zekebuilds-lab/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 or HTTP. 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	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: "0.1.0" }.

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.

How this compares to other MCP agent-auth primitives

A side-by-side breakdown against x402-mcp, @agentauth/mcp, and Cloudflare ARC/ACT is published at powforge.dev/mcp/compare/x402-mcp. Short version: captcha-mcp is the only entrant that ships a free PoW tier alongside a Lightning paid skip on an MCP transport. The other three price every call (USDC) or require platform-issued credentials.

License

MIT