Install this Claude Code hook.
Name: Auto-format on Save
Save to: ~/.claude/hooks/auto-format-on-save.sh
Make executable: yes

---
#!/usr/bin/env node
// Hook type: PostToolUse
// Trigger: Write, Edit (file tool calls)
//
// Reads the tool result from stdin, extracts the file path,
// and runs Prettier on it if a Prettier config is present.

import { execSync } from "child_process"
import { existsSync } from "fs"
import { join } from "path"
import { readFileSync } from "fs"

const input = JSON.parse(readFileSync("/dev/stdin", "utf8"))

// Only act on Write / Edit tool calls
const toolName = input?.tool_name ?? ""
if (!["Write", "Edit"].includes(toolName)) process.exit(0)

const filePath = input?.tool_input?.file_path ?? input?.tool_input?.path
if (!filePath) process.exit(0)

// Detect Prettier config in project root (walk up from file)
function findPrettierConfig(start: string): boolean {
  const configs = [
    ".prettierrc",
    ".prettierrc.json",
    ".prettierrc.js",
    ".prettierrc.cjs",
    "prettier.config.js",
    "prettier.config.cjs",
    "prettier.config.mjs",
  ]
  let dir = start
  while (dir !== "/") {
    if (configs.some((c) => existsSync(join(dir, c)))) return true
    dir = join(dir, "..")
  }
  return false
}

const dir = filePath.replace(/\/[^\/]+$/, "")
if (!findPrettierConfig(dir)) process.exit(0)

try {
  execSync(`npx prettier --write "${filePath}"`, { stdio: "inherit" })
} catch {
  // Non-fatal: Prettier may not support this file type
}