[BUG] Context limit lockout — @mention agent also fails, session completely unrecoverable

Summary

When a session hits the context limit, the session becomes completely unrecoverable. Not only does /compact fail, but using @mention to invoke an agent also fails at the limit. The only escape is /clear which destroys all session context.

Behavior

1. Session approaches context limit during complex multi-agent work
2. Context limit reached — 'Context limit reached · /compact or /clear to continue' shown
3. /compact attempted — fails (API call itself hits the limit)
4. @AgentName message attempted — also fails at the context limit
5. Session is completely dead. All work in progress lost.

Compounding failure pattern

This has been observed compounding with API 529 overloaded_error in sequence:

1. API Error 529 overloaded_error (3m 50s churn, no response)
2. Same prompt → 529 again (another 3m 55s wasted)
3. Model switch attempted
4. Next prompt → context limit hit
5. Total session dead: two 4-minute overload churns + context limit lockout in sequence

By the time the context limit hits, the session has already burned 7+ minutes on failed API calls with nothing to show. The combined failure is worse than either bug alone.

Expected behavior

@mention invocation should work at context limit (it invokes a fresh subagent, not a continuation). /compact should have a fallback when the full history can't be sent.

Environment

• Claude Code v2.1.87
• Windows, Max plan
• Reproducible during heavy multi-agent sessions (6+ parallel agents, complex trading system pipeline work)

Related

• #3138 (resume flag fails after context limit)
• #2038 (context low loop with parallel sub-agents)