Errors & Revert

In the Stylus framework, errors are represented as Move structs. To use a struct as a revert reason, it must be annotated with the #[ext(abi_error)] attribute.

Revert

The framework provides a native revert function to halt execution and undo all state changes, returning the abi-encoded error to the caller.

module stylus::error;

/// Reverts the current transaction.
///
/// This function reverts the current transaction with a given error.
public native fun revert<T: copy + drop>(error: T);

The revert function is generic over the type T. For a successful compilation, T must be a struct annotated with #[ext(abi_error)].

Error encoding

The framework ensures that Move errors follow the Solidity ABI specification. This allows Ethereum-compatible tools like Etherscan to decode and display the error correctly.

The error selector is a 4-byte identifier that tells the EVM which specific custom error is being triggered. It is calculated by taking the first 4 bytes of the Keccak256 hash of the error’s signature string. The signature string is composed of the error struct name followed by the field types in parentheses.

#[ext(abi_error)]
public struct ExampleError {
    message: String,
    code: u8
}

For the struct above, the signature string is ExampleError(string,uint8). The selector is derived as: keccak256("ExampleError(string,uint8)") → 0x... (first 4 bytes).

The complete error message consists of the 4-byte selector followed by the ABI-encoded fields of the struct.

Tip

The signature Error(string) is a special, built-in Solidity error type. If you define a struct that results in this signature, the Stylus node and most Ethereum explorers will automatically decode and display the raw string message.