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Auto-format every file Claude Code edits with a PostToolUse hook

#claudecode #ai #productivity #devops

Claude Code writes decent code, but it doesn't always match your formatter. You can keep asking it to "run prettier", or you can make formatting automatic with a PostToolUse hook.

What a PostToolUse hook does

Hooks are shell commands Claude Code runs at fixed points. A PostToolUse hook runs right after a tool call finishes. Point it at the edit tools and your formatter runs every time a file changes.

Add this to .claude/settings.json in your project:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|MultiEdit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | xargs -r npx prettier --write --ignore-unknown"
          }
        ]
      }
    ]
  }
}
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The hook gets the tool call as JSON on stdin. jq pulls out the edited file's path, and Prettier formats just that file. --ignore-unknown keeps it quiet on file types Prettier doesn't handle.

Other languages

Swap the formatter for your stack:

  • Python: xargs -r ruff format
  • Go: xargs -r gofmt -w
  • Rust: xargs -r rustfmt

Tips

  • Keep the command fast. It runs after every edit, so format one file, not the whole repo.
  • Commit .claude/settings.json so the whole team gets the same behavior.
  • Run /hooks in Claude Code to check the hook is loaded.

Shared settings, a risky-git PreToolUse hook, a short CLAUDE.md and a read-only reviewer subagent all ship ready to copy in Vildanden Agent Config. The free sample is enough to get started today.

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