Bun v1.3.11 panic (Internal assertion failure) on Windows 11 with AVX-512 CPU - regression since v2.1.85

Bug Description

Claude Code crashes immediately with panic(main thread): Internal assertion failure (Bun runtime crash) on every new session. This is a regression introduced in v2.1.85 — version v2.1.84 works perfectly.

Environment

• Claude Code: v2.1.85 through v2.1.88 (all crash)
• Last working version: v2.1.84
• Bundled Bun: v1.3.11 (759ce802)
• OS: Windows 11 Pro, Build 26200 (10.0.26200)
• CPU: Intel Core i7-11800H @ 2.30GHz (Tiger Lake-H)
• CPU Features: sse42, avx, avx2, avx512
• System: Dell Inspiron 15 7510, 16.91GB RAM
• Architecture: x64

Steps to Reproduce

1. Install Claude Code v2.1.85 or later on Windows 11 (Build 26200) with AVX-512 capable CPU
2. Open any terminal (cmd, PowerShell)
3. Navigate to ANY directory (even an empty, freshly created folder)
4. Run claude or claude --dangerously-skip-permissions
5. The welcome screen appears successfully
6. Type anything or press Enter → Bun crashes immediately

What I've Tested

| Test | Result |
|------|--------|
| Empty new folder (no git, no files) | ❌ Crash |
| Project folder (Flutter) | ❌ Crash |
| Project folder (C#) | ❌ Crash |
| Without settings.json (no hooks) | ❌ Crash |
| v2.1.84 on same machine | ✅ Works |
| v2.1.85, v2.1.86, v2.1.87, v2.1.88 | ❌ All crash |

Crash Output

panic(main thread): Internal assertion failure
oh no. Bun has crashed. This indicates a bug in Bun, not your code.

Bun crash report: https://bun.report/1.3.11/e_1759ce80mgggEuhogC+pl/M+mx6G06vremk82c4pylbg1ixSm/kgpB___A0eNrzzCtJLcpLzFFILC5OLSrJzM9TSEvMzCktSgUAiSkKPg

The crash report URL is identical across all attempts, suggesting a deterministic crash path.

Full Crash Dump

Bun v1.3.11 (759ce802) Windows x64 (baseline)
Windows v.win11_dt
CPU: sse42 avx avx2 avx512
Args: "claude"
Features: Bun.stderr(2) Bun.stdin(2) Bun.stdout(2) abort_signal(5) fetch(12) jsc spawn(15) standalone_executable yaml_parse(291)
Builtins: "bun:main" "node:assert" "node:async_hooks" "node:buffer" "node:child_process" "node:constants" "node:crypto" "node:dns" "node:events" "node:fs" "node:fs/promises" "node:http" "node:https" "node:module" "node:net" "node:os" "node:path" "node:path/posix" "node:path/win32" "node:perf_hooks" "node:process" "node:stream" "node:stream/consumers" "node:timers/promises" "node:tls" "node:tty" "node:url" "node:util" "node:zlib" "ws" "node:http2"
Elapsed: 3038ms | User: 2578ms | Sys: 1343ms
RSS: 0.55GB | Peak: 0.55GB | Commit: 0.81GB | Faults: 147246 | Machine: 16.91GB

Suspected Root Cause

The crash appears related to AVX-512 instruction set handling in Bun v1.3.11 on Windows. The bundled Bun version likely changed between v2.1.84 and v2.1.85, introducing this regression. CPUs without AVX-512 may not trigger this bug, explaining why it doesn't affect all users.

Workaround

Pinned to Claude Code v2.1.84 with auto-updates disabled.

Related Issues

• #34515 — Bun internal assertion failure on Windows 11
• #40052 — Bun panic on CPUs without AVX support
• #26723 — Bun crash on Windows 11 ARM64
• #16691 — Bun internal assertion failure on Windows 11