Preflight Checklist

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

What's Wrong?

Bug: claude plugin marketplace list crashes with "Illegal instruction" on AVX-only CPUs (Bun baseline build)

Summary

On Windows 10 with a CPU that supports SSE4.2 + AVX but not AVX2, the bundled Claude Code binary (claude.exe, Bun v1.3.14 baseline build) crashes with panic: Illegal instruction whenever a plugin-related subcommand is invoked (e.g. claude plugin marketplace list --json). This blocks the VS Code / Antigravity extension's "Plugins" UI entirely (extension reports Failed to load plugins: Claude CLI exited with code 3).

The crash is reproducible from a plain PowerShell session — it is not specific to the IDE extension. The root cause appears to be the Bun baseline build executing an instruction the CPU does not support, despite Bun correctly detecting no_avx2 at startup.

Environment

Reproduction

1. Open VS Code / Antigravity, open Claude Code extension panel, click Plugins.
2. Extension errors: Failed to load plugins: Claude CLI exited with code 3: ... (full output in "First crash" below).
3. Open plain PowerShell and run:

   & "C:\Users\<user>\.antigravity\extensions\anthropic.claude-code-2.1.140-win32-x64\resources\native-binary\claude.exe" plugin marketplace list --json

4. Process crashes with a Bun panic and exit code 3. Reproduces every time.

Observed behavior

Two consecutive runs produced two different panic messages at different addresses — strong indicator of an instruction-level issue hit on different code paths, not a deterministic logic bug.

First crash (invoked via extension)

panic(main thread): Failed to start HTTP Client thread: Unexpected

Full Bun banner (relevant fields):

Bun v1.3.14 (d2989145) Windows x64 (baseline)
Windows v.win10_cu
CPU: sse42 avx
Features: Bun.stderr(2) Bun.stdin(2) Bun.stdout(2) fetch jsc no_avx2
          spawn(2) standalone_executable yaml_parse claude_code
Elapsed: 3467ms | User: 1265ms | Sys: 453ms
RSS: 0.25GB | Peak: 0.25GB | Commit: 0.29GB | Faults: 64094 | Machine: 6.31GB

Crash report URL emitted by Bun:

https://bun.report/1.3.14/e_1d298914mgggkEugogigB+jmrX4hy5Ruk95mByzq5mByi43mB__ut7l2CA0eNpzS8zMSU1RKMlXKC5JLCpR8AgJCVBwzslMzStRKMkoSk1MsVIIzUutKEhNLklNAQB7XBB9

Second crash (same command, plain PowerShell)

panic(main thread): Illegal instruction at address 0x7FF7E0C6EF90

Banner this time loaded a different feature/builtin set (e.g. fetch, spawn, ws, node:tls, node:dns, node:http, node:zlib etc. were not initialized — the crash happened earlier):

Bun v1.3.14 (d2989145) Windows x64 (baseline)
CPU: sse42 avx
Features: Bun.stderr(2) Bun.stdin(2) Bun.stdout(2) jsc no_avx2
          standalone_executable claude_code
Elapsed: 2768ms | User: 531ms | Sys: 531ms
RSS: 0.21GB | Peak: 0.21GB | Commit: 0.24GB | Faults: 56569 | Machine: 6.31GB

Crash report URL:

https://bun.report/1.3.14/e_1d298914kggggEugggigBg53vG6+n22C6ro42Cqvj22Cwmh22Cwi45iC8yj5iCwvu7iCq8t7iCi022iC2tggsCq29/rCw3g5xCq/m5xCiw544C+0h1gD___A3u//Bhnoknf

Expected behavior

claude plugin marketplace list --json returns a JSON list of available marketplace plugins (or an empty array) without crashing, and the extension's Plugins tab loads.

Root-cause hypothesis

This is a Bun baseline-build vs. AVX-only CPU issue, not a Claude Code logic bug:

• The CPU is an Ivy Bridge i5-3230M: supports SSE4.2 + AVX, does not support AVX2 (AVX2 first shipped on Haswell, 2013).
• Bun detects this correctly — it ships the baseline Windows build and prints no_avx2 in the features list.
• Despite that, the process panics with Illegal instruction at a fixed address (0x7FF7E0C6EF90) on at least one run, and earlier with a thread-startup failure that is consistent with a SIGILL during thread bootstrap.
• The two different panic sites on two runs (different code paths loaded each time) are consistent with the baseline build still containing one or more instructions outside the SSE4.2/AVX baseline ISA — likely in code reached during plugin/marketplace network setup.

In other words: the Bun baseline binary appears to execute an AVX2 (or newer) instruction somewhere along the plugin marketplace list code path, even though it advertises no_avx2 support.

This should be reproducible by Anthropic on any Sandy Bridge / Ivy Bridge / pre-Haswell x64 Windows machine.

Impact

• Plugins feature is completely unusable for any user on a pre-Haswell CPU. The Plugins tab in the extension just errors.
• Other Claude Code functionality appears unaffected so far — the crash is specific to subcommands that touch the marketplace networking path.
• No workaround from user side is possible without downgrading the extension (and even then, only if the previous Bun didn't have the same regression).

Suggested next steps for Anthropic

1. Confirm Bun version: verify whether Bun v1.3.14's Windows baseline build is known to contain non-baseline instructions. If so, pin Claude Code to a Bun version without that regression, or rebuild from source with stricter -march flags.
2. Add a CPU capability gate: at startup, if AVX2 is required by any code path, fail fast with a clear error message rather than crashing in Bun's thread bootstrap or in the middle of an HTTP setup.
3. Forward to Bun: the two Bun crash report URLs above already contain redacted stack traces — file companion issues at oven-sh/bun referencing the addresses.
4. Extension UX: when the underlying CLI exits with code 3 and stderr contains panic(, surface a clearer message in the extension ("Claude CLI crashed — see logs") instead of the raw error blob.

Crash report URLs (Bun-side, contain redacted stack traces)

• https://bun.report/1.3.14/e_1d298914mgggkEugogigB+jmrX4hy5Ruk95mByzq5mByi43mB__ut7l2CA0eNpzS8zMSU1RKMlXKC5JLCpR8AgJCVBwzslMzStRKMkoSk1MsVIIzUutKEhNLklNAQB7XBB9
• https://bun.report/1.3.14/e_1d298914kggggEugggigBg53vG6+n22C6ro42Cqvj22Cwmh22Cwi45iC8yj5iCwvu7iCq8t7iCi022iC2tggsCq29/rCw3g5xCq/m5xCiw544C+0h1gD___A3u//Bhnoknf

Filed by a Claude Code user on Win10 + Ivy Bridge CPU. Happy to provide additional traces or test patches.

What Should Happen?

Running claude plugin marketplace list --json should return a JSON array of available marketplace plugins (or an empty array) and exit cleanly with code 0.

The VS Code / Antigravity extension's "Plugins" tab should populate with the marketplace list instead of showing "Failed to load plugins: Claude CLI exited with code 3".

At minimum, on a CPU/OS combination that the bundled Bun build does not actually support, the CLI should fail fast with a clear human-readable error (e.g. "This build requires AVX2 — your CPU does not support it") rather than crashing with an unhandled Bun panic.

Error Messages/Logs

Run 1 — invoked via the Antigravity extension's Plugins tab:

============================================================
Bun v1.3.14 (d2989145) Windows x64 (baseline)
Windows v.win10_cu
CPU: sse42 avx
Args: "c:\Users\<user>\.antigravity\extensions\anthropic.claude-code-2.1.140-win32-x64\resources\native-binary\claude.exe" "plugin" "marketplace" "list" "--json"
Features: Bun.stderr(2) Bun.stdin(2) Bun.stdout(2) fetch jsc no_avx2 spawn(2) standalone_executable yaml_parse claude_code
Builtins: "bun:main" "node:assert" "node:async_hooks" "node:buffer" "node:child_process" "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:readline" "node:stream" "node:stream/consumers" "node:stream/promises" "node:string_decoder" "node:timers/promises" "node:tls" "node:tty" "node:url" "node:util" "node:vm" "node:zlib" "ws" "node:http2"
Elapsed: 3467ms | User: 1265ms | Sys: 453ms
RSS: 0.25GB | Peak: 0.25GB | Commit: 0.29GB | Faults: 64094 | Machine: 6.31GB

panic(main thread): Failed to start HTTP Client thread: Unexpected

oh no: Bun has crashed. This indicates a bug in Bun, not your code.

Crash report: https://bun.report/1.3.14/e_1d298914mgggkEugogigB+jmrX4hy5Ruk95mByzq5mByi43mB__ut7l2CA0eNpzS8zMSU1RKMlXKC5JLCpR8AgJCVBwzslMzStRKMkoSk1MsVIIzUutKEhNLklNAQB7XBB9

Process exit code: 3


Run 2 — same command from a plain PowerShell session:

============================================================
Bun v1.3.14 (d2989145) Windows x64 (baseline)
Windows v.win10_cu
CPU: sse42 avx
Args: "C:\Users\<user>\.antigravity\extensions\anthropic.claude-code-2.1.140-win32-x64\resources\native-binary\claude.exe" "plugin" "marketplace" "list" "--json"
Features: Bun.stderr(2) Bun.stdin(2) Bun.stdout(2) jsc no_avx2 standalone_executable claude_code
Builtins: "bun:main" "node:assert" "node:async_hooks" "node:buffer" "node:child_process" "node:crypto" "node:events" "node:fs" "node:fs/promises" "node:http" "node:https" "node:module" "node:net" "node:os" "node:path" "node:path/win32" "node:perf_hooks" "node:process" "node:readline" "node:stream" "node:stream/consumers" "node:timers/promises" "node:tls" "node:tty" "node:url" "node:util" "node:zlib" "node:http2"
Elapsed: 2768ms | User: 531ms | Sys: 531ms
RSS: 0.21GB | Peak: 0.21GB | Commit: 0.24GB | Faults: 56569 | Machine: 6.31GB

panic(main thread): Illegal instruction at address 0x7FF7E0C6EF90

Crash report: https://bun.report/1.3.14/e_1d298914kggggEugggigBg53vG6+n22C6ro42Cqvj22Cwmh22Cwi45iC8yj5iCwvu7iCq8t7iCi022iC2tggsCq29/rCw3g5xCq/m5xCiw544C+0h1gD___A3u//Bhnoknf

Process exit code: 3


Note: two consecutive runs of the same command produce two different panics at different points (HTTP thread bootstrap vs. illegal instruction at a fixed address). This is consistent with a CPU-instruction issue in the Bun "baseline" build being hit on slightly different code paths each run — not a deterministic logic bug.

Steps to Reproduce

1. Use a Windows machine with a 64-bit CPU that supports SSE4.2 + AVX but NOT AVX2 (e.g. any Sandy Bridge / Ivy Bridge Intel CPU — in my case an Intel Core i5-3230M, Family 6 Model 58 Stepping 9).

2. Install Antigravity (or VS Code) and the Claude Code extension version 2.1.140-win32-x64. This bundles claude.exe (Bun v1.3.14, baseline Windows build) at: C:\Users<user>.antigravity\extensions\anthropic.claude-code-2.1.140-win32-x64\resources\native-binary\claude.exe

3. Reproduction A — via the extension UI: a. Open the Claude Code panel. b. Click the "Plugins" tab. c. Observe the error: Failed to load plugins: Claude CLI exited with code 3: ... followed by a Bun panic banner.

4. Reproduction B — confirms the bug is in the binary, not the extension: a. Open a plain PowerShell window (no extension involvement). b. Run: & "C:\Users<user>.antigravity\extensions\anthropic.claude-code-2.1.140-win32-x64\resources\native-binary\claude.exe" plugin marketplace list --json c. The process crashes with a Bun panic and exits with code 3.

5. Repeat step 4 once or twice — the panic message may vary between runs (e.g. "Failed to start HTTP Client thread: Unexpected" vs. "Illegal instruction at address 0x7FF7E0C6EF90"), but the process always crashes before producing any JSON.

Frequency: 100% reproducible on this hardware. Never succeeds.

Other Claude Code functionality (chat, file tools, etc.) appears to work normally — the crash is specific to subcommands that exercise the marketplace / plugin network path.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

No response

Claude Code Version

2.1.139

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

Hardware / OS context (full):

• CPU: Intel Core i5-3230M @ 2.60 GHz — Ivy Bridge (2012), 2 cores / 4 threads
• CPU ISA: SSE4.2 + AVX, NO AVX2 (AVX2 first shipped with Haswell, 2013)
• RAM: 6.31 GB
• OS: Windows 10 Pro, version 22H2, build 19045, 64-bit
• Shell tested: Windows PowerShell 5.1 (also reproduced when launched by the Antigravity/VS Code extension)

Why I believe this is a Bun baseline-build bug, not a Claude Code logic bug:

• Bun correctly detects my CPU and ships the "baseline" build — its own banner prints no_avx2 in the Features list.
• Despite that, the process panics with Illegal instruction at a fixed address (0x7FF7E0C6EF90) on one run, and with a thread-bootstrap failure on another. Different code paths, same root cause class: an instruction outside the SSE4.2/AVX baseline ISA is being executed somewhere in the plugin-marketplace code path.
• This should reproduce on any pre-Haswell x64 Windows machine. The Bun crash reports linked in the main body contain redacted stack traces that the Bun team can decode directly.

Scope of impact observed so far:

• 100% reproducible failure on claude plugin marketplace list --json (and therefore the extension's Plugins tab).
• Other Claude Code subcommands I have used (interactive chat, file tools, slash commands) appear to work normally on the same binary — only the marketplace / plugin networking path crashes.

Workarounds I tried (none worked from the user side):

• Running outside the extension (plain PowerShell) — same crash.
• Excluding the install folder from Windows Defender — no change.
• Disabling VPN/proxy — no change.

What I think would help most:

1. Pin Claude Code to a Bun version whose Windows baseline build is verified to contain no non-baseline instructions, OR rebuild Bun with stricter -march=x86-64-v2 flags.
2. At CLI startup, detect missing AVX2 and either (a) fail fast with a clear message if AVX2 is actually required, or (b) guarantee no code path uses AVX2+.
3. In the extension, when the underlying CLI exits with code 3 and stderr contains panic(, surface a friendlier "Claude CLI crashed — open logs" message instead of the raw Bun banner.

Companion Bun crash reports (auto-redacted by Bun, contain stack traces):

• https://bun.report/1.3.14/e_1d298914mgggkEugogigB+jmrX4hy5Ruk95mByzq5mByi43mB__ut7l2CA0eNpzS8zMSU1RKMlXKC5JLCpR8AgJCVBwzslMzStRKMkoSk1MsVIIzUutKEhNLklNAQB7XBB9
• https://bun.report/1.3.14/e_1d298914kggggEugggigBg53vG6+n22C6ro42Cqvj22Cwmh22Cwi45iC8yj5iCwvu7iCq8t7iCi022iC2tggsCq29/rCw3g5xCq/m5xCiw544C+0h1gD___A3u//Bhnoknf

I'm happy to run additional commands, attach more verbose logs, or test a candidate fix on this hardware — just point me at what you need.