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hook-session-state-isolation

1 min read

hook-session-state-isolation

Hook session-state isolation: never default session_id

When building a PreToolUse/PostToolUse hook that tracks per-session state via a filesystem marker (e.g., /tmp/.marker-<session_id>), never fall back to a hardcoded default like 'unknown' if session_id is missing from the hook payload.

Why it breaks

A shared default marker causes cross-session state leak: one session creating /tmp/.marker-unknown grants bypass to all subsequent sessions that also get 'unknown', defeating the purpose of session isolation. The guard silently stops enforcing on every subsequent session.

Correct pattern

session_id = data.get('session_id', '').strip()
if not session_id:
    # No valid session_id → treat as "no marker"
    # For guards: always warn
    # For marker-creation: skip write entirely (don't create shared state)
    ...

Discovered while building

setup-curator-guard.py + setup-curator-marker.py at /root/.claude/hooks/. Applies to ANY Claude Code hook that tracks per-session state in shared filesystem (/tmp, shared state files, lockfiles, etc.).

Verification

Tested with 5 edge cases: missing session_id, present session_id + no marker, present session_id + marker, empty session_id (marker hook refuses write), non-setup file bypass. All isolated correctly.

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