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Atlas Whoff
Atlas Whoff

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Claude Code Hooks: The Feature Nobody Talks About (But Should)

#claudecode #ai #productivity #devtools

Claude Code Hooks: The Feature Nobody Talks About (But Should)

If you use Claude Code, you probably know about slash commands and CLAUDE.md. But there's a layer most people skip: hooks.

Hooks let you execute shell commands in response to Claude's tool calls. They run on the server side, before or after Claude acts. Think of them as middleware for your AI coding session.

What Hooks Actually Do

Hooks fire at these lifecycle events:

  • PreToolUse — before Claude runs any tool (Read, Bash, Edit, etc.)
  • PostToolUse — after a tool completes
  • Stop — when Claude finishes a response
  • Notification — when Claude sends a notification

You configure them in ~/.claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "echo '[HOOK] Bash call incoming' >> ~/claude-audit.log"
          }
        ]
      }
    ]
  }
}
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That's it. Every time Claude tries to run a Bash command, your hook fires first.

Real Use Cases (From Production)

1. Inject Context on Every Tool Call

The most underrated pattern: return instructions from your hook, and Claude reads them as context injection.

The hook output (stdout) is fed back to Claude. So you can do things like:

#!/bin/bash
# PreToolUse hook for all Bash commands
echo "REMINDER: Use parallel execution for independent tasks. Use run_in_background for long operations."
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This is how you enforce coding standards without bloating your CLAUDE.md.

2. Block Dangerous Commands

Exit code 2 from a hook blocks the tool call entirely and feeds the reason back to Claude:

#!/bin/bash
# Block any rm -rf
if echo "$CLAUDE_TOOL_INPUT" | grep -q "rm -rf"; then
  echo "Blocked: rm -rf is not allowed. Use trash instead."
  exit 2
fi
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Now Claude can't accidentally nuke your working directory.

3. Auto-Log Everything 

#!/bin/bash
# PostToolUse hook — append to session log
echo "$(date +%H:%M:%S) | $CLAUDE_TOOL_NAME | $(echo $CLAUDE_TOOL_INPUT | head -c 100)" >> ~/Desktop/claude-session.log
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Get a timestamped audit trail of every tool call automatically.

4. Alert on Session End 

#!/bin/bash
# Stop hook — notify when Claude finishes
osascript -e 'display notification "Claude finished" with title "Claude Code" sound name "Glass"'
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Stop babysitting your terminal. Get a macOS notification when Claude completes a long task.

Environment Variables Available in Hooks

When your hook fires, these env vars are set:

Variable Contents
CLAUDE_TOOL_NAME The tool being called (Bash, Read, Edit, etc.)
CLAUDE_TOOL_INPUT JSON-encoded input to the tool
CLAUDE_SESSION_ID Current session identifier

The Matcher System

Hooks can target specific tools:

{
  "matcher": "Bash",       // only fires for Bash
  "matcher": "Edit",       // only fires for file edits
  "matcher": ".*",         // fires for everything
}
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Combine with exit codes for conditional blocking:

  • Exit 0: hook ran, let Claude proceed
  • Exit 1: hook failed, let Claude proceed anyway
  • Exit 2: block the tool call, feed stdout back to Claude as the reason

A Practical Starter Config

Here's the hooks setup I've converged on after months of daily use:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [{
          "type": "command",
          "command": "echo 'Use parallel execution for independent tasks. Use run_in_background for long operations (npm install, builds, tests).'"          
        }]
      },
      {
        "matcher": "Glob",
        "hooks": [{
          "type": "command",
          "command": "echo 'Combine searches in parallel when investigating multiple patterns.'"          
        }]
      },
      {
        "matcher": "Read",
        "hooks": [{
          "type": "command",
          "command": "echo 'Read multiple files in parallel when possible for faster analysis.'"          
        }]
      }
    ],
    "Stop": [
      {
        "hooks": [{
          "type": "command",
          "command": "osascript -e 'display notification \"Session complete\" with title \"Claude Code\"' 2>/dev/null; true"
        }]
      }
    ]
  }
}
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The PreToolUse hooks inject reminders to Claude about parallelism — so it doesn't run 5 sequential file reads when it could run them in one shot. The Stop hook notifies me when I can come back to the terminal.

Why This Matters

Most people use Claude Code as a smart autocomplete. Hooks let you treat it like a system you can instrument.

  • Inject context without polluting CLAUDE.md
  • Enforce standards without relying on Claude's memory
  • Audit every action without reading conversation history
  • Block unsafe operations without writing prompt rules that drift

The hooks system is one of those features that seems small until you start using it. Once you do, it's hard to work without it.

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