Gas tokens: the other spend your USDC tracker doesn't see
GetAgentCap team · published 10/13/2026
An agent that pays in USDC on Base or Solana usually also needs a second asset to move it. Fees on Base are paid in ETH and fees on Solana are paid in SOL. A tracker that only reads USDC transfers is looking at the price of the goods and ignoring the delivery charge.
Two assets, two ledgers
A USDC transfer on Base is an ERC-20 call, and the transaction that carries it pays its network fee in ETH. On Solana the transfer is an SPL token instruction, and the fee is paid in SOL. The fee never shows up in the USDC transfer event, because it is a property of the transaction, not of the token movement.
That means a wallet's USDC balance and its fee-token balance move independently. You can have plenty of USDC and still be unable to send a payment because the fee token ran out.
When the agent isn't the one paying the fee
Some x402 flows use a signed transfer authorization (EIP-3009 on EVM chains), where the payer signs and a facilitator submits the transaction. In that shape the facilitator's address pays the network fee, not the agent's wallet.
If you watch only the agent's wallet you will see the USDC leave and no fee at all. If the agent submits its own transactions, you will see both. Which one applies depends on how each service you call is wired, so don't assume it holds across vendors.
Why it matters for budgets
A per-agent budget denominated in USDC is silent about fees. For small payments the fee can be a visible share of the amount, and a retry loop pays it every time, including for attempts that fail after the fee is charged.
Failed transactions can still cost a fee, because the network charges for the computation it performed. Cheap per call, but not free, and invisible in a USDC-only view.
What to track
At minimum, watch the fee-token balance of any wallet that submits its own transactions, and alert before it reaches zero, since an empty fee balance stops payments as surely as an empty USDC balance.
If you reconcile against an accountant's books, record fees as their own line rather than folding them into the payment amount. They are paid in a different asset, at a different price, to a different counterparty.