Optimizing Claude Code's Xcode build token usage

This post is based on Antoine van der Lee’s great article How to reduce token usage in Claude Code, Codex, and Cursor on his blog SwiftLee. All credit goes to him. I’m just writing this down as a note to my future self.

Background

In his article, Antoine explains how AI agents like Claude Code spend a lot of tokens reading Xcode’s verbose build output, which is mostly noise.

Antoine therefore uses xcsift to compress the Xcode build output into a compact format that keeps errors, warnings and test results. You can install it with Homebrew:

brew install xcsift

Antoine tried adding instructions to use xcsift to his agents, but found that this was mostly ignored. Instead, he therefore uses hooks to automatically pipe all build commands through xcsift.

The Claude Code Hook Script

While Antoine’s article covers how to register hooks for Codex, Cursor, and Claude Code, I will only include a simpler script that’s needed for Claude Code.

First, save this script as ~/.claude/hooks/xcode-output.py. It runs before every Bash command, and rewrites xcodebuild, swift build and swift test commands to pipe their output through xcsift:

#!/usr/bin/env python3
"""A Claude Code PreToolUse hook that pipes Xcode build output through xcsift."""

import json
import re
import sys

BUILD = re.compile(r"^(xcodebuild|swift\s+(build|test))\b")
FILTERS = ("xcsift", "xcbeautify", "xcpretty")
UNSAFE = re.compile(r"[|;<>`\n]|\$\(|(?<!&)&(?!&)")


def rewrite(command):
    """Return a rewritten command, or None if it should be left untouched."""
    if any(f in command for f in FILTERS) or UNSAFE.search(command):
        return None
    *prefix, build = [part.strip() for part in command.split("&&")]
    if not BUILD.match(build) or any(not p.startswith("cd ") for p in prefix):
        return None
    steps = prefix + [f"{build} 2>&1 | xcsift -f toon -w"]
    return "set -o pipefail && " + " && ".join(steps)


def main():
    data = json.load(sys.stdin)
    tool_input = data.get("tool_input", {})
    command = rewrite(tool_input.get("command", "").strip())
    if command is None:
        print("{}")
        return
    print(json.dumps({
        "hookSpecificOutput": {
            "hookEventName": "PreToolUse",
            "permissionDecision": "allow",
            "permissionDecisionReason": "Piping build output through xcsift",
            "updatedInput": {**tool_input, "command": command},
        }
    }))


if __name__ == "__main__":
    main()

The script leaves commands untouched if they use a formatter, or if they contain pipes, redirects or other command chains that would be unsafe to rewrite.

The set -o pipefail makes builds still report failures, even though the output goes through xcsift.

Registering the Hook

Register the hook as a PreToolUse hook for the Bash tool in ~/.claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "python3 ~/.claude/hooks/xcode-output.py",
            "timeout": 5
          }
        ]
      }
    ]
  }
}

If your settings file already has a hooks section, just add the PreToolUse entry to it. Restart Claude Code, and every build command will now return compressed output.

Conclusion

This agent hook can save a lot of tokens when working on Xcode projects with Claude Code. Make sure to read Antoine’s article for more tips, and how to set up the same hook for Codex and Cursor.