@powforge/captcha-mcp

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

OpenAI's Sora API does not let you charge per call. Anthropic's billing does not pass through to your tools. If you ship an MCP server today and an autonomous agent finds it, you eat the bill.

This is the gate. Three tools over stdio. Stdlib only.

Quickstart

npx -y @powforge/captcha-mcp

That is it. 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	Agent-friendly
API keys	$0	yes	no
OAuth	$0	yes	no
Stripe metering	high overhead	yes	no
PoW + L402	seconds or 3 sats	no	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.

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

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: "0.1.0" }.

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":"<token-from-verify-tool>"}'

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.

License

MIT

@powforge/captcha-mcp