Add a PostToolUse hook in `.claude/settings.json` that runs after edits.

```json
{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [{ "type": "command", "command": "{{command}}" }]
      }
    ]
  }
}
```

Set `{{command}}` to your formatter/linter (e.g. `npx prettier --write "$CLAUDE_FILE_PATHS"`).
The hook's stdout/stderr is fed back to the model as feedback — make failures loud and actionable.