> ## Documentation Index
> Fetch the complete documentation index at: https://docs.utexo.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Signing an Intent for Solana

> Cosign the prebuilt Solana versioned transaction while preserving signature order.

Versioned-transaction cosigning lets the user authorize a swap by signing a prebuilt Solana transaction that encodes the exact execution parameters. The user signs it locally, preserving the required signature order, and returns it as the approval. Funds remain secure until the resolver executes the swap according to the authorized transaction.

## Signature order

When `approval_mechanism` is `cosign`, `params_to_sign.transaction` is a hex-encoded versioned transaction. After signing, the signatures must be in this order:

| Position | Signer             |
| -------- | ------------------ |
| `0`      | Resolver           |
| `1`      | User               |
| `2`      | Backend (optional) |

## Example

```ts theme={null}
import { Keypair, VersionedTransaction } from "@solana/web3.js";
import bs58 from "bs58";

const BASE_URL = "https://api-swap.utexo.com/affiliate";
const API_KEY = process.env.UTEXO_SWAP_API_KEY!;
const keypair = Keypair.fromSecretKey(bs58.decode(process.env.PRIVATE_KEY!));
const destinationAddress = process.env.DESTINATION_ADDRESS!;

async function api<T>(path: string, body?: unknown): Promise<T> {
  const response = await fetch(`${BASE_URL}${path}`, {
    method: body === undefined ? "GET" : "POST",
    headers: { "Content-Type": "application/json", "X-API-Key": API_KEY },
    body: body === undefined ? undefined : JSON.stringify(body),
  });
  const text = await response.text();
  if (!response.ok) throw new Error(`${path} failed: ${response.status} ${text}`);
  return (text ? JSON.parse(text) : undefined) as T;
}

const userAddress = keypair.publicKey.toBase58();

// 1. Quote: WSOL on Solana -> WETH on Ethereum
const quote = await api<any>("/v1/quotes/best", {
  source_chain: 4,
  source_token: "So11111111111111111111111111111111111111112",
  dest_chain: 1,
  dest_token: "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
  amount: 0.01,
  slippage_bps: "50",
  swap_type: "standard",
  deposit_type: "escrowed",
  retail_user_id: null,
  user_meta: null,
  affiliate_fees: null,
});

// 2. Intent
const intent = await api<any>("/v1/intents", {
  quote_id: quote.id,
  user_source_address: userAddress,
  user_destination_address: destinationAddress,
  refund_address: userAddress,
  user_source_public_key: userAddress,
});

// 3. Cosign, keeping the resolver (0) and backend (2) signatures in place
const tx = VersionedTransaction.deserialize(
  Uint8Array.from(Buffer.from(intent.params_to_sign.transaction, "hex")),
);
const existing = [...tx.signatures];
tx.sign([keypair]);
tx.signatures[0] = existing[0];
if (tx.signatures[2] !== undefined) {
  tx.signatures[2] = existing[2];
}

// 4. Approval
await api(`/v1/intents/${intent.intent_id}/approvals`, {
  type: "cosign",
  signed_data: {
    transaction: Buffer.from(tx.serialize()).toString("hex"),
    user_address: userAddress,
  },
});
```