Agentic Builders

RelAI Facilitator

Explore gas-sponsored x402 v2 payments on SKALE Base, Avalanche, and Solana

The facilitation process in x402 — verification and settlement of payments — is critical to ensuring the resource is paid for properly.

Why use an x402 facilitator

Consistent verification and settlement across schemes and networks
Offload blockchain operations and complexity from resource servers
Serve multiple merchants/apps via a stable /verify and /settle API
Help clients discover merchants via the /list endpoint

Architecture at a glance

Client:

Calls protected resources and constructs payment payloads

Resource Server:

Advertises payment requirements, verifies/settles payments, fulfills requests

Consistent verification and settlement across schemes and networks
Offload blockchain operations and complexity from resource servers
Serve multiple merchants/apps via a stable /verify and /settle API
Help clients discover merchants via the /list endpoint

Architecture at a glance

Client:

Calls protected resources and constructs payment payloads

Resource Server:

Advertises payment requirements, verifies/settles payments, fulfills requests

Facilitator Server:

Verifies payment payloads and executes settlements via standard endpoints

Blockchain Networks:

Execute and confirm payments

Supported Networks

RelAI Facilitator currently supports the following blockchain networks:

skale-base

avalanche

base

solana

Available Facilitators

Name	Endpoint	Demo
RelAI	https://facilitator.x402.fi	RelAI Demo

How It Works

Standard x402 v2 flow with gas sponsoring:

User requests resource - GET request to protected API endpoint
Backend returns 402 - Payment required with x402 v2 response
User signs permit - Off-chain EIP-2612 signature (free, no gas)
Frontend sends payment - Payment payload with signed permit
Backend calls facilitator - /settle endpoint to execute transaction
Facilitator executes - permit() + transferFrom() (RelAI pays CREDIT/AVAX for gas)
Backend returns resource - 200 OK with requested data

API Reference

GET/facilitator/supported

{
  "kinds": [
    { "x402Version": 2, "scheme": "exact", "network": "skale-base", "feePayer": "0x1892f72..." },
    { "x402Version": 2, "scheme": "exact", "network": "avalanche", "feePayer": "0x1892f72..." },
    { "x402Version": 2, "scheme": "exact", "network": "base", "feePayer": "0x1892f72..." },
    { "x402Version": 2, "scheme": "svm-exact", "network": "solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp", "feePayer": "4x4Zhcqi..." }
  ],
  "extensions": [],
  "signers": { 
    "eip155:*": ["0x1892f72fdB3A966b2AD8595aA5f7741Ef72d6085"],
    "solana:*": ["4x4ZhcqiT1FnirM8Ne97iVupkN4NcQgc2YYbE2jDZbZn"]
  }
}

POST/facilitator/verify

{ "success": true, "verified": true }

POST/facilitator/settle

{ "success": true, "txHash": "0x123...", "blockNumber": 12345678 }

GET/facilitator/discovery/resources

Bazaar discovery endpoint. This returns a catalog of discovered resources.

GET /facilitator/discovery/resources?type=http&limit=20&offset=0

{
  "x402Version": 2,
  "items": [],
  "pagination": {
    "limit": 20,
    "offset": 0,
    "total": 0
  }
}

Analytics & Leaderboard (Public)

These endpoints provide ecosystem visibility based on successful settle events. They are not part of the core x402 v2 facilitator interface.

GET/facilitator/activity

GET /facilitator/activity?window=24h

GET/facilitator/live-settlements

GET /facilitator/live-settlements?limit=20

GET/facilitator/leaderboard

GET /facilitator/leaderboard?window=24h&limit=50

What is payer?

The leaderboard ranks user wallets (payers), not the RelAI sponsor fee payer.

EVM networks: payer is the end-user address from the payment payload (userAddress).
Solana: RelAI sponsors fees as feePayer, but payer is extracted as the user signer from the submitted transaction.

Zauth Protection

Zauth is a trust infrastructure layer that provides endpoint verification and agent-driven testing for x402 payments. It protects users from paying for non-working or malicious endpoints by validating them before payment processing.

Pre-Payment Validation

Zauth validates target endpoints before payment processing using HEAD/GET requests with the same headers and parameters as the actual request.

Payment Protection

If validation fails, Zauth blocks payment and returns a 402 response with detailed error information, preventing funds from being charged.

Smart Validation

Uses HEAD requests for GET endpoints (lighter) and same method for POST/PUT/PATCH with full headers, query params, and body validation.

Telemetry & Monitoring

Comprehensive event tracking and monitoring for endpoint health, validation results, and payment protection statistics.

How Zauth Works

Request Interception

Zauth middleware intercepts all x402 payment requests before processing.

Endpoint Validation

Validates target URL with same method, headers, query params, and body.

Payment Decision

Allows payment if validation passes, blocks with 402 if validation fails.

User Notification

Shows detailed error modal with validation results and protection status.

Error Response Format

{
  "error": "Target endpoint validation failed",
  "status": 404,
  "statusText": "Not Found", 
  "message": "Endpoint returned error before payment - payment blocked",
  "x402Version": 2,
  "accepts": [{
    "scheme": "exact",
    "network": "solana",
    "maxAmountRequired": "0",
    "resource": "https://api.example.com/endpoint",
    "description": "Endpoint validation failed (404) - payment blocked",
    "payTo": ""
  }]
}

Benefits

100% Payment Protection

Users never pay for non-working endpoints

Real-time Validation

Instant endpoint health checking

Developer Friendly

Easy integration with existing x402 setup

Comprehensive Monitoring

Full telemetry and analytics dashboard

Zauth Extensions

Zauth extensions allow facilitators to perform endpoint validation before payment processing. Clients can optionally include target URLs in payment payloads for pre-payment validation.

Optional Validation

Clients can optionally include targetUrl in extensions.zauth for facilitator-side validation before payment settlement.

Payment Protection

If validation fails, facilitator blocks payment with maxAmountRequired: "0" and returns detailed error information.

Standard x402 v2

Uses official x402 v2 extensions format with proper schema validation and backward compatibility.

Flexible Implementation

Facilitators can choose to implement Zauth validation or ignore extensions based on their requirements.

How It Works

Client Receives 402 with Zauth Extensions

Facilitator returns payment requirements with zauth extension info and schema.

Client Includes targetUrl in Payment

Client adds targetUrl to extensions.zauth in payment payload for validation.

Facilitator Validates Endpoint

Facilitator performs HEAD request to targetUrl before payment processing.

Payment Decision

If validation passes, continue payment. If fails, block with maxAmountRequired: "0".

Example: Client Request with Zauth

POST /verify
Content-Type: application/json

{
  "x402Version": 2,
  "payload": {
    "authorization": "0x...",
    "signature": "0x...",
    "from": "0x...",
    "to": "0x...",
    "value": "1000000"
  },
  "extensions": {
    "zauth": {
      "targetUrl": "https://api.example.com/protected-data"
    }
  }
}

Example: Facilitator Response (Validation Failed)

HTTP/1.1 402 Payment Required
Content-Type: application/json

{
  "x402Version": 2,
  "error": "Target endpoint validation failed",
  "status": 404,
  "statusText": "Not Found",
  "message": "Endpoint returned error before payment - payment blocked",
  "accepts": [{
    "scheme": "exact",
    "network": "eip155:84532",
    "maxAmountRequired": "0",
    "asset": "0x036CbD53842c5426634e7929541eC2318f3dCF7e",
    "payTo": "0x209693Bc6afc0C5328bA36FaF03C514EF312287C",
    "description": "Endpoint validation failed (404) - payment blocked"
  }]
}

Implementation Guide

For Facilitator Developers:

Check for Zauth extensions

Look for paymentPayload.extensions?.zauth?.targetUrl

Perform validation

Use HEAD request with proper headers and timeout

Block on failure

Return 402 with maxAmountRequired: "0" and error details

Continue on success

Proceed with normal verify/settle flow

Supported Facilitators

RelAI SVM

Solana (SVM) networks with Zauth extensions and targetUrl validation

RelAI EVM

EVM networks (SKALE, Avalanche, Base) with Zauth extensions and targetUrl validation

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