Claude Code 2.1.15 crashes with illegal instruction on pre-AVX CPUs (Westmere Xeon)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Code 2.1.15 crashes immediately on startup with an illegal instruction error on CPUs that lack AVX support. Version 2.1.14 works correctly on the same system. The bundled Bun 1.3.6 "baseline" build appears to be emitting AVX instructions despite advertising itself as baseline-compatible.

Environment:
OS: Ubuntu 24.04 (Linux 6.8.0) running as Proxmox VM with CPU type host
CPU: Dual Intel Xeon X5690 (Westmere-EP, supports SSE4.2, does NOT support AVX)
glibc: 2.39
Claude Code 2.1.14: Works
Claude Code 2.1.15: Crashes

Root Cause Analysis:
The bundled Bun binary in 2.1.15 is labeled as "baseline" but is executing AVX instructions that trigger SIGILL on pre-AVX processors. Either:

The wrong Bun build was bundled (AVX build instead of true baseline)
Bun 1.3.6 has a regression in their baseline build

What Should Happen?

Expected Result:
Claude Code should start normally, as it did with version 2.1.14. The "baseline" Bun build should not require AVX.
Workaround:
Revert to 2.1.14 by updating the symlink:
bashln -sf /home/$USER/.local/share/claude/versions/2.1.14 /home/$USER/.local/bin/claude

Error Messages/Logs

Bun v1.3.6 (d530ed99) Linux x64 (baseline)
Linux Kernel v6.8.0 | glibc v2.39
CPU: sse42 popcnt
Args: "claude"
Features: Bun.stderr(2) Bun.stdin(2) Bun.stdout(2) fetch(9) jsc no_avx2 no_avx spawn(4) standalone_executable
...
CPU lacks AVX support. Please consider upgrading to a newer CPU.
panic(main thread): Illegal instruction at address 0x4001FB4
oh no: Bun has crashed. This indicates a bug in Bun, not your code.
Illegal instruction (core dumped)

Steps to Reproduce

Install Claude Code 2.1.14 on a system with a pre-Sandy Bridge CPU (no AVX support)
Allow auto-update to 2.1.15
Run claude

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.14

Claude Code Version

2.1.15

Platform

Anthropic API

Operating System

linux

Terminal/Shell

Termius

Additional Information

Westmere Xeons (X5690, X5680, etc.) are still common in enterprise/homelab environments
The Bun crash report URL generated: https://bun.report/1.3.6/B_1d530ed9GugogigD+ypR_s1nggEs1nggEs1nggE__67/0/D_wyo1/Dwyo1/D_wyo1/Dwyo1/Dwyo1/Dwyo1/D_67/0/Dmsnt/DwujpiFA3Ao7vggE
CPU flags from /proc/cpuinfo confirm SSE4.2 present, AVX absent