Agent tool: K.length crash on spawn at any context size

Description

The Agent tool crashes with undefined is not an object (evaluating 'K.length') during agent initialization. This is a JavaScript error in the CLI, not related to context window limits.

Reproduction

The crash occurs:

• At any transcript size (observed at 13KB through 2.3MB)
• On the first agent spawn in a fresh session (after /clear)
• More frequently after prior hook denials (PreToolUse deny → retry → crash)

Evidence

Analyzed 72 sessions across 2 projects with agent spawns:

| Transcript size | Sessions | Crashed | Rate |
|----------------|----------|---------|------|
| 100-500KB | 13 | 4 | 31% |
| 500KB-1MB | 21 | 2 | 10% |
| >1MB | 38 | 18 | 47% |

Key observations:

• The safest zone is 500KB-1MB (10% crash rate)
• Crashes happen at 13KB transcript (session 2fe1a50e) — first agent spawn after /clear
• Sessions with prior hook denials (Agent PreToolUse → deny) show higher crash rates
• Once a K.length crash occurs, all subsequent Agent spawns in the same session crash

Workaround

Adding a systemMessage on hook denials saying "Do NOT retry Agent — execute directly via Bash" reduces crash frequency by preventing the retry loop.

Environment

• Claude Code CLI (latest)
• Linux (Kali 6.18.12+)
• Multiple agent types affected (linux-admin, Explore, backend-dev, etc.)
• Both bypassPermissions and default permission modes

Error

Error: undefined is not an object (evaluating 'K.length')

This appears to be an unhandled undefined in the agent initialization JavaScript code path, possibly related to message array handling or context construction.