Ethereum Paymaster Gas: Who Pays the Fee?
The paymaster contract signs the UserOperation, the bundler submits it, and somewhere between those two events the gas fee evaporates from a sponsor's balance. That is the entire mechanism in one breath, and most teams still get the accounting wrong. π― Ethereum Paymaster gas is not a donation and it is not free β it is a prepaid credit line that a smart contract extends to a user's transaction, and someone always settles the bill at the end of the entry point's execution loop.
Strip away the abstraction and you find three parties. The user signs intent. The bundler fronts ETH to the network. The paymaster reimburses the bundler through the EntryPoint's deposit ledger. When a Ethereum Paymaster validates a UserOperation, it is not paying the base fee directly β it is locking a portion of its staked deposit as collateral against the gas units the operation will consume. If validation passes and execution succeeds, the EntryPoint deducts actual gas from the paymaster's deposit and credits the bundler. If execution reverts, the paymaster still eats the validation gas. That asymmetry is where sponsors bleed.
The Deposit Ledger Nobody Audits
Every paymaster holds a balance inside the EntryPoint contract, and that balance is the only thing standing between a user and a failed transaction. Fund it wrong and the op bounces at simulation. Fund it right and you have effectively written an uncollateralized option to every address that can reach your validation logic. π§ The Paymaster gas fee flow is deterministic: validation gas plus execution gas plus a postOp overhead, all priced at the effective gas price the bundler negotiated. Miss the postOp overhead in your budget model and your deposit drains faster than your dashboard shows.
Staked paymasters with a throttle limit get preferential treatment in the bundler mempool. Unstaked ones get rate-limited or dropped entirely during congestion. This is not a soft preference β bundlers rank operations by profitability and risk, and an unstaked paymaster that touches storage during validation is a red flag that gets your op deprioritized. If you are sponsoring high-volume activity, stake first, throttle second, and only then think about pricing.
Who Actually Bears the Cost
Here is the blunt truth most sponsorship decks hide: the user pays nothing at the point of transaction, but the cost does not vanish. It lands on the paymaster's treasury, and it compounds with every op that passes validation but fails execution. A malicious user can burn your deposit by submitting operations that validate cleanly and revert on execution β you cover validation gas every single time. That is a griefing vector, not a bug, and the only defense is aggressive simulation, per-sender rate limits, and allowlists for the contracts your paymaster will sponsor.
Bundlers profit from the spread between the gas they pay the network and the gas they charge the paymaster. Watch that spread during volatile blocks β a bundler that fronted ETH at 30 gwei and settles at 55 gwei captures the difference, and your deposit absorbs it. The gas sponsorship model only stays solvent when the sponsor prices in volatility, not just the median gas price. π°
ERC-4337 Economics and the Prefund Trap
The prefund calculation is where sloppy implementations die. Before validation even runs, the EntryPoint computes a maximum cost β verification gas limit times max fee per gas, plus the call gas ceiling. That amount must be available in the paymaster deposit or the op is rejected outright. Overestimate your limits and you lock capital unnecessarily; underestimate and legitimate ops fail during spikes. The paymaster that survives scaling is the one that tunes these limits per contract interaction, not one global ceiling for everything.
Sponsorship is a liability instrument. Treat the paymaster deposit like a margin account, reconcile it against actual burn every epoch, and never let a single address drain more validation gas than your risk model allows. The mechanics are unforgiving and the ledger never lies.