> ## Documentation Index
> Fetch the complete documentation index at: https://docs.optimism.io/llms.txt
> Use this file to discover all available pages before exploring further.

# RPC Engine Types

The [`op-alloy-rpc-types-engine`](https://docs.rs/op-alloy-rpc-types-engine/latest/op_alloy_rpc_types_engine/) crate provides Optimism types for interfacing
with the Engine API in the OP Stack.

Optimism defines a custom payload attributes type called [`OpPayloadAttributes`](https://docs.rs/op-alloy-rpc-types-engine/latest/op_alloy_rpc_types_engine/struct.OpPayloadAttributes.html).
`OpPayloadAttributes` extends alloy's [`PayloadAttributes`](https://docs.rs/alloy-rpc-types-engine/latest/alloy_rpc_types_engine/payload/struct.PayloadAttributes.html) with a few fields: transactions,
a flag for enabling the tx pool, the gas limit, and EIP 1559 parameters.

Optimism also returns a custom type for the `engine_getPayload` request for both V3 and
V4 payload envelopes. These are the [`OpExecutionPayloadEnvelopeV3`](https://docs.rs/op-alloy-rpc-types-engine/latest/op_alloy_rpc_types_engine/payload/v3/struct.OpExecutionPayloadEnvelopeV3.html) and
[`OpExecutionPayloadEnvelopeV4`](https://docs.rs/op-alloy-rpc-types-engine/latest/op_alloy_rpc_types_engine/payload/v4/struct.OpExecutionPayloadEnvelopeV4.html) types, which both wrap payload envelope types
from [`alloy-rpc-types-engine`](https://crates.io/crates/alloy-rpc-types-engine).