Post Conditions
The Big Picture
Post-conditions are constraints you attach to a transaction that define exactly what assets (STX, SIP-010 tokens, NFTs) are allowed to move and how much.
If the underlying smart contract execution would violate your declared limits, the entire transaction aborts.
Even if a smart contract contains unexpected logic, it cannot move assets beyond what your post-conditions permit.
Post-conditions are constructed on the client-side usually by the client-side developer. They are part of the signed transaction. Contracts cannot modify them.
Wallets will parse the signed transaction and display the declared post-conditions to the user before broadcasting.
What are post-conditions?
Post-conditions are assertions about an on-chain transaction that must be met; otherwise, the transaction will abort during execution. In other words, post-conditions act as a safety net, allowing you to specify what state changes can occur in a transaction.
At times, the client-side developer is not the same person who wrote the underlying smart contract and may not be deeply familiar with its internal logic, nested external contract calls, or edge cases. This logic helps limit the amount of damage that can be done to a user and their assets, whether due to a bug or malicious behavior.
Put simply, post conditions are a set of conditions that must be met before a user's transaction will execute. The primary goal behind post conditions is to limit the amount of damage that can be done to a user's assets due to a bug, intentional or otherwise.
Post conditions are an additional safety feature built into the Stacks protocol itself that help to protect end users. Rather than being a feature of Clarity smart contracts, they are implemented on the client side and meant to be an additional failsafe against malicious contracts.
They are sent as part of the transaction when the user initiates it, meaning we need to implement post-conditions on the frontend. Whenever you are transferring an asset (fungible or non-fungible) from one address to another, you should take advantage of post conditions.
Post-conditions act as a safety net, allowing you to specify what state changes can and should occur during the execution of a transaction.
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