agent-payment-mcp — LemonCake

⚡️ 60-second quickstart

# 1. Register at https://lemoncake.xyz, grab a BUYER_JWT

# 2. Issue a Pay Token (scoped, expiring, revocable)
curl -X POST https://lemoncake.xyz/api/tokens \
  -H "Authorization: Bearer $BUYER_JWT" \
  -H "Content-Type: application/json" \
  -d '{"serviceId":"svc_xxx","limitUsdc":"2.00","sandbox":true}'
# → { "jwt": "<pay_token>", ... }

# 3. Hand it to your agent. Done.
#    Claude / Cursor:
npx agent-payment-mcp
#    Eliza v2:
npm install eliza-plugin-lemoncake
#    Anything else:
curl -X POST https://lemoncake.xyz/api/proxy/svc_xxx/your/endpoint \
  -H "Authorization: Bearer <pay_token>" \
  -H "Idempotency-Key: $(uuidgen)"

Budget exhausted? 402. Token revoked? 422. No runaway agents. No stolen keys.

What is LemonCake?

LLM agents are getting powerful — but they still can't pay for things autonomously.

LemonCake solves this with Pay Tokens: short-lived JWTs that give an agent a scoped spending limit. The agent calls paid APIs through our proxy, gets charged per call in USDC, and stops automatically when the budget runs out.

You                    Agent                   Paid API
 │                       │                        │
 ├─ issue Pay Token ──▶  │                        │
 │   limit: $2.00        │                        │
 │                       ├─ call /api/proxy ────▶ │
 │                       │   Authorization: Bearer <pay_token>
 │                       │                        │
 │                       │  ◀─ response + charge ─┤
 │                       │    X-Charge-Id: ch_...  │
 │                       │    X-Amount-Usdc: 0.001 │
 │                       │                        │
 │                       ├─ (budget exhausted)     │
 │                       │   402 Payment Required  │
 │                       ✗   agent stops cleanly   │

🔌 Integrations

MCP サーバー — agent-payment-mcp

Claude Desktop・Cursor に npx 一発で接続できる公式 MCP サーバー。Demo Mode で env vars 空のまま動作（Wikipedia / FX / httpbin が即試せる）。x402-compatible interface。

npx agent-payment-mcp

旧パッケージ名 lemon-cake-mcp は薄いラッパーとして維持中なので、既存の npx -y lemon-cake-mcp configs もそのまま動きます（stderr に deprecation 通知のみ）。

claude_desktop_config.json に追記するだけ：

{
  "mcpServers": {
    "pay-per-call": {
      "command": "npx",
      "args": ["-y", "agent-payment-mcp"],
      "env": {
        "LEMON_CAKE_PAY_TOKEN": "<Pay Token JWT>",
        "LEMON_CAKE_BUYER_JWT": "<Buyer JWT>"
      }
    }
  }
}

Eliza v2 プラグイン — eliza-plugin-lemoncake

@elizaos/core v2 対応の公式プラグイン。character.plugins に追加するだけで Eliza エージェントが自律決済を実行できます。

npm install eliza-plugin-lemoncake

import { lemonCakePlugin } from "eliza-plugin-lemoncake";

const character = {
  name: "MyAgent",
  plugins: [lemonCakePlugin],
};

# どちらか一方を設定
LEMONCAKE_PAY_TOKEN=eyJhbGci...   # クイックスタート（事前発行トークン）
LEMONCAKE_BUYER_JWT=eyJhbGci...   # 本番運用（呼び出しごとに都度発行）

自然言語でそのまま動く：

"LemonCake の demo_search_api を 0.50 USDC で呼び出して"
"serviceId: svc_invoice に 0.10 USDC 支払いを実行して"

→ 詳細: eliza-plugin-lemoncake/README.md

✨ Features

Packages (5)

For AI Agents (Buyers)

• Pay Token (JWT) — Scoped, expiring spend authorization. One token per task or session.
• 402-first design — Agents receive structured 402 Payment Required errors with machine-readable codes when budget runs out.
• Idempotency keys — Prevent double charges on retries (auto-assigned by plugins).
• Real-time balance — Check remaining USDC before committing to expensive calls.
• Kill Switch — Atomic one-click token revocation. Race-condition-free.

For API Providers (Sellers)

• Service registry — Register any REST API. Set price-per-call in USDC.
• Instant revenue — Get paid per call with no invoicing, no net-30, no chargebacks.
• Usage analytics — See call counts, revenue, and error rates per service.

Infrastructure

• JPYC on-chain deposit — Charge balance with JPYC (Polygon ERC-20). Auto-verified via TX hash.
• KYA (Know Your Agent) — Tiered spending limits: 10 → 1,000 → 50,000 USDC/day.

🏗 Architecture

┌──────────────────────────────────────────────────────────┐
│                    lemoncake.xyz                          │
│                                                          │
│  ┌─────────────┐    ┌──────────────┐   ┌─────────────┐  │
│  │  Dashboard  │    │   Hono API   │   │  Pay Proxy  │  │
│  │  (Next.js)  │◀──▶│  + OpenAPI   │◀──│  Middleware │  │
│  └─────────────┘    └──────┬───────┘   └─────────────┘  │
│                            │                             │
│               ┌────────────┼────────────┐                │
│               ▼            ▼            ▼                │
│          PostgreSQL      Redis       Polygon             │
│          (Prisma)       (Queue)    USDC / JPYC           │
└──────────────────────────────────────────────────────────┘
        ▲                              ▲
        │                              │
   MCP / Eliza                   Upstream APIs
   (Claude, Cursor, Eliza...)    (registered services)

Key design decisions:

• HMAC-SHA256 (HS256, RFC 7518) for JWT signing today; Ed25519 (EdDSA, RFC 8037) migration planned for v0.1.0 to enable asymmetric verification by external partners
• Optimistic locking on usedUsdc — prevents double charges under concurrent agent calls
• Proxy-first — upstream API keys never leave the server; agents only hold Pay Tokens

🔌 Public API (selected endpoints)

Full OpenAPI spec available at /api/doc after signing in.

Auth

POST /api/auth/register
{ "name": "string", "email": "string", "password": "string (min 8)" }
→ { "token": "<buyer_jwt>", "expiresIn": 2592000 }

POST /api/auth/buyer-login
{ "email": "string", "password": "string" }
→ { "token": "<buyer_jwt>" }

Tokens (Pay Token)

POST /api/tokens
Authorization: Bearer <buyer_jwt>
{
  "serviceId": "<id>",
  "limitUsdc": "5.00",
  "buyerTag": "my-agent-session-42",
  "expiresAt": "2026-05-01T00:00:00Z"
}
→ { "tokenId": "...", "jwt": "<pay_token>", "limitUsdc": "5.000000", "expiresAt": "..." }

Proxy (Pay-per-call)

ANY /api/proxy/<serviceId>/<upstream-path>
Authorization: Bearer <pay_token>
Idempotency-Key: <uuid>

→ upstream response
  + X-Charge-Id: ch_...
  + X-Amount-Usdc: 0.001000

Error responses:

{ "error": "Token limit exceeded", "used": "4.999", "limit": "5.000" }   // 402
{ "error": "Insufficient balance: 1.23 USDC available" }                 // 402
{ "error": "Token expired" }                                              // 401

Services

GET /api/services?reviewStatus=APPROVED&limit=50
→ [{ "id": "...", "name": "...", "pricePerCallUsdc": "0.001", ... }]

🧑‍💻 Local Development

# 1. Clone
git clone https://github.com/evidai/agent-payment-mcp.git
cd agent-payment-mcp

# 2. API server
cd api
cp .env.example .env
npm install && npx prisma migrate dev
npm run dev                    # http://localhost:3000

# 3. Dashboard
cd ../dashboard
cp .env.example .env.local    # NEXT_PUBLIC_API_URL=http://localhost:3000
npm install && npm run dev     # http://localhost:3001

# 4. MCP server
cd ../mcp-server
npm install && npm run build

# 5. Eliza plugin
cd ../eliza-plugin-lemoncake
npm install && npm run build

Seed demo data

node api/seed_demo.js
# Creates demo provider, 4 approved services, and 9,000+ sample charge records

🛡 Security

• Brute-force protection — 10 failed logins triggers a 15-minute lockout per email
• Pay Token scoping — Each token is bound to a single serviceId; cross-service reuse is rejected
• No JWT storage — Only the jti (token ID) is stored in the DB; the signed JWT never persists
• CSRF protection — OAuth state parameter with 10-minute TTL and single-use consumption
• On-chain deposit verification — JPYC transfers are verified against Polygon event logs before balance is credited

🗺 Roadmap

Shipped:

• agent-payment-mcp — USDC pay-per-call for any HTTP API, Demo Mode included
• xstocks-mcp — Jupiter DEX xStocks (AAPLx/TSLAx/SPYx) on Solana mainnet
• alpaca-guard-mcp — Alpaca Markets proxy with spend caps and kill switch
• tokenized-stock-mcp — Dinari dShares with USDC, sandbox default
• @lemon-cake/mcp-sdk — SDK for MCP developers to add Pay Token billing in 3 lines
• Kill Switch — atomic one-click token revocation (race-condition-free)
• KYA (Know Your Agent) — tiered spending limits: 10 → 1,000 → 50,000 USDC/day
• Sandbox / Demo Mode — full-flow dry-run without moving real USDC
• JPYC on-chain deposit with Polygon event-log verification

Next:

• Panic button — revoke all active tokens for a buyer in one click
• LemonCake marketplace — register and monetize any REST API
• Streaming / token-count billing for LLM APIs
• Agent-to-agent sub-token delegation (parent caps child)
• Self-hostable edition

📄 License

Proprietary — All rights reserved © 2026 LemonCake MCP server and Eliza plugin source are available for review. Core API and payment engine are closed source.

Built for the agentic web. lemoncake.xyz