[Windows] Bash tool: MSYS2 builtins and POSIX-compiled binaries cannot write to stdout; native Windows exes work fine

Environment

• OS: Windows 11 Enterprise 10.0.26100
• Claude Code: 2.1.45
• Shell: Git for Windows
• CLAUDE_CODE_GIT_BASH_PATH: C:\Users\iwheeler\AppData\Local\Programs\Git\bin\bash.exe

Summary

On Windows, the Bash tool fails silently for all bash builtins and MSYS2-compiled binaries. Native Windows executables (git.exe, python.exe, where.exe) work correctly. This makes the Bash tool almost entirely unusable on Windows.

Reproduction

These fail with no output:

echo hello          # exit 1, no output
ls -la              # exit 2, no output
pwd                 # exit 1, no output
/usr/bin/printf "test"  # exit 1, no output

These work:

git --version       # exit 0, correct output
python -c "print('hello')"  # exit 0, correct output
where python        # exit 0, correct output

Root Cause (diagnosed)

Node.js creates Windows anonymous pipes for subprocess I/O. MSYS2's POSIX emulation layer has its own pipe-type detection: it checks whether stdout is an MSYS2 pipe, a Windows console, or a generic Windows pipe. When the pipe is a Node.js anonymous pipe, MSYS2's CRT write() call fails with EPIPE — so bash builtins and MSYS2-compiled binaries exit 1 with no output.

Native Windows executables bypass this entirely — they call WriteFile()/WriteConsole() directly against the Windows HANDLE, which works fine regardless of pipe type.

Secondary issue: MSYS2 path conversion mangles flags passed to native Windows programs. Running cmd /c "echo hello" launches cmd.exe interactively instead of executing the command, because MSYS2 converts /c to C:\ before handing off to cmd.exe. This is controlled by MSYS_NO_PATHCONV.

This is a known Node.js + MSYS2/Cygwin interop issue.

Suggested Fix

One or more of:

1. Spawn bash via winpty to provide a PTY that MSYS2 can properly write to
2. Set MSYS2 environment variables in the subprocess environment:

• MSYS_NO_PATHCONV=1
• MSYS2_ARG_CONV_EXCL=*
• MSYS2_TTY_HACK=1

1. Use a native PTY allocator (e.g. node-pty) rather than anonymous pipes

Workaround

Use native Windows executables directly. python -c "..." works; echo, ls, pwd do not.