Post-Conditions

In Stacks, transactions can have post-conditions.

These offer additional security to ensure a transaction was executed as expected.

More precisely, adding post-conditions to a transaction can ensure that:

  • STX tokens were transferred from an address
  • FTs/NFTs were transferred from an address
Info

Post-conditions aren't perfect. They can't say anything about the end-state after a transaction. In other words, they can't guarantee the receipt of FTs/NFTs, since they only check for sending.

Here's an example of a post-condition where the principal must send 1000 uSTX, or else the transaction will abort:

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

const tx = await makeContractCall({
  // ...
  postConditions: [
    Pc.principal('STB44HYPYAT2BB2QE513NSP81HTMYWBJP02HPGK6').willSendEq(1000).ustx(),
  ],
});

Post-condition mode

In addition to the post-condition itself, we can also specify a mode for the transaction to verify asset transfers. The mode can be either Allow or Deny.

  • Allow means that the transaction can transfer any asset (assuming no conflicting post-conditions).
  • Deny means the transaction will fail if any asset transfers (not specified in the post-conditions) are attempted.

In either case, all post-conditions will still be checked. By default, transactions are set to Deny mode for additional security.

Transactions

Broadcasting

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Post-condition mode