How to Auto-Format Files After Edits with a PostToolUse Hook

A PostToolUse hook fires after a tool finishes. A common use is formatting: every time Claude Code edits or writes a file, run your formatter so the diff stays clean and you never have to ask. This guide wires Prettier to run automatically on each file write.

• Prettier installed in the project (npm install -D prettier)
• Claude Code with hooks enabled
• A jq install or willingness to parse JSON in a small script

Step 1: Read the edited file path from the payload

PostToolUse passes the tool result on stdin as JSON. For Edit and Write the file path lives at tool_input.file_path. A tiny wrapper script extracts it and runs Prettier on just that file.

#!/usr/bin/env bash
set -euo pipefail

file=$(jq -r '.tool_input.file_path // empty')

if [[ -n "$file" && "$file" =~ \.(ts|tsx|js|jsx|json|css|md)$ ]]; then
  npx prettier --write "$file" >/dev/null 2>&1 || true
fi

Step 2: Register it for Edit and Write

Use a matcher that targets the file-changing tools. The pipe in the matcher value lets one hook cover both Edit and Write.

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write|MultiEdit",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/format.sh"
          }
        ]
      }
    ]
  }
}

Step 3: Watch it run

Ask Claude to make a change. After the edit completes, the hook runs Prettier on the touched file. The next time you look at the diff, the formatting is already correct.

Result: every file Claude Code edits is formatted automatically right after the write, keeping diffs clean without a single manual prettier run.