Implementing Post Conditions

Learn how to add post-conditions to protect your Stacks transactions.

Post-conditions are a powerful security feature in Stacks that protect users from unexpected transaction outcomes. This tutorial will walk you through implementing post-conditions in your applications to ensure transactions behave exactly as users expect.

What you'll learn

• Construct post-conditions using the Pc helper API

• Add post-conditions to different transaction types

• Configure post-condition modes for transaction security

• Implement post-conditions for STX, fungible tokens, and NFTs

• Handle semi-fungible tokens (SFTs) with post-conditions

Prerequisites

• Basic understanding of Stacks transactions

• Stacks.js library installed (npm install @stacks/transactions)

• A development environment set up for Stacks

Constructing post-conditions

The Pc helper in Stacks.js provides a fluent, BDD-inspired API for constructing post-conditions. Start with Pc.principal() to specify which address will be verified, then chain methods to define the condition.

import { Pc } from '@stacks/transactions';

// Basic structure of a post-condition
const postCondition = Pc
  .principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
  .willSendEq(1000)
  .ustx();

The Pc helper uses method chaining for intuitive condition building. Your IDE will provide auto-completion for available methods at each step.

Available transfer methods

Post-conditions support different comparison operators and asset types. Choose the appropriate method based on your security requirements.

STX and fungible token methods

// Exact amount
Pc.principal(address).willSendEq(1000).ustx();

// Greater than or equal
Pc.principal(address).willSendGte(500).ustx();

// Less than
Pc.principal(address).willSendLt(2000).ustx();

Comparison methods available:

• .willSendEq(amount) - Exactly equal to amount

• .willSendGte(amount) - Greater than or equal to amount

• .willSendGt(amount) - Greater than amount

• .willSendLte(amount) - Less than or equal to amount

• .willSendLt(amount) - Less than amount

Asset type methods

// STX transfers
.ustx()

// Fungible token transfers
.ft(contractAddress, tokenName)

// NFT transfers
.nft(assetIdentifier, tokenId)

NFT-specific methods

// Ensure NFT is sent
Pc.principal(address).willSendAsset().nft(...);

// Ensure NFT is NOT sent
Pc.principal(address).willNotSendAsset().nft(...);

Setting the post-condition mode

The post-condition mode determines how the Stacks blockchain handles asset transfers not explicitly covered by your post-conditions. This is a critical security setting.

import { PostConditionMode, makeContractCall } from '@stacks/transactions';

const tx = await makeContractCall({
  // ... other transaction properties
  postConditionMode: PostConditionMode.Deny, // Recommended default
  postConditions: [
    // your post-conditions here
  ],
});

Mode options:

• PostConditionMode.Deny (default): Transaction fails if any unspecified transfers occur

• PostConditionMode.Allow: Transaction allows transfers beyond specified post-conditions

Common implementation patterns

STX transfer post-conditions

Protect STX transfers by specifying exact amounts or ranges.

import { Pc, makeSTXTokenTransfer } from '@stacks/transactions';

// Exact amount post-condition
const exactAmountCondition = Pc
  .principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
  .willSendEq(1000)
  .ustx();

// Use in a transaction
const tx = await makeSTXTokenTransfer({
  recipient: 'ST1PQHQKV0RJXZFY1DGX8MNSNYVE3VGZJSRTPGZGM',
  amount: 1000,
  postConditions: [exactAmountCondition],
  postConditionMode: PostConditionMode.Deny,
  // ... other properties
});

Fungible token post-conditions

Ensure fungible tokens are transferred as expected in contract calls.

import { Pc, makeContractCall } from '@stacks/transactions';

// Minimum amount condition
const ftCondition = Pc
  .principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
  .willSendGte(500)
  .ft('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6.token-ft', 'token');

// Use in a contract call
const tx = await makeContractCall({
  contractAddress: 'STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6',
  contractName: 'token-transfer',
  functionName: 'transfer',
  functionArgs: [
    // ... function arguments
  ],
  postConditions: [ftCondition],
  // ... other properties
});

NFT transfer post-conditions

Control NFT ownership changes with specific post-conditions.

import { Pc, Cl } from '@stacks/transactions';

// Ensure NFT is sent
const sendNftCondition = Pc
  .principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
  .willSendAsset()
  .nft('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6.nft-contract::nft-name', Cl.uint(1));

// Ensure NFT is NOT sent (protection against unwanted transfers)
const keepNftCondition = Pc
  .principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
  .willNotSendAsset()
  .nft('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6.nft-contract::nft-name', Cl.uint(1));

Use willNotSendAsset() to protect valuable NFTs from being transferred unexpectedly.

Semi-fungible token (SFT) post-conditions

SFTs require special handling as they have both fungible and non-fungible properties.

import { Cl, Pc } from '@stacks/transactions';

// SFT as NFT (specific token ID)
const sftNftCondition = Pc
  .principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
  .willSendAsset()
  .nft(
    'ST1PQHQKV0RJXZFY1DGX8MNSNYVE3VGZJSRTPGZGM.sft-contract::sft-id',
    Cl.tuple({
      'token-id': Cl.uint(1),
      owner: Cl.principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
    })
  );

// SFT as FT (amount-based)
const sftFtCondition = Pc
  .principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6')
  .willSendEq(500)
  .ft('ST1PQHQKV0RJXZFY1DGX8MNSNYVE3VGZJSRTPGZGM.sft-contract', 'sft-token');

Multiple post-conditions

Complex transactions often require multiple post-conditions to fully protect all asset transfers.

const tx = await makeContractCall({
  // ... transaction properties
  postConditions: [
    // Sender must send exactly 1000 uSTX
    Pc.principal(senderAddress).willSendEq(1000).ustx(),

    // Contract must send at least 100 tokens to user
    Pc.principal(contractAddress).willSendGte(100)
      .ft(contractAddress + '.my-token', 'my-token'),

    // User must not lose their NFT
    Pc.principal(senderAddress).willNotSendAsset()
      .nft(nftContract + '::my-nft', Cl.uint(1)),
  ],
  postConditionMode: PostConditionMode.Deny,
});