Quanta

Pay-per-call structured data for autonomous AI agents. x402-metered, MCP-native.

A small, real reference service: a FastAPI API serves a structured dataset, every data call is metered by the x402 protocol (a USDC micropayment on Base or Solana), and the same data is exposed as MCP tools so an agent can discover and call it natively. Backed by Postgres/Supabase (SQLite locally).

This is a working slice of the agent-commerce stack: agent -> MCP tool -> x402 paywall -> data.

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Why it exists

It's the exact shape of the systems I build for: a live API that sells structured data to autonomous agents, monetized per call, with an MCP front door. It runs the real protocols (official x402 SDK + official MCP SDK), not a mock.

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Architecture

                      ┌──────────────────────────────────────────┐
   AI agent  ───────▶ │  MCP tools (FastMCP)   /mcp              │
   (Claude, etc.)     │    list_assets · get_asset · get_signal  │
                      └───────────────┬──────────────────────────┘
                                      │  same data layer
   HTTP client ─────▶ ┌──────────────▼──────────────────────────┐
   (curl, SDK)        │  FastAPI  /v1/*   ── x402 middleware ────┤
                      │    402 challenge ▶ verify ▶ settle ▶ data │
                      └───────────────┬──────────────────────────┘
                                      │
                      ┌───────────────▼──────────────┐   ┌──────────────────┐
                      │  data.py (single source)      │   │  x402 facilitator │
                      │  assets + usage_log           │   │  verify + settle  │
                      └───────────────┬──────────────┘   │  Base / Solana    │
                                      │                   └──────────────────┘
                      ┌───────────────▼──────────────┐
                      │  Postgres / Supabase (SQLite) │
                      └───────────────────────────────┘

The x402 request loop

1. Agent calls a /v1/* route with no payment.
2. Server replies 402 Payment Required + the accepted rails (USDC on Base Sepolia and Solana devnet), price, and pay-to address.
3. Agent signs a payment and retries with an X-PAYMENT proof.
4. The facilitator verifies + settles on-chain; the server returns the data and writes a row to usage_log (who paid, which chain, which record).

Metering is a feature flag. If the x402 wheel isn't present (local dev on an unsupported Python), the app logs a warning and serves unmetered so you can still work on everything else. It never fails to boot.

Layout

app/
  main.py        FastAPI app, /v1/* data routes, x402 wired in
  x402_setup.py  multi-chain (EVM + Solana) metering, import-safe
  data.py        seed data + shared queries + usage log (one source of truth)
  models.py      Asset + UsageLog (SQLAlchemy 2.0 async)
  db.py          async engine; DATABASE_URL swaps SQLite <-> Supabase
  config.py      typed settings
mcp_server/
  server.py      FastMCP tools (x402-gated per tool), stdio or HTTP
client/
  pay_and_fetch.py  agent demo: unpaid -> 402, then paid -> data
scripts/seed.py  create tables + load sample data
tests/test_api.py
Dockerfile · railway.json

Run it locally

python3.12 -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt
cp .env.example .env            # defaults to SQLite + testnet
python -m scripts.seed
uvicorn app.main:app --port 4021

Prove the metering is live:

curl -i http://localhost:4021/v1/assets/BTC      # -> 402 Payment Required + rails
curl    http://localhost:4021/internal/usage     # -> audit trail (free)
python -m client.pay_and_fetch                   # unpaid 402, then paid loop

MCP (stdio, for Claude Desktop / MCP Inspector):

python -m mcp_server.server

Deploy

• Railway / Cloud Run / Render: Docker build (Python 3.12). railway.json is included; health check is /health.
• Database: set DATABASE_URL to your Supabase Postgres string (postgresql+asyncpg://...).
• Payments: testnet works with the public x402.org facilitator and no keys. For mainnet settlement, set CDP_API_KEY_ID / CDP_API_KEY_SECRET (Coinbase CDP) and switch the network.

Honest scope

• The dataset is a clearly-labelled sample snapshot, not a live market feed - enough to prove the pay-per-call mechanics end to end.
• Settlement runs on testnet (Base Sepolia / Solana devnet). Flipping to mainnet is config, not code.
• The full paid loop in client/pay_and_fetch.py needs a funded test wallet; the unpaid 402 proves the contract without one.

Stack

FastAPI · SQLAlchemy 2.0 (async) · Postgres/Supabase · official x402 SDK (EVM + SVM) · official MCP SDK (FastMCP) · Docker.