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Killswitch Gate — PreToolUse + UserPromptSubmit

Killswitch Gate — PreToolUse + UserPromptSubmit

Comprehensive token burn prevention via fail-closed killswitch gate in BOTH hook events.

MC #101650 — PreToolUse Consolidation (2026-05-21)

Verdict: PASS
Task: [P3-4] Hook consolidation: merge PreToolUse matchers, eliminate 6x killswitch + duplicate fires

Summary

~/.claude/settings.json had killswitch-gate.sh repeated in every PreToolUse matcher group. For Task, multiple matcher groups also matched, causing duplicate killswitch execution. The PreToolUse killswitch is now centralized in one universal matcher while specialized gates remain in their original matcher groups.

Change

  • Added first PreToolUse entry: matcher .* → bash $HOME/system/hooks/killswitch-gate.sh.
  • Removed killswitch-gate.sh from the specific PreToolUse matcher entries:
    • Bash
    • Task|WebSearch|WebFetch
    • Task
    • mcp__playwright__.*
    • Write|Edit|MultiEdit
  • Backed up settings to ~/.claude/settings.json.bak-101650.
  • Restored uchg immutable flag on ~/.claude/settings.json.

Validation

  • settings.json JSON-valid.
  • PreToolUse killswitch occurrences reduced from 5 to 1.
  • Representative matching analysis shows exactly one PreToolUse killswitch for:
    • Bash
    • Task
    • WebSearch
    • WebFetch
    • mcp__playwright__click
    • Write/Edit/MultiEdit
  • Killswitch OFF smoke: killswitch-gate.sh exits 0 with empty stdout/stderr.

Evidence

  • /tmp/101650-hook-inventory.txt
  • /tmp/101650-post-consolidation-analysis.txt
  • /tmp/101650-smoke-validation.txt
  • /tmp/101650-evidence/

MC #103690 — UserPromptSubmit Gate Addition (2026-06-19)

Verdict: PASS
Task: killswitch-gate.sh added to UserPromptSubmit — halt prompts when killswitch engaged

Problem

killswitch-gate.sh was registered only in PreToolUse (settings.json), NOT UserPromptSubmit. An engaged killswitch (~/system/state/killswitch) blocked tool use but NOT prompt submission — prompts still went through and burned tokens.

Fix

  • Added killswitch-gate.sh as the FIRST hook in the UserPromptSubmit chain in ~/.claude/settings.json:
    • Command: bash $HOME/system/hooks/killswitch-gate.sh
    • Timeout: 5000
  • settings.json is uchg-immutable (anti-tamper). Edit procedure:
    • chflags nouchg ~/.claude/settings.json → edit → chflags uchg ~/.claude/settings.json (re-lock)
    • Back up first

Gate Behavior (Verified)

  • Fast-path exit 0 if ~/system/state/killswitch absent (fail-open, no stdin parse) → safe in UserPromptSubmit
  • When engaged → exit 2 + stderr KILLSWITCH:ENGAGED + UserPromptSubmit JSON {"hookEventName":"UserPromptSubmit", "permissionDecision":"deny"}
  • Verified via:
    • Direct test: no-killswitch → exit 0, engaged → exit 2
    • lint-killswitch-preamble.sh PASS: "OK (first): UserPromptSubmit[]"

Result

Engaged killswitch now halts BOTH prompts (UserPromptSubmit) and tool use (PreToolUse).

Tools

  • Canonical install: ~/system/tools/install-killswitch-settings.sh
  • Lint: ~/system/tools/lint-killswitch-preamble.sh
  • CLI: ~/system/tools/killswitch.sh on|off|status

Evidence

  • ~/.claude/settings.json (UserPromptSubmit first hook)
  • ~/system/tools/lint-killswitch-preamble.sh PASS
  • Direct test: exit 0 (no killswitch), exit 2 (engaged)

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