[BUG] Single web search request spawned 280 parallel subagent sessions, consuming entire daily limit + $51 extra usage

Environment

• Claude Code version: 2.1.110
• OS: macOS Darwin 25.3.0
• Model: claude-opus-4-6 (1M context)
• Plan: Max with $50 extra usage allowance

What happened

I asked Claude Code a single question: to research the practical risks of P2P cryptocurrency trading in Russia. A normal research task that should spawn 3-8 subagent sessions.

Instead, Claude Code spawned 280 parallel subagent sessions between 14:46 and 15:44 (under 1 hour). Each session performed nearly identical web searches with minor wording variations (e.g., "Тинькофф блокировка P2P крипто 2025", "Tinkoff bank P2P crypto block Russia 2025", "115-ФЗ криптовалюта блокировка" repeated hundreds of times).

The system became unresponsive. I could not stop the runaway agents.

Evidence

Session log analysis from ~/.claude/projects/:

• 280 sessions created in the 14:46-15:52 window
• 11.1 MB of session log data generated
• Each session is a separate .jsonl file with a unique UUID
• First user messages are all auto-generated subagent prompts like "Ты исследователь. Используй WebSearch чтобы найти...", "Search the web for:...", "Search the web thoroughly for:..."
• The cascade kept spawning new sessions even after 100+ were already running

Billing impact

• /usage shows: 95% of usage was while 4+ sessions ran in parallel
• /usage shows: 51% of usage came from subagent-heavy sessions
• Daily limit: 100% consumed (from a single question)
• $51.20 / $50.00 extra usage spent on this one incident
• Weekly usage jumped to 43% in one day

Expected behavior

A web research request should spawn a bounded number of subagent sessions (3-8 max), collect results, synthesize, and return. Not 280.

Request

1. This is a clear runaway agent bug. Please refund the $51.20 extra usage charge caused by this incident
2. Consider implementing maxParallelAgents (ref: #15487) or a circuit breaker for subagent spawning

Related issues

• #15487 (Add maxParallelAgents config)
• #34449 (Stop button fails to halt runaway agent)
• #13996 (Kill switch for runaway Task agents)
• #34476 (No way to cancel spawned agent team)
• #35741 (Interrupted firing across multiple parallel sessions)