Build a transaction

When you call sdk.buy() or sdk.sell(), the SDK calls this endpoint (POST /v1/tx/swap) to build an unsigned transaction. This page explains what happens under the hood.

Building Transactions

You build an unsigned transaction via REST, then sign it locally. The API constructs an optimized transaction with the correct accounts, instructions and a fresh blockhash.

Endpoint

POST /v1/tx/swap

Request Body

{
  "inputMint":  "So11111111111111111111111111111111111111112",
  "outputMint": "TOKEN_MINT_ADDRESS",
  "amount":     0.002,
  "swapMode":   "exactIn",
  "slippage":   0.05,
  "priority":   "fast"
}

Fields:

• inputMint: mint address of the token you are spending
• outputMint: mint address of the token you are receiving
• amount: amount of the exact side — input when swapMode is exactIn, output when exactOut
• swapMode: exactIn (spend a fixed amount) or exactOut (receive a fixed amount) — exactIn is the right default
• slippage: maximum allowed slippage (0.05 = 5%)
• priority: transaction priority (economy, fast, faster, fastest) — see Priority

What You Get Back

The API returns several critical pieces:

• submissionToken: Links this transaction server-side. You’ll need this when submitting.
• unsignedTransaction: Base64-encoded wire transaction ready for signing. Don’t modify this.
• expiresAt: ~30 second validity window due to Solana blockhash expiry. Sign and submit quickly.
• estimates: Pre-execution estimates for input/output amounts and price impact.
• priorityCost: The actual priority fee for this transaction. Check this before signing if you have cost limits.

Act Fast

The transaction expires in ~30 seconds. Build, sign and submit in quick succession.

For complete API reference, see: POST /v1/tx/swap.

Code Examples

TypeScript

import axios from 'axios';

const API_KEY = 'api_your_key_here';
const API_BASE_URL = 'https://api.darkfibre.dev/v1';
const WSOL_MINT = 'So11111111111111111111111111111111111111112';

const api = axios.create({
  baseURL: API_BASE_URL,
  headers: { 'Authorization': `Bearer ${API_KEY}` },
  timeout: 30000,
});

async function buildBuyTransaction() {
  const response = await api.post('/tx/swap', {
    inputMint: WSOL_MINT,
    outputMint: 'TOKEN_MINT_ADDRESS',
    amount: 0.002,
    swapMode: 'exactIn',
    slippage: 0.05,
    priority: 'fast',
  });

  const { submissionToken, unsignedTransaction, expiresAt, priorityCost } = response.data.data;

  console.log('Submission token:', submissionToken);
  console.log('Expires at:', expiresAt);
  console.log('Priority cost:', priorityCost, 'SOL');

  return { submissionToken, unsignedTransaction };
}

buildBuyTransaction().catch(console.error);

Python

import requests

API_KEY = "api_your_key_here"
API_BASE_URL = "https://api.darkfibre.dev/v1"
WSOL_MINT = "So11111111111111111111111111111111111111112"

def build_buy_transaction():
    response = requests.post(
        f"{API_BASE_URL}/tx/swap",
        headers={"Authorization": f"Bearer {API_KEY}"},
        json={
            "inputMint": WSOL_MINT,
            "outputMint": "TOKEN_MINT_ADDRESS",
            "amount": 0.002,
            "swapMode": "exactIn",
            "slippage": 0.05,
            "priority": "fast",
        },
        timeout=30,
    )
    response.raise_for_status()

    data = response.json()["data"]

    print(f"Submission token: {data['submissionToken']}")
    print(f"Expires at: {data['expiresAt']}")
    print(f"Priority cost: {data['priorityCost']} SOL")

    return {
        "submissionToken": data["submissionToken"],
        "unsignedTransaction": data["unsignedTransaction"],
    }

build_buy_transaction()

cURL

curl -X POST https://api.darkfibre.dev/v1/tx/swap \
  -H "Authorization: Bearer api_your_key_here" \
  -H "Content-Type: application/json" \
  -d '{
    "inputMint":  "So11111111111111111111111111111111111111112",
    "outputMint": "TOKEN_MINT_ADDRESS",
    "amount":     0.002,
    "swapMode":   "exactIn",
    "slippage":   0.05,
    "priority":   "fast"
  }'

Understanding the Response

The response contains several important fields:

• submissionToken: Links this transaction server-side. You’ll need this when submitting.
• unsignedTransaction: Base64-encoded wire transaction ready for signing.
• expiresAt: ~30s validity window due to blockhash expiry.
• estimates: Pre-execution estimates for input/output amounts and price impact.
• priorityCost: The actual priority fee for this transaction. Check this before signing if you have cost limits.