Summary
I’d like to propose BuilderNet/rbuilder to support sponsored bundles, matching Titan (eth_sendBundle | Titan Builder Docs), Quasar (eth_sendBundle | Quasar Builder Docs), and many other block builders functionality.
How
When a bundle tx fails with LackOfFundForGasLimit (returned when the EOA can’t cover gas + value), the builder injects a transaction funding the EOA with the required amount so it executes. The builder only includes the bundle if its net effect increases the builder balance by more than the sponsored amount. This feature is automatically activated on the funding error, so no new field is required in the bundle.
Sponsored ETH is builder-fronted capital, so it should be netted out before refundable value is computed. Inclusion would require builder balance after >= builder balance before - sponsored gas, which keeps the “builder never loses on a bundle” invariant.
Why
This feature serves flows where the value arrives inside the bundle, but the sender starts with no ETH. Today the only path for this on BuilderNet is a self-funded bundle with a prepended sponsor tx, which forces every such sender to run and fund a separate EOA. BuilderNet is one of the few major builders without this feature currently.
I would be open to providing the initial feature implementation to the rbuilder repository if the feature