claude -r: 100% CPU for 20+ minutes when resuming a 12MB session (4801 lines)

Summary

claude -r (resume last session) causes 100% CPU on the main thread for 20+ minutes when the session JSONL is ~12MB. The CLI is completely frozen during this time — no output, no prompt, no way to interact.

Environment

• Claude Code: 2.1.37
• macOS 15.3 (Darwin 25.2.0, arm64)
• Apple M2 Pro, 32 GB RAM

Reproduction

1. Work in a session with ~247 user turns, tool calls that produce large outputs (backtest results, screenshots, etc.)
2. Exit
3. claude -r
4. Observe: 100%+ CPU, no output for 20+ minutes

Session characteristics

File:       12.4 MB, 4801 lines
progress:   4041 messages
user:       247 messages (6.8 MB total — tool_results with large outputs)
assistant:  385 messages (0.6 MB)
system:     71 messages

Messages > 100KB:       11
Largest single message: 877 KB (tool_result containing base64 image)

Profiling data

/usr/bin/sample (5s, 3670 samples):

• 93% of samples in a single JIT-compiled JS function on the main thread
• All Bun Pool worker threads idle (__ulock_wait2)
• HTTP Client idle (kevent64)
• JSC GC scavenger intermittently calling madvise

Resource usage during resume:

Elapsed   CPU      RSS       CPU Time
08:00     102.9%   1,578 MB  —
12:00     103.6%   1,344 MB  9:46
16:00     102.1%   1,104 MB  12:02
20:00     108.2%   1,401 MB  16:40

RSS oscillates 1.1–1.6 GB (GC churn). Terminal I/O offset stays constant at ~5.3 MB — no rendering happening, pure CPU computation.

Expected

Resume a 12 MB session in seconds, not 20+ minutes.

Possible causes

• Full synchronous JSONL parse + deserialization on the main thread
• No streaming/incremental parsing — 877 KB base64 image payloads are fully materialized
• Conversation tree reconstruction scales poorly with message count

Suggestions

• Stream-parse JSONL incrementally, skip/truncate large tool_result payloads
• Offload parsing to a worker thread
• Show a progress indicator during load
• Auto-compact sessions exceeding a size threshold