SessionStart hook crashes with AttributeError when stdin payload is None

[hahnpierre]

Bug

mgrep_watch.py (SessionStart hook) crashes on startup when read_hook_input() returns None.

Error

Displayed in Claude Code CLI header:

SessionStart:startup hook error    Failed with non-blocking status code: Traceback (most recent call last):

Root cause

In hooks/mgrep_watch.py, lines 34-35:

payload = read_hook_input()
cwd = payload.get("cwd")  # AttributeError if payload is None

read_hook_input() returns None when stdin is empty or JSON parsing fails (lines 22-29):

def read_hook_input():
    raw = sys.stdin.read()
    if not raw.strip():
        return None       # ← returns None, not {}
    try:
        return json.loads(raw)
    except json.JSONDecodeError as exc:
        debug_log(f"Failed to decode JSON: {exc}")
        return None       # ← returns None, not {}

Line 35 then calls payload.get("cwd") on None, raising AttributeError: 'NoneType' object has no attribute 'get'.

Suggested fix

Either guard against None:

payload = read_hook_input() or {}

Or have read_hook_input() return {} instead of None as the fallback.

Environment

• Claude Code v2.1.104
• macOS (Darwin 24.6.0)
• Python 3 (system)
• mgrep plugin version 0.0.0