Auto-compact fails on Opus 4.6 [1m]: session dies at 100% context (regression since ~2.1.172)

Description

Auto-compact does not trigger when context reaches 100% on Opus 4.6 with 1M context window. The session becomes unresponsive/dies instead of compacting.

Environment

• OS: Windows 11
• Claude Code version with issue: 2.1.172+
• Claude Code version without issue: 2.1.89
• Model: claude-opus-4-6[1m] (Opus 4.6 with 1M context)
• Auth: Claude Max subscription

Steps to Reproduce

1. Start a session with Opus 4.6 [1m]
2. Work normally until context usage approaches 100%
3. Observe that auto-compact does not fire — the session dies/becomes unresponsive

Expected Behavior

Auto-compact should trigger before context reaches 100%, compacting the conversation and allowing the session to continue.

Actual Behavior

Session dies when context hits 100%. No auto-compact is triggered.

Notes

• This is a regression. Version 2.1.89 does NOT have this issue.
• The fix in 2.1.172 ("Fixed sessions using 1M context without usage credits getting permanently stuck") may have introduced or not fully resolved this.
• Reproducible consistently during normal extended coding sessions.