ArcBox

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

What it does

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

2. The user creates an ArcBox account or logs in.

3. ArcBox 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 — App Kit kit.send() (requires confirm: true)

   • list_transfers — recent sends from this app

Related MCP server: PayPls MCP Server

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: ArcBox

5. Server URL: the /mcp URL

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

7. Save, connect, and complete the ArcBox 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 ArcBox 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.

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://your-project.vercel.app
COOKIE_SECRET=
CIRCLE_API_KEY=
CIRCLE_ENTITY_SECRET=
MCP_DANGEROUSLY_ALLOW_INSECURE_ISSUER_URL=false

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

Vercel functions are stateless. The local JSON user/wallet store lives in /tmp there, so accounts do not survive cold starts. Fine for a smoke test; for real use add a database next.

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