Standalone Service

The standalone service example is the most comprehensive Key0 deployment pattern. It runs Key0 as its own microservice, decoupled from your backend, handling the full payment lifecycle: agent discovery, challenge issuance, on-chain verification, token delivery, payment notifications, and automatic refunds. This is the recommended architecture for production deployments where you want to isolate payment infrastructure from your application logic.

Architecture

                  ┌─────────────────────────────┐
                  │         AI Agent             │
                  └──────┬───────────┬───────────┘
                         │           │
            1. Discover  │           │  5. Use token
            + Pay (A2A)  │           │     (Bearer JWT)
                         ▼           ▼
              ┌──────────────┐   ┌──────────────────┐
              │ Key0 Service │   │  Your Backend     │
              │              │──>│                   │
              │ - Challenge  │ 3.│  - Protected APIs │
              │ - Verify tx  │   │  - /internal/*    │
              │ - Issue JWT  │   │                   │
              │ - Refund     │   │                   │
              └──────────────┘   └──────────────────┘
                     │  ▲              ▲
                  2. │  │ 4.           │
                     ▼  │              │
              ┌──────────────┐         │
              │  Base (USDC) │         │
              └──────────────┘         │
                                       │
              ┌──────────────┐         │
              │ Redis/Postgres├────────┘
              └──────────────┘

Flow:

Agent discovers the Key0 service via /.well-known/agent.json and requests access via A2A
Key0 issues an x402 challenge; agent pays USDC on Base
Key0 verifies the on-chain transfer and notifies your backend
Key0 issues a JWT (locally or via your backend) and returns it to the agent
Agent uses the JWT to call your protected API directly

What This Example Demonstrates

Capability	Options
Storage backend	Redis or Postgres, selected at startup
Auth strategies	`noAuth`, `sharedSecretAuth`, `signedJwtAuth` for backend communication
Token issuance	Native (local JWT) or remote (delegates to backend)
Gas wallet	Optional self-contained settlement mode
MCP support	`mcp: true` enables MCP tool discovery alongside A2A
Automatic refunds	BullMQ cron processes refunds for failed deliveries
Multi-plan pricing	Pay-as-you-go, monthly, and yearly plans

Configuration Reference

Core Settings

Variable	Required	Default	Description
`KEY0_PORT`	No	`3001`	Port the service listens on
`KEY0_NETWORK`	No	`testnet`	`testnet` (Base Sepolia) or `mainnet` (Base)
`KEY0_ACCESS_TOKEN_SECRET`	Yes	—	Secret for JWT signing. Must be at least 32 characters.
`KEY0_WALLET_ADDRESS`	Yes	—	Wallet address to receive USDC payments
`KEY0_PUBLIC_URL`	No	`http://localhost:{port}`	Public URL for agent card and endpoint URLs
`CHALLENGE_TTL_SECONDS`	No	`900`	How long a payment challenge remains valid

Backend Communication

Variable	Required	Default	Description
`BACKEND_API_URL`	Yes	—	Base URL of your backend service
`BACKEND_AUTH_STRATEGY`	No	`none`	Auth strategy: `none`, `shared-secret`, or `jwt`
`INTERNAL_AUTH_SECRET`	Conditional	—	Required when `BACKEND_AUTH_STRATEGY=shared-secret`
`TOKEN_MODE`	No	`native`	`native` (local JWT) or `remote` (call backend)

Storage

Variable	Required	Default	Description
`STORAGE_BACKEND`	No	`redis`	`redis` or `postgres`
`REDIS_URL`	Conditional	—	Required when `STORAGE_BACKEND=redis`
`DATABASE_URL`	Conditional	—	Required when `STORAGE_BACKEND=postgres`

Gas Wallet and Refunds

Variable	Required	Default	Description
`USE_GAS_WALLET`	No	`false`	Enable gas wallet settlement mode
`GAS_WALLET_PRIVATE_KEY`	Conditional	—	Required when `USE_GAS_WALLET=true`
`KEY0_WALLET_PRIVATE_KEY`	No	—	Private key for sending refunds. Refund cron is disabled without this.
`REFUND_INTERVAL_MS`	No	`15000`	How often the refund cron runs (ms)
`REFUND_MIN_AGE_MS`	No	`30000`	Minimum age before a failed delivery is eligible for refund (ms)

Agent Card Metadata

Variable	Required	Default	Description
`AGENT_NAME`	No	`Key0 Service`	Name shown in the A2A agent card
`AGENT_DESCRIPTION`	No	`Payment-gated API access for AI agents`	Description in the agent card
`PROVIDER_NAME`	No	`Example Corp`	Provider name in the agent card
`PROVIDER_URL`	No	`https://example.com`	Provider URL in the agent card

Code Walkthrough

Storage Backend Selection

The service selects its storage layer at startup based on a single environment variable. Both backends implement the same IChallengeStore and ISeenTxStore interfaces, so the rest of the code is storage-agnostic.

Redis
Postgres

const redis = new Redis(process.env.REDIS_URL!);
store = new RedisChallengeStore({ redis, challengeTTLSeconds: 900 });
seenTxStore = new RedisSeenTxStore({ redis });

Redis uses atomic Lua scripts for state transitions and SET NX for double-spend prevention. Best for low-latency, single-region deployments.

const postgres = (await import("postgres")).default;
const sql = postgres(process.env.DATABASE_URL!);
store = new PostgresChallengeStore({ sql });
seenTxStore = new PostgresSeenTxStore({ sql });

Postgres uses row-level locking for atomicity. Best when you already run Postgres and want a single data store.

Regardless of which storage backend you choose, the BullMQ refund cron always requires a Redis connection. If you use Postgres for storage, set REDIS_URL or BULLMQ_REDIS_URL separately for the job queue.

Auth Strategy Configuration

When Key0 communicates with your backend (to notify payments or request token issuance), it authenticates using one of three strategies:

let authProvider: AuthHeaderProvider;

if (BACKEND_AUTH_STRATEGY === "jwt") {
  authProvider = signedJwtAuth(serviceTokenIssuer, "backend-service");
} else if (BACKEND_AUTH_STRATEGY === "shared-secret") {
  authProvider = sharedSecretAuth("X-Internal-Auth", INTERNAL_AUTH_SECRET);
} else {
  authProvider = noAuth();
}

Strategy	When to use	What it sends
`noAuth()`	Local development, trusted internal networks	No auth headers
`sharedSecretAuth()`	Simple production setups	`X-Internal-Auth: <secret>` header
`signedJwtAuth()`	Zero-trust environments	`Authorization: Bearer <signed-jwt>` header

Never use noAuth() in production. Use shared-secret as a minimum, or jwt for environments where services communicate across network boundaries.

Token Issuance Modes

After payment verification, Key0 needs to issue a credential. The standalone service supports two modes:

Native (local JWT)
Remote (delegate to backend)

Key0 signs a JWT locally using AccessTokenIssuer. The token includes the challenge ID, resource ID, plan ID, and transaction hash:

const localTokenIssuer = new AccessTokenIssuer(SECRET);
fetchResourceCredentials = async (params) => {
  const ttl = 3600;
  return localTokenIssuer.sign(
    {
      sub: params.requestId,
      jti: params.challengeId,
      resourceId: params.resourceId,
      planId: params.planId,
      txHash: params.txHash,
    },
    ttl,
  );
};

Your backend validates the token using the same KEY0_ACCESS_TOKEN_SECRET. Best when Key0 and your backend share a secret.

Key0 calls your backend’s /internal/issue-token endpoint, letting your backend control the token format entirely:

const remoteTokenIssuer = createRemoteTokenIssuer({
  url: `${BACKEND_API_URL}/internal/issue-token`,
  auth: authProvider,
  timeoutMs: 10000,
});
fetchResourceCredentials = remoteTokenIssuer;

Best when your backend already has its own token system (OAuth, API keys, session tokens) and you want Key0 to integrate with it.

Plan Catalog

The service defines a multi-tier pricing structure that agents discover via the A2A agent card:

const plans = [
  {
    planId: "basic",
    unitAmount: "$0.015",
    description: "Pay-as-you-go. 2 concurrent agents, 10 req/min.",
  },
  {
    planId: "starter-monthly",
    unitAmount: "$15.00",
    description: "1,650 req/month, 10 concurrent agents, 100 req/min.",
  },
  {
    planId: "starter-yearly",
    unitAmount: "$168.00",
    description: "Same as starter-monthly, billed yearly (save 7%).",
  },
  {
    planId: "pro-monthly",
    unitAmount: "$150.00",
    description: "16,500 req/month, 50 concurrent agents, 1,000 req/min.",
  },
];

Agents select a plan when requesting access. Key0 issues a challenge for the corresponding unitAmount, and the entire payment/verification/delivery flow proceeds automatically.

Payment Notifications

After a payment is verified and a credential is issued, Key0 notifies your backend so you can activate the subscription, update billing records, or trigger downstream workflows:

onPaymentReceived: async (grant) => {
  const headers = await authProvider();
  await fetch(`${BACKEND_API_URL}/internal/payment-received`, {
    method: "POST",
    headers: { "Content-Type": "application/json", ...headers },
    body: JSON.stringify(grant),
  });
},

The payment notification is fire-and-forget. If the backend is unreachable, the error is logged but the agent still receives their token. Design your backend to handle idempotent replays if needed.

Refund Cron Setup

The BullMQ-based refund cron automatically processes refunds for challenges that reached the PAID state but failed delivery. It runs on a configurable interval and only processes challenges older than a grace period:

const refundQueue = new Queue("refund-cron", { connection: bullConnection });
await refundQueue.add("process-refunds", {}, {
  repeat: { every: REFUND_INTERVAL_MS },
});

const cronWorker = new Worker("refund-cron", () => runRefundCron(), {
  connection: bullConnection,
});

The refund processor transitions challenges through PAID -> REFUND_PENDING -> REFUNDED (or REFUND_FAILED), sending USDC back to the payer on-chain.

The refund cron requires KEY0_WALLET_PRIVATE_KEY to sign refund transactions. Without it, the cron starts but skips processing. Never commit private keys to source control — use a secrets manager.

Running the Example

Start infrastructure

You need Redis running (required for BullMQ, and optionally for storage). If using Postgres for storage, start that as well.

docker run -d --name redis -p 6379:6379 redis:7-alpine

Configure environment variables

Create a .env file in the examples/standalone-service directory:

# Core
KEY0_PORT=3001
KEY0_NETWORK=testnet
KEY0_ACCESS_TOKEN_SECRET=your-secret-at-least-32-characters-long
KEY0_WALLET_ADDRESS=0xYourWalletAddress

# Backend
BACKEND_API_URL=http://localhost:3000
BACKEND_AUTH_STRATEGY=shared-secret
INTERNAL_AUTH_SECRET=your-internal-secret

# Storage
STORAGE_BACKEND=redis
REDIS_URL=redis://localhost:6379

# Token mode
TOKEN_MODE=native

Install dependencies

cd examples/standalone-service
bun install

Start the service

bun run start

You should see:

Key0 Standalone Service
   Port: 3001
   Network: testnet
   Storage: REDIS
   Token Mode: native
   Facilitation Mode: Standard
   Backend URL: http://localhost:3000
   Agent Card: http://localhost:3001/.well-known/agent.json
   A2A Endpoint: http://localhost:3001/agent

Refund cron:
  Interval     : 15s
  Grace period : 30s
  Status       : DISABLED (set KEY0_WALLET_PRIVATE_KEY)

Endpoints

Once running, the service exposes:

Endpoint	Description
`GET /.well-known/agent.json`	A2A agent card with capabilities and pricing
`POST /agent`	A2A JSON-RPC endpoint for challenge/proof flow
`GET /.well-known/mcp.json`	MCP discovery (when `mcp: true`)
`POST /mcp`	MCP Streamable HTTP endpoint
`GET /health`	Health check with storage, token mode, and network info
`GET /.well-known/token-info`	Token format metadata for backend integration

Testing

Verify the agent card

curl http://localhost:3001/.well-known/agent.json | jq .

Check health

curl http://localhost:3001/health | jq .

Expected response:

{
  "status": "ok",
  "service": "key0",
  "storage": "redis",
  "tokenMode": "native",
  "network": "testnet"
}

Run a full payment flow

Point an agent at this service to run a complete discovery-to-payment-to-access flow.

Test MCP discovery

curl http://localhost:3001/.well-known/mcp.json | jq .

Production Checklist

Before deploying to production:

Set KEY0_NETWORK=mainnet
Use a strong, randomly generated KEY0_ACCESS_TOKEN_SECRET (64+ characters)
Set BACKEND_AUTH_STRATEGY to shared-secret or jwt
Configure KEY0_PUBLIC_URL to your actual domain
Set KEY0_WALLET_PRIVATE_KEY to enable automatic refunds
Use a managed Redis (e.g., Upstash, ElastiCache) or managed Postgres
Run behind a reverse proxy (nginx, Cloudflare) with TLS
Set KEY0_WALLET_ADDRESS to a dedicated payment-receiving wallet

Source code

examples/standalone-service/server.ts

Examples

​Architecture

​What This Example Demonstrates

​Configuration Reference

​Core Settings

​Backend Communication

​Storage

​Gas Wallet and Refunds

​Agent Card Metadata

​Code Walkthrough

​Storage Backend Selection

​Auth Strategy Configuration

​Token Issuance Modes

​Plan Catalog

​Payment Notifications

​Refund Cron Setup

​Running the Example

​Endpoints

​Testing

​Verify the agent card

​Check health

​Run a full payment flow

​Test MCP discovery

​Production Checklist

Source code

Architecture

What This Example Demonstrates

Configuration Reference

Core Settings

Backend Communication

Storage

Gas Wallet and Refunds

Agent Card Metadata

Code Walkthrough

Storage Backend Selection

Auth Strategy Configuration

Token Issuance Modes

Plan Catalog

Payment Notifications

Refund Cron Setup

Running the Example

Endpoints

Testing

Verify the agent card

Check health

Run a full payment flow

Test MCP discovery

Production Checklist