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TIP-1007: Fee Token Introspection

Protocol Version: TBD
Status: In Review
Authors: Georgios Konstantopoulos

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Abstract

TIP-1007 adds a getFeeToken() view function to the FeeManager precompile that returns the fee token address being used for the current transaction. This enables smart contracts to introspect which TIP-20 token is paying for gas fees during execution, allowing for dynamic logic based on the fee token choice.

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Motivation

Tempo transactions support paying gas fees in any USD-denominated TIP-20 token via the fee token preference system. However, prior to this TIP, there was no way for a smart contract to determine which fee token is being used for the current transaction during execution. This capability was requested by a partner. It could be useful for contracts that want to:
  • Adjust their internal logic based on which fee token is being used
  • Provide fee token-aware pricing or routing decisions
  • Emit events or logs that include the fee token for off-chain indexing
  • Implement fee token-specific behavior in cross-chain messaging

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Specification

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New Function

The following function is added to the IFeeManager interface: 
interface IFeeManager {
    // ... existing functions ...

    /// @notice Returns the fee token being used for the current transaction
    /// @return The address of the TIP-20 token paying for gas fees
    /// @dev This value is set by the protocol before transaction execution begins.
    ///      Returns address(0) if no fee token has been set (e.g., in eth_call
    ///      simulations where the transaction handler does not run).
    function getFeeToken() external view returns (address);
}

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Behavior

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Fee Token Resolution

The fee token returned by getFeeToken() is the same token that was resolved by the protocol during transaction validation, following the fee token preference rules.

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Storage

The fee token is stored in transient storage (EIP-1153) within the FeeManager precompile. This means:
  • The value is automatically cleared at the end of each transaction
  • No persistent storage writes occur, minimizing gas costs
  • The value is consistent across all calls within a transaction (including internal calls and subcalls)

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Timing

The fee token is set by the protocol in the validate_against_state_and_deduct_caller handler phase, before any user code executes. This ensures the value is available throughout the entire transaction execution.

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Gas Cost

Reading the fee token costs the standard warm transient storage read cost (100 gas for TLOAD). This is the cost of calling getFeeToken() itself; callers should account for additional gas used by the CALL opcode to invoke the precompile.

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Edge Cases

ScenarioReturn Value
Normal transactionThe resolved fee token address
Free transaction (zero gas price)The resolved fee token (may still be set)
eth_call simulationaddress(0) (no transaction context)
The only case where address(0) is returned is in simulation contexts (e.g., eth_call) where the protocol handler does not execute.

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Example Usage

import { IFeeManager } from "./interfaces/IFeeManager.sol";

contract FeeTokenAware {
    IFeeManager constant FEE_MANAGER = IFeeManager(0xfeeC000000000000000000000000000000000000);
    address constant PATH_USD = 0x20C0000000000000000000000000000000000000;

    function doSomething() external {
        address feeToken = FEE_MANAGER.getFeeToken();

        if (feeToken == PATH_USD) {
            // User is paying fees in pathUSD
        } else if (feeToken != address(0)) {
            // User is paying fees in a different USD stablecoin
        } else {
            // No fee token context (e.g., eth_call simulation)
        }
    }
}

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Invariants

  • getFeeToken() must return a consistent value across all calls within the same transaction
  • getFeeToken() must return address(0) in simulation contexts (e.g., eth_call) where no transaction handler runs
  • getFeeToken() must be callable from staticcall contexts without reverting
  • The fee token returned must match the token used for actual fee deduction in collectFeePreTx and collectFeePostTx
  • Reading the fee token must not modify any state (view function)