Writing Reliable Claude Code Hooks

This skill teaches you how to write hooks that never get aborted by Claude Code's internal timeout mechanism.

The Problem: "Operation Was Aborted"

You've probably seen this error:

Plugin hook ... failed to start: The operation was aborted.
Check that the command exists and is executable.

This happens because Claude Code has an internal timeout (via JavaScript's AbortController) that expects hooks to respond quickly. If your hook doesn't output JSON fast enough, Claude Code aborts the process.

The Solution: Immediate Response Pattern

The fix is simple but critical: Output JSON immediately after reading stdin, then fork the actual work to a subprocess.

The Wrong Way (Gets Aborted)

#!/usr/bin/env python3
import json
import sys

def main():
    # Read stdin
    input_data = json.load(sys.stdin)

    # DO WORK HERE (takes 100-500ms)
    process_data(input_data)  # TOO SLOW!
    do_more_work()            # STILL WORKING...

    # Output JSON - TOO LATE! Already aborted!
    print(json.dumps({"event": "UserPromptSubmit"}))

if __name__ == "__main__":
    main()

Timeline:

0ms    - Hook starts
10ms   - Reads stdin
100ms  - Still doing work...
200ms  - ABORTED by Claude Code's internal timeout
500ms  - Would have output JSON (never reached)

The Right Way (Never Aborted)

#!/usr/bin/env python3
import json
import os
import subprocess
import sys
from pathlib import Path

def main():
    # Step 1: Read stdin IMMEDIATELY
    try:
        input_data = json.load(sys.stdin)
    except json.JSONDecodeError:
        print(json.dumps({"event": "UserPromptSubmit"}), flush=True)
        sys.exit(0)

    # Step 2: Output JSON IMMEDIATELY (satisfies Claude Code's timeout)
    print(json.dumps({"event": "UserPromptSubmit"}), flush=True)
    sys.stdout.flush()

    # Step 3: Fork subprocess to do actual work
    env = os.environ.copy()
    env["MY_DATA"] = json.dumps(input_data)  # Pass data via env var

    worker_script = Path(__file__).parent / "worker.py"
    subprocess.Popen(
        [sys.executable, str(worker_script)],
        stdout=subprocess.DEVNULL,
        stderr=subprocess.DEVNULL,
        stdin=subprocess.DEVNULL,
        start_new_session=True,  # Detach from parent
        env=env,
    )

    sys.exit(0)

if __name__ == "__main__":
    main()

Timeline:

0ms    - Hook starts
10ms   - Reads stdin
20ms   - Outputs JSON (Claude Code satisfied!)
30ms   - Forks subprocess
40ms   - Hook exits cleanly
...    - Worker does actual work in background

Hook Configuration (hooks.json)

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "matcher": ".*",
        "hooks": [
          {
            "type": "command",
            "command": "python3 ${CLAUDE_PLUGIN_ROOT}/scripts/my_hook.py",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

Important notes:

• Use python3 prefix (not just the script path)
• ${CLAUDE_PLUGIN_ROOT} is expanded by Claude Code
• timeout is YOUR timeout (in seconds), separate from Claude Code's internal timeout
• Don't use suppressOutput: true during debugging - you need to see errors

Script Setup Checklist

1. Make Scripts Executable

CRITICAL: All hook scripts must be executable!

chmod +x scripts/*.py

Without this, you'll get cryptic errors like "command not found" or "permission denied".

2. Choose Your Invocation Method

There are three ways to invoke Python hooks. Each has tradeoffs:

Method A: python3 prefix (RECOMMENDED)

"command": "python3 ${CLAUDE_PLUGIN_ROOT}/scripts/my_hook.py"

Pros: Works reliably, python3 is always in PATH Cons: None significant Shebang: #!/usr/bin/env python3

Method B: Direct script execution

"command": "${CLAUDE_PLUGIN_ROOT}/scripts/my_hook.py"

Pros: Cleaner command Cons: Requires executable permission, relies on shebang Shebang: #!/usr/bin/env python3 (REQUIRED)

Method C: uv run with inline dependencies (ADVANCED)

"command": "uv run ${CLAUDE_PLUGIN_ROOT}/scripts/my_hook.py"

Pros: Can specify dependencies inline via PEP 723 Cons: uv may not be in Claude Code's PATH on all systems Shebang: Use PEP 723 format:

#!/usr/bin/env -S uv run --script
# /// script
# requires-python = ">=3.11"
# dependencies = [
#     "requests>=2.31.0",
# ]
# ///

WARNING: On macOS, /opt/homebrew/bin may not be in Claude Code's PATH. If uv fails, fall back to Method A.

3. Shebang Reference

Shebang	Use Case
#!/usr/bin/env python3	Standard Python script
#!/usr/bin/env -S uv run --script	Script with inline dependencies
#!/bin/bash	Shell script hooks

Common mistake: Escaped shebangs like #\!/usr/bin/env python3 - the backslash breaks it!

4. The suppressOutput Setting

{
  "type": "command",
  "command": "python3 ${CLAUDE_PLUGIN_ROOT}/scripts/my_hook.py",
  "timeout": 30,
  "suppressOutput": true  // REMOVE THIS WHEN DEBUGGING!
}

• suppressOutput: true - Hides all output (clean but hides errors)
• suppressOutput: false or omitted - Shows errors (use during development)

Debugging workflow:

1. Remove suppressOutput to see errors
2. Fix the errors
3. Add suppressOutput: true back for production

Output Visibility Reference

Who Sees What

Output Channel	Visible To	When	How to Use
stdout (JSON)	Claude Code	Always	print(json.dumps({...}), flush=True)
stdout (text)	User (in chat)	When suppressOutput: false	print("message")
stderr	User (in chat)	When suppressOutput: false	print("error", file=sys.stderr)
File log	Developer (via file)	Always	Write to .claude/hook_debug.log
Exit code	Claude Code	Always	sys.exit(0) or sys.exit(2)

Output Behavior Matrix

suppressOutput	stdout JSON	stdout text	stderr	File log
true	Parsed by Claude Code	Hidden	Hidden	Always works
false/omitted	Parsed by Claude Code	Shown to user	Shown to user	Always works

JSON Output Fields (stdout)

Claude Code parses JSON from stdout to control behavior:

Field	Type	Effect
decision	"block"	Blocks the operation (PreToolUse/UserPromptSubmit)
reason	string	Shown to user when blocking
additionalContext	string	Added to Claude's context
suppressOutput	boolean	In JSON response, not hooks.json

Example blocking response:

print(json.dumps({
    "decision": "block",
    "reason": "Operation not allowed: dangerous command detected"
}), flush=True)
sys.exit(2)  # Exit code 2 = block

Example adding context:

print(json.dumps({
    "additionalContext": "Note: User is in production environment"
}), flush=True)
sys.exit(0)

Exit Codes

Exit Code	Meaning	Effect
0	Success	Operation proceeds normally
2	Block	Operation is blocked, stderr shown to user
Other	Error	Non-blocking error, stderr shown in verbose mode

Redirecting Output

To hide all user-visible output:

"suppressOutput": true

To log to file (always works):

def debug_log(message: str) -> None:
    log_file = Path(".claude/hook_debug.log")
    log_file.parent.mkdir(parents=True, exist_ok=True)
    timestamp = datetime.now().isoformat()
    with open(log_file, "a") as f:
        f.write(f"[{timestamp}] {message}\n")

To show message to user (when not suppressed):

# Via stderr (recommended for errors)
print("Warning: something happened", file=sys.stderr)

# Via stdout text (before JSON)
print("Info: processing...")
print(json.dumps({"event": "UserPromptSubmit"}), flush=True)

To send context to Claude (not shown to user):

print(json.dumps({
    "additionalContext": "This info goes to Claude's context only"
}), flush=True)

Summary: Where to Put Output

What You Want	Where to Put It
Debug during development	stderr + suppressOutput: false
Permanent debug log	File (.claude/hook_debug.log)
Block operation with message	JSON decision: "block" + reason + exit 2
Add context for Claude	JSON additionalContext
Silent production operation	suppressOutput: true + file logging

Hook Events Reference

Event	When It Fires	Input Schema
SessionStart	When Claude Code starts	{session_id, cwd}
UserPromptSubmit	User sends a message	{prompt, transcript_path, session_id, cwd}
Stop	Claude finishes responding	{transcript_path, stop_reason}
PreToolUse	Before a tool runs	{tool_name, tool_input}
PostToolUse	After a tool runs	{tool_name, tool_input, tool_output}

Event-Specific Output Handling

IMPORTANT: Each event type handles JSON output differently!

UserPromptSubmit (SPECIAL HANDLING)

This event has unique output behavior:

Exit Code	JSON Field	Effect
0	(none)	Prompt proceeds normally
0	additionalContext	Text added to Claude's context (user doesn't see)
0	systemMessage	Shown as system message in chat
2	decision: "block"	Prompt is erased from context, stderr shown to user
2	reason	Shown to user explaining why blocked

# Allow prompt but add context for Claude
print(json.dumps({
    "additionalContext": "User is working on production database"
}), flush=True)
sys.exit(0)

# Block prompt completely (erased from history!)
print(json.dumps({
    "decision": "block",
    "reason": "Cannot execute destructive commands in production"
}), flush=True)
sys.exit(2)

Key difference: When UserPromptSubmit blocks with exit code 2, the prompt is erased from context - Claude never sees it!

PreToolUse

Exit Code	JSON Field	Effect
0	(none)	Tool executes normally
0	permissionDecision: "allow"	Explicitly allow
0	permissionDecision: "deny"	Block tool, show reason
2	decision: "block"	Block tool execution
2	reason	Shown to user

# Block dangerous command
if "rm -rf" in tool_input.get("command", ""):
    print(json.dumps({
        "decision": "block",
        "reason": "Dangerous command blocked: rm -rf"
    }), flush=True)
    sys.exit(2)

PostToolUse

Exit Code	JSON Field	Effect
0	(none)	Continue normally
0	additionalContext	Add info about tool result

# Add context about what happened
print(json.dumps({
    "additionalContext": f"Tool {tool_name} modified 5 files"
}), flush=True)
sys.exit(0)

Stop

Exit Code	JSON Field	Effect
0	(none)	Session ends normally
0	additionalContext	Added for next turn

# Add reminder for next turn
print(json.dumps({
    "additionalContext": "Remember to run tests before committing"
}), flush=True)
sys.exit(0)

SessionStart

Exit Code	JSON Field	Effect
0	(none)	Session starts normally
0	additionalContext	Added to initial context
0	systemMessage	Shown to user at start

# Welcome message and context
print(json.dumps({
    "systemMessage": "GHE plugin loaded. Transcription active.",
    "additionalContext": "Project: my-app, Branch: feature/new-ui"
}), flush=True)
sys.exit(0)

Event Comparison Matrix

Event	Can Block?	Erases Context?	additionalContext	systemMessage
SessionStart	No	No	Yes	Yes
UserPromptSubmit	Yes (exit 2)	Yes (when blocked)	Yes	Yes
PreToolUse	Yes (exit 2)	No	Yes	No
PostToolUse	No	No	Yes	No
Stop	No	No	Yes	No

Complete Example: Message Logger

Hook Script (log_messages.py)

#!/usr/bin/env python3
"""UserPromptSubmit hook that logs messages without getting aborted."""

import json
import os
import subprocess
import sys
from pathlib import Path

def main():
    # CRITICAL: Read stdin immediately
    try:
        input_data = json.load(sys.stdin)
    except json.JSONDecodeError:
        print(json.dumps({"event": "UserPromptSubmit"}), flush=True)
        sys.exit(0)

    # CRITICAL: Output JSON immediately to prevent abort
    print(json.dumps({"event": "UserPromptSubmit"}), flush=True)
    sys.stdout.flush()

    # Fork the actual logging work
    prompt = input_data.get("prompt", "")
    if prompt:
        env = os.environ.copy()
        env["LOG_MESSAGE"] = prompt

        worker = Path(__file__).parent / "log_worker.py"
        if worker.exists():
            subprocess.Popen(
                [sys.executable, str(worker)],
                stdout=subprocess.DEVNULL,
                stderr=subprocess.DEVNULL,
                stdin=subprocess.DEVNULL,
                start_new_session=True,
                env=env,
            )

    sys.exit(0)

if __name__ == "__main__":
    main()

Worker Script (log_worker.py)

#!/usr/bin/env python3
"""Background worker that does the actual logging."""

import os
from datetime import datetime
from pathlib import Path

def main():
    message = os.environ.get("LOG_MESSAGE", "")
    if not message:
        return

    log_file = Path(".claude/message_log.txt")
    log_file.parent.mkdir(parents=True, exist_ok=True)

    timestamp = datetime.now().isoformat()
    with open(log_file, "a") as f:
        f.write(f"[{timestamp}] {message}\n")

if __name__ == "__main__":
    main()

Debugging Tips

1. Remove suppressOutput: true during development to see errors
2. Add debug logging to a file (stdout goes to Claude Code)
3. Test hooks manually before deploying:

   echo '{"prompt": "test"}' | python3 my_hook.py
   

4. Check the debug log:

   tail -f .claude/hook_debug.log
   

Common Mistakes

Mistake	Fix
Doing work before outputting JSON	Output JSON first, fork work
Using time.sleep() in hook	Never sleep in the main hook
Reading large files before responding	Fork to worker, read there
Making API calls in hook	Fork to worker, call API there
Forgetting flush=True	Always flush stdout immediately
Not using start_new_session=True	Worker might get killed with parent
Script not executable	Run chmod +x script.py
Escaped shebang #\!	Use #! (no backslash)
Using uv without checking PATH	Fall back to python3 prefix
suppressOutput: true hiding errors	Remove it during debugging
Relying on shell environment	Claude Code has minimal PATH
Multiple plugins with same hook event	Can cause race conditions

Troubleshooting Guide

Error: "failed to start: The operation was aborted"

Cause: Hook didn't output JSON fast enough before Claude Code's internal timeout.

Fix: Use the immediate response pattern - output JSON first, fork work to subprocess.

Error: "command not found" or "permission denied"

Cause: Script not executable or bad shebang.

Fix:

chmod +x scripts/*.py
# Check shebang is correct (no backslash!)
head -1 scripts/my_hook.py  # Should show: #!/usr/bin/env python3

Error: "uv: command not found"

Cause: uv not in Claude Code's PATH (common on macOS with Homebrew).

Fix: Use python3 prefix instead:

"command": "python3 ${CLAUDE_PLUGIN_ROOT}/scripts/my_hook.py"

Hook runs but nothing happens

Causes:

1. suppressOutput: true hiding errors
2. Worker subprocess failing silently
3. Wrong working directory

Fix:

1. Remove suppressOutput temporarily
2. Add debug logging to worker script
3. Use absolute paths or ${CLAUDE_PLUGIN_ROOT}

Hook works manually but fails in Claude Code

Cause: Different environment (PATH, working directory, etc.)

Fix:

1. Don't rely on shell aliases or custom PATH entries
2. Use python3 or full paths to executables
3. Pass data via environment variables, not stdin to subprocess

Key Takeaways

1. Output JSON within 50ms - Claude Code's internal timeout is aggressive
2. Fork heavy work - Use subprocess with start_new_session=True
3. Pass data via environment variables - Safer than command args
4. Always exit 0 - Non-zero exit codes have special meanings
5. Test manually first - Easier to debug outside Claude Code

Bundled Template Scripts

This skill includes ready-to-use template scripts in the scripts/ directory:

• scripts/example_hook.py - Template hook with the immediate response pattern
• scripts/example_worker.py - Template worker for background processing

To use these templates:

1. Copy them to your plugin's scripts/ directory
2. Rename and customize for your use case
3. Update hooks.json to point to your hook script

Related Resources

• Claude Code Hooks Documentation
• GitHub Issue #5468 - AbortError investigation
• GHE Plugin source code: plugins/ghe/scripts/capture_user.py