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Immutables via Salt

Aztec contracts can commit immutable values directly into the contract's address by encoding them into the deployment salt, removing the need for a separate initialization transaction.

Overview​

Rather than storing immutables in private storage (which requires an initializer function and an extra transaction), the aztec-immutables-macro library encodes them into the contract's salt:

salt = poseidon2_hash([actual_salt, constant_0, constant_1, ...])

Since the salt is part of the address derivation, the immutable values become cryptographically bound to the contract's address itself.

Key benefits​

  • No initialization transaction — immutables are committed at deployment time, not in a separate setup call
  • Runtime verification — at execution time, capsule data is loaded and verified against the stored salt, ensuring data integrity
  • Persistent storage — immutables are persisted to the PXE's CapsuleStore after deployment, so capsules don't need to be attached to every transaction
  • Compatible with standard storage — works alongside #[storage] and initializers when needed

Performance​

Initialization cost is completely eliminated (no constructor transaction). The per-transaction overhead is approximately 1,098 gates (+0.2%) in the account entrypoint.

Getting started​

For installation instructions, usage examples, and a reference implementation of an initializerless Schnorr account contract, see the aztec-immutables-macro README.

The protocol ships a built-in account contract using the same pattern: see account deployment for how initializerless accounts work and creating accounts for using them from Aztec.js.