[BUG] Latest version of Claude Code on Windows desktop errors on every prompt submit - aka does not work (on my version of windows) - Bun 1.3.14 "Internal assertion failure" (ntdll) crashes BOTH desktop-bundled 2.1.170 and client CLI

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?

One panic signature — panic: Internal assertion failure in Bun v1.3.14 Windows x64 baseline, ntdll frame — crashes two independent Claude Code binaries on the same Windows 11 machine:

Surface 1: Claude Desktop bundled binary (2.1.170) — every session command fails.
Desktop app updated 2026-06-09 (the Fable 5 client push). Since then, any message in a Code session crashes with "Claude Code process exited with code 3":

Bun v1.3.14 (521eedd6) Windows x64 (baseline), win11_dt
Args: "<home>\AppData\Roaming\Claude\claude-code\2.1.170\claude.exe" "--output-format" "stream-json"
  "--verbose" "--input-format" "stream-json" "--effort" "medium" "--model" "claude-fable-5[1m]"
  "--permission-prompt-tool" "stdio"...
panic(thread 12900): Internal assertion failure
Elapsed: 414ms | RSS: 0.31GB | Faults: 75921
bun.report: https://bun.report/1.3.14/<blob>.DLLu10LCSntdll.<blob>   (ntdll frame)

Note: the desktop app redacts the bun.report URL to <blob> in both its crash dialog and its own main.log (36 panics logged, all redacted) — so the full encoded stack never reaches disk on the desktop side. An un-redacted link from the CLI crash is available on request.

Surface 2: Standalone native-install CLI — only --resume fails; plain claude works.

Bun v1.3.14 (2a41ca97) Windows x64 (baseline), win11_dt
Args: "C:\Users\<user>\.local\bin\claude.exe" "--resume"
panic(thread 28828): Internal assertion failure
Elapsed: 294ms | RSS: 0.30GB | Faults: 73358

CLI version: <paste claude --version>.

Key diagnostics already eliminated

• Not session-data corruption: Surface 2 reproduces identically with ~\.claude\projects renamed away entirely (empty store). Largest session file is only ~20MB.
• Not cwd-dependent: identical panic + fingerprint from a brand-new project dir and an established one.
• Not machine-wide Bun breakage: plain interactive claude (Surface 2 binary) runs fine; Cowork mode (Linux VM runtime in the same desktop app) works perfectly.
• Two different Bun 1.3.14 builds (521eedd6, 2a41ca97), two install channels, one signature, near-identical resource fingerprint (~0.3GB RSS, ~73–76k page faults, <450ms) → single upstream Bun 1.3.14 baseline-Windows regression, hit via different code paths (desktop: stream-json session spawn; CLI: resume picker).

Platform / OS / Shell

Windows 11 25H2, build 10.0.26200.8655 (same 26200 build line reported in #55219)
CPU: Intel Core Ultra 7 255H (Arrow Lake-H) — supports SSE4.2/AVX/AVX2, no AVX-512 (so distinct from #41498)
32GB RAM · PowerShell + Claude Desktop app

Additional Information

Possible triage clue: this CPU supports AVX2, yet both binaries shipped/selected the Bun baseline build (intended for non-AVX2 CPUs). If the regression is baseline-variant-specific, the build mis-selection on a hybrid P/E-core Arrow Lake CPU may be why this machine is exposed at all.
Related: #55219, #61647, #65754, upstream oven-sh/bun#30031. Happy to run instrumented repros — the empty-data deterministic crash makes this an easy bisect target.

Key diagnostics already eliminated

Not session-data corruption: Surface 2 reproduces identically with ~\.claude\projects renamed away entirely (empty store). Largest session file is only ~20MB.
Not cwd-dependent: identical panic + fingerprint from a brand-new project dir and an established one.
Not machine-wide Bun breakage: plain interactive claude (Surface 2 binary) runs fine; Cowork mode (Linux VM runtime in the same desktop app) works perfectly.
Two different Bun 1.3.14 builds (521eedd6, 2a41ca97), two install channels, one signature, near-identical resource fingerprint (~0.3GB RSS, ~73–76k page faults, <450ms) → single upstream Bun 1.3.14 baseline-Windows regression, hit via different code paths (desktop: stream-json session spawn; CLI:

What Should Happen?

Desktop Code sessions should run commands; claude --resume should open the session picker. Neither should panic the bundled Bun runtime.

Error Messages/Logs

Claude Code process exited with code 3

============================================================
Bun v1.3.14 (521eedd6) Windows x64 (baseline)
Windows v.win11_dt
CPU: sse42 avx avx2
Args: "<home>\AppData\Roaming\Claude\claude-code\2.1.170\claude.exe" "--output-format" "stream-json" "--verbose" "--input-format" "stream-json" "--effort" "medium" "--model" "claude-fable-5[1m]" "--permission-prompt-tool" "stdio"...
Features: Bun.stderr(2) Bun.stdin(2) Bun.stdout(2) fetch(2) jsc spawn(3) standalone_executable yaml_parse(13) claude_code 
Builtins: "bun:ffi" "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" "no
Elapsed: 358ms | User: 265ms | Sys: 125ms
RSS: 0.31GB | Peak: 0.31GB | Commit: 0.41GB | Faults: 75568 | Machine: 33.78GB
panic(thread 33604): Internal assertion failure
oh no: Bun has crashed. This indicates a bug in Bun, not your code.
To send a redacted crash report to Bun's team,
please file a GitHub issue using the link below:
 https://bun.report/1.3.14/<blob>.DLLu10LCSntdll.<blob>

Steps to Reproduce

1. Windows 11 x64 (build below), AVX2 CPU — no AVX-512 involved, unlike #41498.
2. Update Claude Desktop to the 2026-06-09 build (bundles Claude Code 2.1.170) → send any message in a Code session → exit code 3.
3. Or with the native-install CLI: run claude --resume (crashes even with ~\.claude\projects empty/removed).

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

1.11187.4

Claude Code Version

1.11847.5

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

Possible triage clue: this CPU supports AVX2, yet both binaries shipped/selected the Bun baseline build (intended for non-AVX2 CPUs). If the regression is baseline-variant-specific, the build mis-selection on a hybrid P/E-core Arrow Lake CPU may be why this machine is exposed at all.
Related: #55219, #61647, #65754, upstream oven-sh/bun#30031. Happy to run instrumented repros — the empty-data deterministic crash makes this an easy bisect target.

<img width="1008" height="1468" alt="Image" src="https://github.com/user-attachments/assets/77b1d0ba-082e-42ad-a4d2-a8d280ddebfb" />