Syscall Handlers Overview

Syscall handlers are the Rust side of the Cairo VM syscall interface. They decode CallContract requests and return the data your Cairo module expects.

Core types

CallContractRequest and CallContractResponse define the memory layout.
SyscallHandler owns the routing logic and per-chain handlers.
CallHandler in crates/syscall_handler/src/traits.rs defines the handler interface.

Keccak syscall

The Keccak syscall selector is handled by KeccakHandler, which exposes Cairo keccak hashing to the VM.

Relevant code:

crates/syscall_handler/src/lib.rs
crates/types/src/cairo/new_syscalls.rs

Routing behavior

The handler inspects the contract_address and calldata:

Special handlers: 'debug', 'arbitrary_type', 'injected_state', 'unconstrained'
All other calls use the chain ID encoded in calldata to select EVM vs Starknet handlers.

The EVM and Starknet handlers use separate internal CallHandlerId enums to select header vs storage vs receipt logic.

EVM call handler IDs

When routing to the EVM handler, the numeric contract_address selects the call handler:

0: header
1: account
2: storage
3: transaction
4: receipt
5: log

Handler state

Handlers store: