Fee Layer

aztec-fee produces the ExecutionPayload that pays a transaction’s fee.

Context

Aztec transactions pay fees in Fee Juice. The payment payload has to be built and merged with the user’s call payload before the wallet passes the combined request to the account provider.

Several scenarios require different strategies:

• Established account holding Fee Juice.
• Sponsored onboarding (e.g. sandbox, faucet).
• First-time payer whose Fee Juice was deposited on L1.

Design

A single trait unifies the strategies:

#[async_trait]
pub trait FeePaymentMethod: Send + Sync {
    async fn get_asset(&self) -> Result<AztecAddress, Error>;
    async fn get_fee_payer(&self) -> Result<AztecAddress, Error>;
    async fn get_fee_execution_payload(&self) -> Result<ExecutionPayload, Error>;
}

Shipped implementations:

A private-FPC strategy is on the roadmap but is not currently shipped; consumers needing it must implement FeePaymentMethod themselves.

Implementation

Each strategy’s get_fee_execution_payload returns an ExecutionPayload containing:

• The protocol-contract calls that move Fee Juice to the sequencer.
• Any auth witnesses needed to satisfy those calls.

aztec-wallet::SendOptions::fee selects the strategy; the wallet merges the payload with the user’s payload and hands the combined payload to the account provider, which wraps everything in the entrypoint.

AccountEntrypointMetaPaymentMethod in aztec-account is a specialised method that piggybacks the payment through the account’s own entrypoint, avoiding a separate public call.

Edge Cases

• Estimate drift: fee estimates can diverge between simulation and inclusion; GasSettings::max_fee_per_gas SHOULD include a safety margin.
• L1 not yet ready: for FeeJuicePaymentMethodWithClaim, the L1 deposit MUST be observable on L2 (via is_l1_to_l2_message_ready) before proving; use wait_for_l1_to_l2_message_ready when building onboarding flows.
• Sponsor refusal: SponsoredFeePaymentMethod may be refused by the sponsor contract at inclusion; applications SHOULD handle the resulting revert.

Security Considerations

• The fee payer can always be inspected via get_fee_payer before sending — use this to display the intended payer in UIs.
• Claim messages MUST NOT be replayed; consumption is bound to the specific tx by the kernel.
• Sponsored flows create trust relationships with the sponsor contract — review its policy before depending on it in production.

References

• aztec-fee reference
• aztec-ethereum reference — prepare_fee_juice_on_l1 for claim-based flows.
• Concepts: Fees