autoCompact: false ignored - compaction triggers at ~78%

Summary

Claude Code triggers automatic context compaction at approximately 78% context usage even when autoCompact: false is explicitly set in settings.json.

Environment

• Claude Code Version: 2.1.7
• OS: Ubuntu Linux (6.8.0-58-generic)
• Date: 2026-01-14

Configuration

// ~/.claude/settings.json
{
  "autoCompact": false,
  // ... other settings
}

Expected Behavior

With autoCompact: false, automatic compaction should NEVER trigger. Compaction should only occur:

1. When manually requested by user (/compact)
2. When hitting the hard API context limit (100%)

Actual Behavior

• Compaction triggered automatically at ~78-79% context usage
• CONFIRMED: Status line shows "68% used" + "Context left until auto-compact: 11%" = 79% trigger point
• PreCompact hook fired with trigger: auto
• User attempted to cancel with Ctrl+C multiple times, but compaction kept restarting
• 5 compaction attempts occurred within ~25 minutes (17:39, 17:59, 18:00, 18:01, 18:02)

Evidence

PreCompact hook backups show:

trigger: auto
context_used: unknown  # Hook couldn't read percentage from input JSON

The trigger: auto field confirms this was automatic compaction, not user-initiated.

Impact

• Session context corruption: Compaction degrades Claude's memory of the conversation
• Work interruption: User cannot cancel compaction effectively with Ctrl+C
• Data loss risk: If user has session handoff workflow (save to external notes before exit), premature compaction bypasses this

Steps to Reproduce

1. Set "autoCompact": false in ~/.claude/settings.json
2. Start a Claude Code session
3. Work until context reaches ~78%
4. Observe compaction triggering despite the setting

Workaround

Using UserPromptSubmit hook to warn at 70%/75% and manually exit before 78%:

# In hook, warn at 75% to allow manual save before buggy compaction
if [ "$USED_INT" -ge 75 ]; then
    echo "MANDATORY: Save session and exit before compaction triggers at ~78%"
fi

Additional Notes

• The PreCompact hook receives JSON that doesn't include context_window.used_percentage field (shows as "unknown"), making it impossible to log the actual context level when compaction fires
• Ctrl+C during compaction should abort, not restart compaction

Requested Fix

1. Honor autoCompact: false - never trigger automatic compaction when disabled
2. Ctrl+C during compaction should cleanly abort
3. Include context percentage in PreCompact hook input JSON