Polymarket Guard

🚀 Get started

Monetizing an MCP/API? Start here — one command:

npx create-lemon-mcp my-paid-mcp     # a paid MCP server, running in sandbox now# then add a Seller Key in /app and set LEMONCAKE_SELLER_KEY → it charges for real (no code change)

Pick your path:

npx -y agent-payment-mcp

{ "mcpServers": { "lemon": { "command": "npx", "args": ["-y", "agent-payment-mcp"] } } }

What is LemonCake?

LemonCake is an x402 payment rail for monetizing MCP servers and HTTP APIs. Sellers register an endpoint and set a price per call. Buyers prepay by card and receive a spend-capped Pay Token. Agents call the gateway with that token, and LemonCake verifies, meters, forwards, and records usage.

sequenceDiagram    participant A as 🤖 AI Agent    participant G as 🍋 LemonCake Gateway    participant API as Your API    A->>G: POST /g/:id (no token)    G-->>A: 402 + accepts[] (price, mintUrl)    A->>G: mint Pay Token (off-session, capped)    A->>G: Bearer :jwt    G->>API: forward (upstream key hidden)    API-->>A: 200 + result    Note over A,G: budget exhausted → 402 → agent self-funds → continues

Sellers register any HTTP API and set a price per call. Buyers / agents prepay with a card → Pay Token issued automatically → agent calls the API within budget. Budget exhausted → 402 challenge → agent self-funds → continues. No humans.

🧱 Open core

💳 How payment works

For buyers (human or agent)

1. Open a buy link (lemoncake.xyz/buy/<shortId>)
2. Pay with a card → Pay Token (JWT) issued automatically
3. Pass Authorization: Bearer <token> to the gateway
4. Gateway verifies, meters, forwards to the real API
5. Budget exhausted → 402 challenge returned with payment instructions

For agents (fully autonomous)

1. Issue a Buyer Key (bk_...) at /app → Pay Tokens pane
2. Save a card once at /agent/fund
3. Agent calls POST /api/lc/agent/tokens (Bearer bk_) → off-session card charge → JWT
4. Agent uses JWT to call gateway — hard-capped, no human in the loop

// MCP config for agent with pre-issued Pay Token{  "mcpServers": {    "lemon": {      "command": "npx",      "args": ["-y", "agent-payment-mcp"],      "env": {        "LC_PAY_TOKEN": "<jwt from Pay Token>"      }    }  }}

🏗 Publish your API on LemonCake (sellers)

Monetize any HTTP API or MCP server:

1. Sign in at lemoncake.xyz/app
2. Add API — paste your URL, set price per call (e.g. $0.01)
3. Share the buy link, or issue a Seller Key (sk_live_…) to charge from your own server
4. You keep 97%. LemonCake takes 3% (Stripe Connect Direct Charge). Never holds funds.

Scaffold a paid MCP in one command — sandbox by default, production with one env var:

npx create-lemon-mcp my-paid-mcp     # demo runs with no key# then: set LEMONCAKE_SELLER_KEY=sk_live_… → it charges for real (no code change)

Add billing to any tool with the SDK (@lemon-cake/mcp-sdk v1, no crypto):

import { createLemonCakeSDK } from "@lemon-cake/mcp-sdk";
const lc = createLemonCakeSDK();   // reads LEMONCAKE_SELLER_KEY (demo without it)
server.tool("my_premium_tool", "desc", { q: z.string() },  lc.charge({ price: 0.01 })(async ({ q }) => {    return { content: [{ type: "text", text: "result" }] };  }),);

lc.charge wraps the handler: preflight (reserve) → run → settle (confirm on success, refund on failure). Or route existing traffic through https://lemoncake.xyz/g/<shortId> — no code changes required.

🪪 Agent Identity

Give each AI agent its own spend-capped identity — so a fleet can pay for APIs without a shared card or runaway cost. Built on top of Pay Tokens; no balance pool, custody-free (an agent's "budget" is just the Pay Tokens bound to it).

• Bind a Pay Token to an agent when issuing it (agentId) → spend is attributed to that agent.
• Per-agent rollup — budget, spend, calls, last-used in the dashboard.
• Kill switch — pause / resume / revoke an agent; bound tokens are rejected at the gateway instantly (AGENT_PAUSED / AGENT_REVOKED), even with budget remaining.
• Server-authoritative — price is set on the endpoint; agents never carry a card or a seller key.

# manage agents (owner-authenticated)POST /api/agents            # create  → { agent_id, ... }POST /api/agents/:id/pause  # kill switch (also /resume, /revoke)GET  /api/agents            # list + per-agent spend rollup

✨ Features

For agents / buyers

• ✅ x402 native — 402 challenge returns accepts[] with price + mintUrl
• ✅ Hard-capped — per-mint / daily / monthly limits, server-enforced
• ✅ Off-session top-up — agent self-funds via Buyer Key (bk_...), no prompts
• ✅ Demo Mode — 8 free tools, try without any setup

For API providers / sellers

• ✅ 5-min go-live — npx create-lemon-mcp, or lc.charge() on any tool, or route through the gateway
• ✅ Custody-free — Stripe Connect Direct Charge, 97% goes directly to seller
• ✅ Usage ledger — every call recorded, revenue visible in dashboard
• ✅ Buy link — share one URL, buyers self-serve

For agent fleets / operators

• ✅ Per-agent identity — bind Pay Tokens to an agent, attribute spend
• ✅ Per-agent kill switch — pause / resume / revoke, enforced at the gateway
• ✅ Spend rollup — budget / spent / calls per agent
• ✅ No balance pool — custody-free; budget = the agent's bound Pay Tokens

Infrastructure

• 🔧 Stripe Connect Direct Charge (no custody)
• 🔧 x402 gateway with WWW-Authenticate: Lemoncake-Prepaid
• 🔧 EN / 日本語 / Español dashboard
• 🔧 JP FSA: registration not required (confirmed 2026-06)

🏗 Architecture

┌─────────────────────────────────────────────────┐│  Buyer / Agent                                  ││   ↳ prepays via card OR Buyer Key (bk_...)      ││   ↳ receives Pay Token (signed JWT)             │└──────────────────────┬──────────────────────────┘                       │  Authorization: Bearer <jwt>                       ▼┌─────────────────────────────────────────────────┐│  LemonCake Gateway  /g/<shortId>                ││   ↳ verify JWT signature                        ││   ↳ check budget + calls + rate limit           ││   ↳ decrement spend, write to ledger            │└──────────────────────┬──────────────────────────┘                       │  HTTPS + upstream_auth (hidden)                       ▼┌─────────────────────────────────────────────────┐│  Your API / MCP server (unchanged)              │└─────────────────────────────────────────────────┘

LemonCake is the middle box. It never holds funds — money flows Stripe → seller via Direct Charge.

🌍 Compliance

Japan FSA Fintech Support Desk (2026-06) confirmed: no registration required. Custody-free design (Stripe Connect Direct Charge, no pooled balance).

See lemoncake.xyz/security

🔌 Package family

🛡 Security

• Server-side hard caps — per-mint / daily / monthly, cannot be exceeded
• Pay Token = signed JWT — HS256, verified on every gateway call
• upstream_auth never exposed — seller's real API key hidden from buyers
• RLS on all DB tables — Supabase row-level security enabled
• Stripe Connect Direct Charge — LemonCake never holds funds