Large paste in long-context session causes unrecoverable TUI freeze

Summary

When a large paste (~196KB) pushes a long-context session past the compaction threshold, auto-compact fails, the suggested recovery ("Press esc twice to go up a few messages and try again") also fails, and the TUI freezes permanently — requiring kill -9 of the terminal emulator.

Reproduction Steps

1. Run a session until token count reaches ~145K+ (near the compaction threshold of ~167K)
2. Paste a large block of text (~150KB+, ~4,400 lines)
3. Token count jumps past the effective window (e.g., 198K > 180K)
4. Auto-compact fires and fails: Error: Conversation too long. Press esc twice to go up a few messages and try again.
5. Follow the suggested recovery: press Esc twice, use the message-selector to rewind to before the paste
6. Manual compact also fails with the same error
7. TUI becomes unresponsive — no input accepted, no recovery possible
8. Only fix: kill -9 from another terminal

Debug Log Evidence

Session 61bd4f39-7241-4b51-9f0d-7523e384ac1f, version 2.1.39, macOS Darwin 25.2.0.

Token state before paste:

autocompact: tokens=146295 threshold=167000 effectiveWindow=180000

After paste (196KB, 4,466 lines):

autocompact: tokens=198718 threshold=167000 effectiveWindow=180000

Auto-compact failure:

[ERROR] Error: Error: Conversation too long. Press esc twice to go up a few messages and try again.
    at yhT (/$bunfs/root/claude:3303:939)
    at async tPB (/$bunfs/root/claude:3388:2545)

Manual compact failure (after following the Esc-Esc suggestion):

[ERROR] Error: Error: Conversation too long. Press esc twice to go up a few messages and try again.
    at yhT (/$bunfs/root/claude:3303:939)
    at async <anonymous> (/$bunfs/root/claude:3602:56)
    ...

[ERROR] Error: Error: Error during compaction: Error: Conversation too long. Press esc twice to go up a few messages and try again.
    at <anonymous> (/$bunfs/root/claude:3603:189)
    at async n08 (/$bunfs/root/claude:1989:3696)
    at async YXB (/$bunfs/root/claude:1989:1064)
    at async vk8 (/$bunfs/root/claude:7276:2509)
    at async HyR (/$bunfs/root/claude:7274:3516)
    at async Zx8 (/$bunfs/root/claude:7415:23321)
    at async OkR (/$bunfs/root/claude:7415:22473)

Third attempt — same result:

[ERROR] Error: Error: Conversation too long. Press esc twice to go up a few messages and try again.
    at LPB (/$bunfs/root/claude:3305:808)
    at async <anonymous> (/$bunfs/root/claude:7430:8084)
    at async j (/$bunfs/root/claude:7277:263)

Rendering death spiral — the paste expanded the virtual screen to ~6,000 rows:

High write ratio: blit=0, write=849312 (100.0% writes), screen=5962x158
Slow render: 100.2ms, screen: 5962x158, damage: 158x5962, changes: 1623525

Every render frame wrote 849K+ cells with 0% blit reuse. This continued until the process was killed.

Root Causes

1. Compaction death loop: When the paste pushes tokens past the effective window, compact needs to make an API call — but the conversation is already too long for that API call. The error suggests "go up a few messages," but compact fails from the rewound position too. There is no escape from this state.

1. Rendering death spiral: The 196KB paste creates a ~6,000-row virtual screen. At 849K writes per frame and 100ms+ per render, the TUI spends 100% CPU painting and becomes unresponsive.

1. No paste size guard: 196KB / 4,466 lines enters the conversation without pre-validation. Large pastes should be rejected, truncated, or stored as file references.

Suggested Fixes

• Paste size validation: Reject or warn on pastes that would push past the effective window
• Graceful compaction fallback: If compact fails, offer to discard the last message rather than looping on the same error
• Render throttling: Cap rendering for pathologically large screen buffers (849K writes/frame is unsustainable)
• The Esc-Esc recovery path must actually work: If the UI suggests rewinding as recovery, that path needs to handle the "too long" state gracefully

Environment

• Claude Code version: 2.1.39 (auto-updated to 2.1.42 during the crash)
• OS: macOS Darwin 25.2.0
• Terminal: iTerm2
• Platform: darwin arm64