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Fast Fill is a feature that allows you to accelerate the destination fill of a cross-chain request. This is useful if you want to speed up the completion of a bridge or swap operation before it’s landed onchain.

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Requirements

Before you can start fast filling requests, you need:
  1. An API Key — Required to authenticate your fast fill requests. Request an api key.
  2. Enterprise Partnership — Reach out to the Relay team to become an Enterprise Partner. Learn more about the Enterprise Partner Program.
  3. An Outstanding Balance Limit - When reaching out to the Relay team to enable this feature, you’ll need to set an outstanding balance limit. This is the maximum amount of funds that can be fast filled at any given time. These will clear as transactions land onchain.
Once you’re set up, you can begin fast filling requests.

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How to use it?

To Fast Fill a request you simply need to call the Fast Fill API with the request ID of the request you want to fast fill after you’ve submitted it onchain but before it finalizes. The request will be instantly filled and the total amount will be added to your outstanding balance. Once the transaction lands on chain and Relay indexes your transaction, the funds will be cleared from your outstanding balance.

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Example

This example demonstrates the full fast fill flow: getting a quote, submitting the transaction on-chain, calling the fast fill API, and monitoring the status. 
import { getClient, createClient } from "@relayprotocol/relay-sdk";
import { createWalletClient, createPublicClient, http } from "viem";
import { base } from "viem/chains";

// 1. Setup Wallet - Initialize your wallet using your preferred method
const account = {}; // Your wallet account (e.g., privateKeyToAccount, or injected wallet)
const wallet = createWalletClient({
  account,
  chain: base,
  transport: http(),
});

const publicClient = createPublicClient({
  chain: base,
  transport: http(),
});

// 2. Initialize the Relay client
createClient({
  baseApiUrl: "https://api.relay.link",
  source: "YOUR_APP_NAME",
});

// 3. Get a quote using the SDK
const quote = await getClient().actions.getQuote({
  chainId: 8453, // Base
  toChainId: 42161, // Arbitrum
  currency: "0x0000000000000000000000000000000000000000", // ETH
  toCurrency: "0x0000000000000000000000000000000000000000", // ETH
  amount: "100000000000000", // 0.0001 ETH
  tradeType: "EXACT_INPUT",
  wallet,
});

const requestId = quote.steps[0].requestId;
console.log(`Request ID: ${requestId}`);

// 4. Execute the quote and fast fill once the transaction is submitted
let txHash: string;

await getClient().actions.execute({
  quote,
  wallet,
  onProgress: async ({ currentStep, currentStepItem, txHashes }) => {
    // Once transaction is submitted, trigger fast fill immediately
    if (txHashes && txHashes.length > 0 && !txHash) {
      txHash = txHashes[0];
      console.log(`Transaction submitted: ${txHash}`);

      // 5. Call Fast Fill immediately after submitting (before waiting for confirmation)
      const fastFillResult = await getClient().actions.fastFill({
        requestId,
      });
      console.log("Fast fill initiated:", fastFillResult);
    }

    // Monitor progress
    if (currentStep && currentStepItem) {
      console.log(`Step: ${currentStep.action}, Status: ${currentStepItem.status}`);
    }
  },
});

console.log("Bridge completed successfully!");

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Caveats

  • Slippage may lead to a surplus or shortage in your outstanding balance. This may eventually max out your outstanding balance. In order to clear this reach out to the Relay team to resolve this. If you have a good estimate of the final amount you can use the solverInputCurrencyAmount parameter to specify the exact amount of input currency you want to use for the fill, thus minimizing slippage.
  • We recommend protecting your API key on the backend by not exposing it to the client.