Assert / Assert_eq

Leo provides assert and assert_eq for runtime checks. If an assertion fails, the entire transaction is rejected - no state changes take effect.

assert(amount > 0u64);            // Fails if amount is zero
assert_eq(self.caller, owner);    // Fails if caller isn't the owner
assert_neq(sender, receiver);     // Fails if sender equals receiver

These are your primary tools for access control, input validation, and invariant enforcement. Unlike Solidity's require, failed assertions in Aleo don't consume gas - the transaction simply doesn't produce a valid proof.