exit 0 Means Pass, exit 2 Means Block — Taming AI with Claude Code Hooks

CLAUDE.md guidance is strong, but long sessions can dilute adherence. Hooks enforce behavior at execution time.

Semantics

• exit 0: allow
• exit 2: block
• stderr: feedback to Claude
• stdout: inject context

Common production patterns

1. PreToolUse dangerous command block (rm -rf, destructive SQL)
2. SessionStart re-injection after compaction
3. PreCompact async backups
4. StatusLine token visibility
5. PostToolUse auto-format/lint

Use CLAUDE.md for “what to do.”
Use hooks for “what must never happen.”

One exit 2 is often more reliable than 100 lines of “please don’t.”

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• Claude Code Hooks Guide
• Claude Code Hooks Reference
• Trail of Bits Claude Code Config