calls an online service itself).

Is it still maintained?

The GitHub repo last changed –, and the latest version (0.1.1) reached the registry 6 months ago.

Which apps can use it?

Apps that support MCP, such as Claude Desktop, Claude Code, Cursor and VS Code. You also need Node.js on your computer.

What can I use instead?

In the same category: Spec Driven Development, Remote Desktop Commander, Winecellarbuilders

Alternatives

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Remote Desktop Commander

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Remote Desktop Commander: use files, terminal and processes on your own computers from any AI app.

Runs in the cloud★ 211Updated 3 days ago

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Runs in the cloudUpdated today

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README from the maker

Open the README

Copied from the maker’s repo as written. PopNx MCP has not reviewed or endorsed it. HTML, images and scripts are removed. Links lead off this site.

@powforge/captcha-mcp

Your MCP server returns 429 when agents pound it. captcha-mcp makes them earn their next call instead. Hand the agent a proof-of-work puzzle (free, ~5s of CPU) or a 3-sat Lightning invoice — both are machine-readable backoff signals an autonomous caller can satisfy without an account, email, or API key.

Three tools over stdio or HTTP. Stdlib only. No signup, free fallback, self-hosted, no revenue share.

Why not 429?

429 Too Many Requests is the wrong shape for the agent era. Three patterns recur across MCP server reports:

Agent frameworks treat 429 as a connection failure. They retry immediately, often with exponential backoff that is still too aggressive, and amplify the overload that triggered the limit in the first place.
There is no per-caller signal. A 429 fires for the bucket, not the agent. One noisy caller gets every other caller throttled, and the server has no way to ask the noisy one to slow down specifically.
Retry-After is advisory and frequently ignored. Agents do not consistently parse it, do not consistently respect it, and have no incentive to wait — the cost of retrying is zero.

captcha-mcp replaces the 429 with a 402-style challenge. The next call costs the caller something (CPU seconds or 3 sats). That cost is per-caller, machine-readable, and self-throttling — an agent that cannot solve the puzzle cannot flood the endpoint.

Quickstart

npx -y @powforge/captcha-mcp

No install, no config, no API key. The server starts on stdio and waits for an MCP client.

To wire it into Claude Code, Cursor, or any MCP-compatible host, add to your config:

"mcpServers": { "powforge-captcha": { "command": "npx", "args": ["-y", "@powforge/captcha-mcp"] } } }

Or run npx @powforge/captcha-mcp --install to print the config block.

What it does

Wraps the PowForge pow-captcha service (captcha.powforge.dev) as three MCP tools:

Tool	Purpose
`challenge`	Request a fresh proof-of-work puzzle. Returns `{id, salt, difficulty, signature}`.
`verify`	Submit a solved nonce. Returns a 5-minute HMAC-signed access token.
`status`	Server health, lifetime stats, L402 endpoint metadata.

The free tier costs the agent ~5-10 seconds of CPU time (SHA-256, default 14 leading zero bits). The paid tier costs 3 sats over Lightning via L402 (RFC 7235 + bolt11 invoice in WWW-Authenticate).

Why this and not OAuth, API keys, or Stripe

Approach	Per-call cost	Account required	Self-hosted	Agent-friendly
API keys	$0	yes	n/a	no
OAuth	$0	yes	n/a	no
Stripe metering	high overhead	yes	n/a	no
Managed MCP auth platform	100–2000 sats	no	no	yes
PoW + L402 (this)	seconds or 3 sats	no	yes	yes

Agents do not have email addresses. They do not click confirmation links. They do not enter credit cards. PoW + Lightning is the only auth primitive that works for fully autonomous callers.

Managed MCP auth platforms work, but they charge 100–2000 sats per call on vendor infrastructure — your revenue flows through their rails. This package runs on your server, your Lightning node, your keys. You keep the sats.

Configuration

Set CAPTCHA_URL to point at a different captcha backend. Default is http://localhost:3077 so you can run the full stack locally for development. Production deployments point it at https://captcha.powforge.dev.

CAPTCHA_URL=https://captcha.powforge.dev npx @powforge/captcha-mcp

HTTP Streamable transport

Hosted MCP clients (Smithery, browser-based hosts) need HTTP, not stdio. Pass --http or set HTTP_MODE=1:

HTTP_MODE=1 PORT=3200 npx @powforge/captcha-mcp

or

npx @powforge/captcha-mcp --http

The server then listens on:

Endpoint	Method	Purpose
`/mcp`	POST	Single JSON-RPC request, single JSON-RPC response. Notifications return 202.
`/mcp`	GET	SSE stream for server-pushed notifications (kept open with a 25s heartbeat).
`/health`	GET	Liveness probe — returns `{ok, server, transport}`. Not part of MCP.

Stateless. No session ids. CORS open (Access-Control-Allow-Origin: *) so browser clients work. Stdio mode is unchanged and remains the default — npx @powforge/captcha-mcp with no flag still talks JSON-RPC over stdin/stdout.

Smoke test the HTTP transport:

HTTP_MODE=1 PORT=3200 node src/server.js &

curl -X POST http://localhost:3200/mcp \ -H "Content-Type: application/json" \ -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1"}}}'

Returns {jsonrpc:"2.0", id:1, result:{protocolVersion:"2024-11-05", capabilities:{tools:{}}, serverInfo:{...}}}.

Local development

Clone the captcha widget repo or run the public service. The MCP server only needs HTTP access to the captcha endpoints listed under status.

git clone https://github.com/zekebuilds-lab/captcha-mcp

cd captcha-mcp node src/server.js

It prints ready to stderr and waits for JSON-RPC on stdin.

Smoke-test the protocol manually:

echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1"}}}' | node src/server.js

You should see a JSON response with serverInfo: { name: "@powforge/captcha-mcp", version: "0.2.5" }.

Token verification from your own backend

When an agent submits a token to your service, verify it without trusting the agent:

curl -X POST https://captcha.powforge.dev/api/token/verify \

-H "Content-Type: application/json" \ -d '{"token":" "}'

Returns {valid: true, method, issued_at, expires_at} or {valid: false, reason}.

Related packages

@powforge/captcha — the browser widget for the same service.
@powforge/mcp-l402-gate — Express middleware to gate any MCP server with L402 + Depth-of-Identity scoring.
@powforge/mcp-identity — agent reputation oracle. Pair with this gate for first-call abuse protection.

How this compares to other MCP agent-auth primitives

The gate-the-MCP-server space is filling up. Here is the honest landscape, ranked by how directly each tool overlaps with what captcha-mcp does.

Tool	Payment rail	Auth model	Self-host	Free PoW tier	No account to pay
PayGated	Stripe credits	API key + OAuth 2.1 + PKCE + M2M	yes (MIT)	no	no (Stripe customer record per caller)
APort	none disclosed	W3C verifiable creds, pre-tool hook	design-partner	no	n/a (audits, does not charge)
AgentSign	none disclosed	Ed25519 signed passport + trust gate	unknown	no	n/a
x402-mcp	USDC on-chain	wallet signature	yes	no	no (needs funded wallet)
Managed MCP auth (Auth0 for AI, MintMCP)	SaaS	OAuth 2.0 / SAML / SSO	no	no	no
captcha-mcp (this)	Lightning (L402)	PoW gate + L402 skip + free-tier	yes	yes	yes

PayGated is the closest collision. Same "monetize MCP tools per call" pitch, same self-host + open-source posture, but it settles on Stripe. That means you need a Stripe account in good standing (KYC, a bank, a supported country) to collect, and every caller needs a Stripe customer record before it can pay you a cent. captcha-mcp's differentiator is the no-account path: a non-US agent author pays 3 sats per call in about 200ms with no KYC, or solves a free PoW puzzle if it will not pay at all.

APort and AgentSign sit at a different layer. They record who used a tool under what authority; they do not price the call. They compose with a gate like this one rather than replace it.

None of them price the act of interacting. Every other row assumes the caller is already an authorized identity and meters or audits after that. The PoW tier here is the only mechanism in the table that puts a cost on the interaction itself, not on the identity of the actor. That is the position this package defends.

A longer breakdown against x402-mcp, @agentauth/mcp, and Cloudflare ARC/ACT is at powforge.dev/mcp/compare/x402-mcp.

License

MIT

Captcha MCP

Overview

PopNx MCP’s three pillars

What you can do with it

Install

Before you install

Values you fill in

What it can do

Basic checksi

Maker matches the repo

Still maintained

Has a licence

Endpoint responds

Sign-in with OAuth

No hidden instructions in tool descriptions

Compared with others in Otheri

Common questions

Alternatives

Spec Driven Development

Remote Desktop Commander

Winecellarbuilders

README from the maker

@powforge/captcha-mcp

Why not 429?

Quickstart

What it does

Why this and not OAuth, API keys, or Stripe

Configuration

HTTP Streamable transport

or

Local development

Token verification from your own backend

Related packages

How this compares to other MCP agent-auth primitives

License