---
description: Address event parameters should be indexed
---

## Address event parameters should be indexed

**Severity**: `Info`
**ID**: `event-fields`

Flags events whose `address` parameters are not declared `indexed`.

### What it does

For each event, identifies unindexed `address` parameters and reports a single warning
listing them. The lint respects the EVM cap on indexed parameters (3 for normal events,
4 for `anonymous`) and does not flag events that are already at capacity. Events that
already have at least one indexed parameter are left alone: the author has clearly chosen
what to index, so we stay silent.

### Why is this bad?

Indexed event parameters are stored as topics in the transaction log, which lets
off-chain indexers, explorers, and clients efficiently filter events by sender or
recipient. Leaving filterable fields unindexed forces consumers to scan and decode every
event, which is slow and brittle.

### Example

#### Bad

```solidity
event Transfer(address from, address to, uint256 value);
event Mint(address to, uint256 tokenId);
```

#### Good

```solidity
event Transfer(address indexed from, address indexed to, uint256 value);
event Mint(address indexed to, uint256 tokenId);
```

### Limitations

This lint is intentionally conservative:

- Only explicit `address` parameters are checked. `address payable`, custom types,
  interfaces, and user-defined value types are not unwrapped.
- Only actionable suggestions are reported. If an event has no remaining indexed
  slots (3 for normal events, 4 for `anonymous`), no warning is emitted. If only
  some slots remain, only the first parameters that could still be indexed are
  reported; already-indexed parameters are never suggested for change.
