Bun 1.3.6 in Claude Code 2.1.15 crashes with SIGILL on CPUs without AVX support

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 SIGILL (Illegal Instruction) on CPUs that lack AVX support. The embedded Bun runtime (v1.3.6) reports "CPU lacks AVX support" despite being labeled as a "baseline" build which should not require AVX.

What Should Happen?

Claude Code should run on CPUs without AVX support, as the "baseline" Bun build is specifically designed for older CPUs. Previous versions (2.1.9, 2.1.11) with Bun 1.3.5 work correctly on the same hardware.

Error Messages/Logs

Bun v1.3.6 (Linux x64 baseline)
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.
Error: Claude Code process terminated by signal SIGILL

Steps to Reproduce

1. Use a CPU without AVX support (e.g., Intel Westmere, pre-2011 CPUs)
2. Install Claude Code 2.1.15 in VS Code (local or via Remote SSH)
3. Try to start Claude Code
4. Observe SIGILL crash

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.11 (uses Bun v1.3.5)

Claude Code Version

2.1.15 (crashes on startup, cannot run claude --version)

Platform

Other

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

Environment details

• OS: Ubuntu 24.04.3 LTS
• Kernel: 6.8.0-90-generic
• CPU: Intel Westmere (virtualized on QNAP Virtualization Station)
• glibc: 2.39
• VS Code: Remote SSH extension

Version comparison

| Version | Bun | Status |
|---------|-----|--------|
| 2.1.9 | v1.3.5 (baseline) | ✅ Works |
| 2.1.11 | v1.3.5 (baseline) | ✅ Works |
| 2.1.15 | v1.3.6 (baseline) | ❌ SIGILL crash |

Workaround

Downgrade to Claude Code 2.1.11 and disable auto-update.

Root cause

Likely a regression in Bun 1.3.6 "baseline" build that inadvertently requires AVX instructions.