Post-Conditions

In Stacks, transactions can have post-conditions.

These are additional security to ensure the 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 we transferred from an address

:::caution Post-conditions aren't perfect. They can't say anything about the end-state after a transaction. So, they can't guarantee the receival of FTs/NFTs, since they only check for sending. :::

An example adding a post-condition (of an address sending 1000 uSTX).

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

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

Post-Condition Mode

...aka "allow transfer of unspecified assets?"

In addition to the post-conditions 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.

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