Build a STX post-condition

A helper function that creates a post-condition for STX token transfers using the Pc builder class, ensuring exact amounts are transferred as expected.

import {

  AnchorMode,

  broadcastTransaction,

  makeSTXTokenTransfer,

  Pc,

  PostConditionMode,

} from "@stacks/transactions";



// Create a post-condition that ensures exactly 10 STX is sent

const pc = Pc.principal("ST1PQHQKV0RJXZFY1DGX8MNSNYVE3VGZJSRTPGZGM")

  .willSendEq(10000000)  // 10 STX in micro-STX

  .ustx();



const txOptions = {

  recipient: "ST1SJ3DTE5DN7X54YDH5D64R3BCB6A2AG2ZQ8YPD5",

  amount: 10000000,

  senderKey: "753b7cc01a1a2e86221266a154af739463fce51219d97e4f856cd7200c3bd2a601",

  network: "testnet",

  postConditions: [pc],

  postConditionMode: PostConditionMode.Deny,

  anchorMode: AnchorMode.Any,

};



const transaction = await makeSTXTokenTransfer(txOptions);

const broadcastResponse = await broadcastTransaction(transaction);

console.log("Transaction ID:", broadcastResponse.txid);

Use cases

  • Securing STX token transfers with transfer amount validation
  • Protecting users from unexpected token transfers
  • Ensuring contract interactions behave as expected

Key concepts

The Pc builder provides a fluent interface for creating post-conditions:

  • Pc.principal() - Specify the principal that will send tokens
  • .willSendEq() - Ensure exactly this amount is sent (also supports willSendGte, willSendLte, willSendGt, willSendLt)
  • .ustx() - Specify the token type (micro-STX)

Post-condition types

// Exact amount

const exactPC = Pc.principal(address).willSendEq(1000000).ustx();



// Greater than or equal

const gtePC = Pc.principal(address).willSendGte(1000000).ustx();



// Less than or equal

const ltePC = Pc.principal(address).willSendLte(1000000).ustx();
Post-condition modes

Always use PostConditionMode.Deny to ensure transactions fail if post-conditions aren't met. This protects users from unexpected token transfers.