Onix

A PayBox-style remote MCP server for ChatGPT: each ChatGPT user signs in with OAuth, gets a Circle developer-controlled wallet on Arc Testnet, can send USDC, and can pay x402 APIs.

What it does

1. ChatGPT connects to https://<your-host>/mcp with OAuth (not “No Auth”).

2. The user signs in with Google or an email code.

3. Onix creates one Circle wallet on ARC-TESTNET for that user (or reuses the existing one).

4. ChatGPT can then call:

   • get_wallet — address, explorer, faucet

   • get_balance — USDC on Arc Testnet

   • estimate_send_usdc — fee preview

   • send_usdc — Circle REST transfer (requires confirm: true)

   • x402_probe — inspect an HTTP 402 paywall

   • x402_pay — sign and pay an x402 API on Arc Testnet

   • list_transfers — recent sends from this app

   • mint_probe — paste a collection URL; check whitelist and remaining mints

   • mint_nft — preview, then mint or reserve until drop time (confirm: true)

   • cancel_mint — cancel an upcoming reservation

   • list_mints — this user's mint history

Related MCP server: PayPls MCP Server

NFT mint sites

Onix does not scrape an unknown HTML mint button. Your collection site should expose a JSON spec (the page URL, /.well-known/onix-mint.json, or <link rel="onix-mint" href="...">):

{
  "version": 1,
  "name": "My collection",
  "network": "eip155:5042002",
  "chainId": 5042002,
  "contract": "0x…",
  "mintStartsAt": "2026-09-01T18:00:00.000Z",
  "priceUsdc": "0",
  "eligibilityUrl": "https://your-site.com/api/mint/eligibility",
  "abiFunctionSignature": "mint(uint256)",
  "abiParameters": ["$quantity"],
  "maxPerWallet": 3
}

abiParameters placeholders: $to (the user's Onix wallet), $quantity, $proof (from eligibility).

Eligibility (GET eligibilityUrl?address=0x…):

{
  "address": "0x…",
  "whitelisted": true,
  "maxMints": 3,
  "minted": 0,
  "remaining": 3,
  "proof": []
}

Onix only mints if whitelisted is true, quantity is 1..remaining, and the user confirmed. If mintStartsAt is in the future, it reserves and mints automatically at that time.

Example spec for discovery testing: https://onixmpc.vercel.app/mint/example.json (does not mint on-chain).

Prerequisites

• Node.js 22+

• A Circle Console testnet API key

• A registered entity secret

• A public HTTPS URL so ChatGPT can reach this server (Cloudflare Tunnel or ngrok)

Setup

cd arcbox
copy .env.example .env

Edit .env:

PUBLIC_URL=https://your-tunnel-host
PORT=8787
COOKIE_SECRET=a-long-random-string
CIRCLE_API_KEY=TEST_API_KEY:...
CIRCLE_ENTITY_SECRET=your-64-char-entity-secret

npm install
npm run dev

Open PUBLIC_URL in a browser and create an account. Confirm a wallet address appears on /dashboard. Fund it from the Circle faucet (Arc Testnet USDC).

Connect ChatGPT

1. Copy https://<your-host>/mcp

2. On ChatGPT web, open Settings and turn on Developer mode

3. Plugins → Browse plugins → + → New Plugin

4. Name: Onix

5. Server URL: the /mcp URL

6. Authentication: OAuth — do not leave No Auth selected

7. Save, connect, and complete the Onix login

Then in chat:

What’s my Arc Testnet wallet and USDC balance?

Send 1.00 USDC to 0x…

send_usdc first returns a preview unless ChatGPT passes confirm: true.

Auth model

• OAuth 2.1 authorization code + PKCE (S256)

• Dynamic client registration and Client ID Metadata Documents

• Refresh tokens (required by ChatGPT)

• resource / audience bound to /mcp

• One Circle EOA per Onix user, inside one shared wallet set

This is developer-controlled custody: Circle holds the keys, your server signs with the entity secret. Do not treat this as a non-custodial PayBox clone.

Firebase auth

Sign-in is Google or email + 6-digit code. Firebase Auth holds the user; Firestore maps email / uid to the Circle wallet.

1. Create a Firebase project and enable Authentication → Google.

2. Create a Firestore database (production mode is fine — the Admin SDK bypasses rules).

3. Deploy firestore.rules (clients have no direct access).

4. Add onixmpc.vercel.app and localhost under Authentication → Settings → Authorized domains.

5. Project settings → service account → generate a private key.

6. Project settings → Your apps → Web app: copy apiKey, authDomain, appId.

For email codes, set RESEND_API_KEY (or install the Firebase Trigger Email extension, which reads the mail collection).

Deploy on Vercel

1. Push this repo to GitHub.

2. Import the repo in Vercel.

3. Set these environment variables in the Vercel project:

PUBLIC_URL=https://onixmpc.vercel.app
COOKIE_SECRET=
CIRCLE_API_KEY=
CIRCLE_ENTITY_SECRET=
MCP_DANGEROUSLY_ALLOW_INSECURE_ISSUER_URL=false
FIREBASE_PROJECT_ID=
FIREBASE_CLIENT_EMAIL=
FIREBASE_PRIVATE_KEY=
FIREBASE_WEB_API_KEY=
FIREBASE_AUTH_DOMAIN=
FIREBASE_APP_ID=
RESEND_API_KEY=
EMAIL_FROM=

PUBLIC_URL must be the live https:// origin (no trailing slash). After the first deploy, copy the Vercel URL into PUBLIC_URL and redeploy if needed.

4. ChatGPT plugin URL: https://your-project.vercel.app/mcp

User accounts and email→wallet maps live in Firestore once Firebase is configured.

Local-only note

ChatGPT will not talk to http://localhost. Use a tunnel, or the Vercel HTTPS URL, and set PUBLIC_URL to that origin.

Safety

• Testnet only

• Daily send cap via SEND_DAILY_LIMIT_USDC (default 100)

• Sends require an explicit confirm: true tool argument