Implementing Gas-Free Transactions with FluxRail Paymaster Transfers: A Practical Tutorial

Introduction

Gas fees have long been a barrier to mainstream blockchain adoption. Users must hold native tokens (ETH, MATIC, etc.) just to pay for transaction costs, creating friction in onboarding and user experience. FluxRail's Paymaster Transfers solve this by abstracting gas entirely. With a single API call, you can execute transfers where FluxRail sponsors the gas, allowing your users to send USDC, USDT, or other tokens without ever needing the chain's native asset. In this tutorial, we'll walk through implementing gas-free transfers using the FluxRail REST API.

Prerequisites

• A FluxRail account. Sign up at fluxrail.io and grab your API key (test keys start with flux_test_).
• Basic familiarity with curl or any HTTP client (fetch, axios, etc.).
• Test tokens (USDC or USDT) on a supported testnet. FluxRail's test mode simulates everything, so no real funds are needed.

Understanding the Pay Transfer Endpoint

The core endpoint for gas-free transfers is POST /api/v1/transfer. It accepts a payload with the source chain, token, recipient, amount, and a gasless flag (or simply use the paymaster feature by default). FluxRail will construct, sign, and broadcast the transaction, covering the gas cost. You can optionally provide a client_reference_id for idempotency and tracking.

Step 1: Create a Transfer

Let's send 10 USDC on Polygon (MATIC) to a recipient address. Replace YOUR_API_KEY with your test key.

curl -X POST https://api.fluxrail.io/api/v1/transfer \
  -H "X-API-Key: flux_test_YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "chain": "polygon",
    "token": "USDC",
    "amount": "10.00",
    "recipient": "0xRecipientAddressHere",
    "client_reference_id": "order-12345",
    "gasless": true
  }'

Response:

{
  "id": "txn_abc123",
  "status": "pending",
  "chain": "polygon",
  "token": "USDC",
  "amount": "10.00",
  "recipient": "0xRecipientAddressHere",
  "gas_sponsored": true,
  "client_reference_id": "order-12345",
  "created_at": "2025-03-15T10:30:00Z"
}

The transfer is created and pending. FluxRail will process it asynchronously. The gas_sponsored field confirms that gas was covered.

Step 2: Check Transfer Status

Poll the status using the transfer ID or your client_reference_id.

curl -X GET https://api.fluxrail.io/api/v1/transfer/txn_abc123 \
  -H "X-API-Key: flux_test_YOUR_API_KEY"

Or by reference ID (prefix with ref:):

curl -X GET https://api.fluxrail.io/api/v1/transfer/ref:order-12345 \
  -H "X-API-Key: flux_test_YOUR_API_KEY"

Once confirmed, the status will be completed:

{
  "id": "txn_abc123",
  "status": "completed",
  "tx_hash": "0xabcd...",
  "gas_fee": "0.002",
  "gas_sponsored": true,
  ...
}

Step 3: Handle Webhooks for Real-Time Updates

FluxRail sends signed webhooks to your endpoint when transfer statuses change. First, configure a webhook URL in the dashboard or via API:

curl -X POST https://api.fluxrail.io/api/v1/webhooks \
  -H "X-API-Key: flux_test_YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://your-app.com/webhooks/fluxrail",
    "events": ["transfer.completed", "transfer.failed"]
  }'

When a transfer completes, you'll receive a payload like this:

{
  "event": "transfer.completed",
  "data": {
    "id": "txn_abc123",
    "status": "completed",
    "tx_hash": "0xabcd...",
    "client_reference_id": "order-12345"
  },
  "livemode": false
}

Verify the signature in the X-FluxRail-Signature header using HMAC SHA-256 with your webhook secret (found in the dashboard).

Step 4: Estimate Bitcoin Fees (Optional)

If you're working with Bitcoin, you can get fee recommendations before sending:

curl -X GET https://api.fluxrail.io/api/v1/transfer/btc/fees \
  -H "X-API-Key: flux_test_YOUR_API_KEY"

Response includes fast, medium, slow fee rates. For gas-free transfers on EVM chains, FluxRail handles fee estimation automatically.

Advanced: Bulk Transfers and Idempotency

Use client_reference_id to prevent duplicate transfers if your request is retried. FluxRail guarantees idempotency for 24 hours. For high-volume scenarios, you can create multiple transfers sequentially; each returns immediately with a pending status.

Testing with Sandbox

All the above works in test mode with a flux_test_ key. No real funds move, and no external chains are called. You can simulate any scenario, including failures, by using specific amounts (e.g., "amount": "0.00" triggers an error). Check the docs for sandbox behaviors.

Conclusion

Gas-free transfers with FluxRail's Paymaster eliminate a major UX hurdle. Your users can transact without ever holding ETH, MATIC, or other native tokens. The integration is a single REST call, and FluxRail handles the rest: gas sponsorship, transaction broadcasting, and webhook notifications. Try it today and remove friction from your dApp or payment flow.

For more details, visit the FluxRail Documentation.