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The x84 Facilitator is a production x402 v2 payment facilitator that verifies and settles USDC payments on Solana. It implements the Coinbase x402 protocol with x84-specific extensions for atomic, attestation, and delegated settlement modes.

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Base URL

https://facilitator.x84.ai

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How it works

The facilitator sits between A2A clients and the Solana network. When a client wants to pay for an agent request:
1

Resource server returns 402

The A2A gateway returns a 402 Payment Required response with payment requirements (amount, token, recipient, facilitator address).
2

Client builds transaction

The client constructs a Solana VersionedTransaction with an SPL Token TransferChecked instruction. The facilitator’s public key is set as the fee payer.
3

Client signs and sends to facilitator

The client partially signs the transaction (payer signature only) and sends it to POST /settle along with the payment requirements.
4

Facilitator verifies

The facilitator deserializes the transaction, validates the SPL instructions, checks amounts and recipients, and confirms the fee payer matches.
5

Facilitator co-signs and submits

The facilitator adds its signature (as fee payer) and submits the fully-signed transaction to Solana. It confirms the transaction on-chain and returns the signature.

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Endpoints

MethodPathDescription
GET/supportedDiscover supported payment kinds, extensions, and signer keys
POST/verifyVerify a payment payload without settling
POST/settleVerify, co-sign as fee payer, and submit to Solana
GET/healthHealth check

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x84 extensions

On top of standard x402 SVM exact payments, the facilitator supports three x84-specific settlement modes via the extensions field in the payment payload:
ExtensionIDDescription
Atomicx84-atomicClient submits transaction directly to Solana. Facilitator verifies the on-chain tx signature and receipt PDA.
Attestationx84-attestationSimilar to atomic, but uses an attestation receipt PDA instead of a CPI receipt.
Delegatedx84-delegatedUses a Delegation PDA for budget-based spending. Supports both client-side (delegation PDA reference) and server-side (ed25519 signature with 60s timestamp window) modes.

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Supported assets

Currently the facilitator supports:
NetworkAssetToken
Solana Devnet (solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1)Gh9ZwEmdLJ8DscKNTkTqPbNwLNNBjuSzaG9Vp2KGtKJrUSDC (devnet)
Query GET /supported for the current list of supported kinds, extensions, and signer keys.

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Verification rules

The /verify and /settle endpoints enforce these validation rules:
  1. Deserialization — Transaction must be a valid Solana VersionedTransaction
  2. Allowed programs — Only SPL Token, ComputeBudget, and Memo programs are permitted
  3. Transfer instruction — Must contain exactly one TransferChecked instruction
  4. Amount match — Transfer amount must match the payment requirements
  5. Mint match — Token mint must match the required asset
  6. Destination match — Transfer destination must match the payTo address
  7. Fee payer — Must be the facilitator’s public key (from /supported signers)
  8. Replay prevention — Already-signed transactions are rejected

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Idempotency

The /settle endpoint is idempotent. If the same transaction is submitted twice, the second call returns the original settlement result (same transaction signature) without resubmitting.

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v1 backward compatibility

The paymentPayload field accepts both formats:
  • v2 (recommended) — Structured PaymentPayload object with x402Version, accepted, payload, and optional extensions
  • v1 (legacy) — Base64-encoded JSON string, automatically normalized to v2 internally

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Integration example

import { X84A2AClient } from "@x84/sdk";

const client = new X84A2AClient({
  wallet: myKeypair,
  rpcUrl: "https://api.devnet.solana.com",
  facilitatorUrl: "https://facilitator.x84.ai",
});

// The SDK handles the full x402 flow automatically:
// 1. Sends request → gets 402 → builds tx → calls /settle → retries with proof
const result = await client.sendMessage(agentUrl, {
  message: { role: "user", parts: [{ type: "text", text: "Hello" }] },
});
For manual integration without the SDK, see the endpoint reference below.