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External wallets use eoa mode (Fusion) and do not support EIP-7702. This is the minimal Supertransaction API flow: request a quote, sign the payload returned by the API, then execute.
Signing the payload is the critical step. The API returns payloadToSign and a quoteType (permit or onchain) that tells you how to sign.

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Minimal EOA (Fusion) flow

import { createWalletClient, custom } from "viem";

const walletClient = createWalletClient({
  account: userAddress,
  transport: custom(window.ethereum),
});

const quoteResponse = await fetch("https://api.biconomy.io/v1/quote", {
  method: "POST",
  headers: {
    "X-API-Key": "YOUR_API_KEY",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    mode: "eoa",
    ownerAddress: userAddress,
    fundingTokens: [
      {
        tokenAddress: "0xToken",
        chainId: 1,
        amount: "1000000",
      },
    ],
    composeFlows: [...],
  }),
});

const quote = await quoteResponse.json();
const payload = quote.payloadToSign[0];

let signature;
if (quote.quoteType === "permit") {
  signature = await walletClient.signTypedData({
    ...payload.signablePayload,
    account: userAddress,
  });
} else {
  const txHash = await walletClient.sendTransaction({
    to: payload.to,
    data: payload.data,
    value: BigInt(payload.value || "0"),
  });
  signature = txHash;
}

await fetch("https://api.biconomy.io/v1/execute", {
  method: "POST",
  headers: {
    "X-API-Key": "YOUR_API_KEY",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    ...quote,
    payloadToSign: [{ ...payload, signature }],
  }),
});