x402 Payment Protocol

x402 is an open protocol for machine-to-machine payments using the HTTP 402 Payment Required status code. It enables AI agents to pay for services programmatically using USDC stablecoin.

x402 is live on api.openaccountant.ai today — every paid skill and chain is gated behind @x402/hono payment middleware in addition to the Polar.sh bearer-token path.

Discovery

Three ways for an agent to find out that Open Accountant speaks x402:

/.well-known/x402 — machine-readable JSON: network, facilitator, pay-to, per-route prices
- API (authoritative): https://api.openaccountant.ai/.well-known/x402
- Site mirror: https://openaccountant.ai/.well-known/x402
- Docs mirror: https://docs.openaccountant.ai/.well-known/x402
Link headers — every response includes Link: </.well-known/x402>; rel="payment"; type="application/json"
/x402/demo — always returns HTTP 402 with a valid payment-requirements body. $0.001 fixed price. Useful for sandbox testing and scanner detection.

Protected Routes

Route	Price range	Bought with
`GET /content/skills/{slug}`	$0.05 – $0.25 per call	x402 or Polar bearer
`GET /content/chains/{slug}`	$0.05 per call	x402 or Polar bearer
`GET /x402/demo`	$0.001	x402 only (sandbox)

Full price table lives at /.well-known/x402.

How It Works

1. Agent GETs api.openaccountant.ai/content/skills/tax-prep
2. API returns HTTP 402 + accepts: [{ scheme, price, network, payTo }]
3. Agent signs a USDC transfer on Base and includes it in X-PAYMENT header
4. API verifies with the x402 facilitator and returns the skill markdown

Via MCP

Agents connected to the Open Accountant MCP server can also invoke paid content as MCP tools:

get_skill_content({ slug })
get_chain_content({ slug })

When called without auth these return an isError result containing a JSON body with code: 402, the payment_requirements, and a pointer to the REST endpoint for clients that need the real X-PAYMENT exchange.

Example — Demo Endpoint

curl -i https://api.openaccountant.ai/x402/demo

HTTP/2 402
content-type: application/json
cache-control: no-store

{
  "x402_version": 2,
  "error": "payment_required",
  "resource": "https://api.openaccountant.ai/x402/demo",
  "accepts": [{ "scheme": "exact", "price": "$0.001", "network": "...", "payTo": "0x..." }],
  "facilitator": "https://x402.org/facilitator"
}

Stack

Middleware: @x402/hono — applied in api/src/middleware/auth.ts
Scheme: exact via @x402/evm/exact/server
Network: Base (Sepolia in dev, Base mainnet in production)
Facilitator: https://x402.org/facilitator (public)
Pay-to address: X402_PAY_TO_ADDRESS secret on the Cloudflare Worker

API Authentication — bearer + x402 dual-auth
MCP Server — invoke paid content as MCP tools
x402 spec