> **Can't find what you're looking for?** Use `search_docs` on the docs MCP server at `https://www.getfoundry.sh/api/mcp` to find what you need.
>
> **Have feedback?** Use `submit_feedback` on the same MCP server.

## Unused error

**Severity**: `Info`
**ID**: `unused-error`

### What it does

Reports custom error declarations that are never used by a revert, `require`, or selector
reference anywhere in the compiled sources.

Errors in interfaces and abstract contracts are excluded because external implementations
and consumers may rely on their ABI.

### Why is this bad?

An unused error is dead code. It suggests a missing revert path or a leftover from a refactor.

### Example

```solidity
error Unauthorized(); // declared but never referenced
```

Use instead:

```solidity
error Unauthorized();

function withdraw() external {
    if (msg.sender != owner) revert Unauthorized();
}
```

### Limitations

A selector hardcoded in inline assembly or built with `abi.encodeWithSignature("Err(...)")` does
not reference the declaration and is not seen as a use.