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What is x402?

x402 is an open protocol that uses HTTP status code 402 Payment Required to enable native, per-request API monetization — with no accounts, API keys, or billing infrastructure.

The Problem

Traditional API monetization requires:

  • Developer account registration
  • API key management
  • Monthly subscription billing
  • Rate limit enforcement
  • Credit card processing

This works for humans with browsers, but completely breaks for AI agents — they can't sign up for accounts or manage billing.

How x402 Fixes It

x402 adds a payment layer directly into the HTTP request cycle. A resource server announces its price inside the 402 response. The client signs a USDC payment and retries the request with a X-PAYMENT header. A facilitator verifies and settles the transaction on-chain.

The result: any HTTP client with a funded wallet can pay for any x402 endpoint — zero setup.

The Protocol Flow

1. Client sends a normal HTTP request
   GET /api/price-feed  HTTP/1.1
 
2. Server responds with 402 + payment requirements
   HTTP/1.1 402 Payment Required
   X-Payment-Required: [{"scheme":"exact","network":"solana:5eykt4...","maxAmountRequired":"10000","asset":"EPjFW...","payTo":"BqVq3..."}]
 
3. Client builds and signs USDC payment
   (using @x402/fetch, @x402/node, or MCP orchus_fetch)
 
4. Client retries with signed payment
   GET /api/price-feed  HTTP/1.1
   X-Payment: <signed-payment-payload>
 
5. Server forwards to facilitator for verification + settlement
   POST x402.agentstrail.ai/settle
 
6. Facilitator submits Solana tx, returns receipt
 
7. Server delivers the response
   HTTP/1.1 200 OK

x402 Versions

VersionHeaderNetwork formatUsed by
v2X-Payment-RequiredCAIP-2 (solana:5eykt4...)@x402/core 2.x
v1X-Payment-Requiredhuman-readable (solana, base)older SDKs

The Orchus MCP server and facilitator support both v1 and v2 on Solana and Base.

Key Concepts

Facilitator — the trusted third party that verifies signed payments and submits Solana transactions. Orchus runs its own at x402.agentstrail.ai.

Scheme — the payment scheme. Currently exact (pay exactly the listed amount in USDC).

Asset — the USDC SPL token mint address. Verified by the facilitator before settlement.

maxAmountRequired — the price in atomic USDC units (6 decimals). 10000 = 0.01 USDC.

Further Reading