[BUG] Bash tool returns exit 1 with no output for all commands on Windows/MINGW (set -o onecmd injection)

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?

Environment:

• Claude Code version: v2.1.45 (native installer)
• Platform: Windows 10/11, MINGW64 (Git Bash)
• Shell: bash via MINGW64_NT-10.0-26200

Symptoms:

• Every Bash tool call returns exit code 1 with empty output
• Affects all commands including echo "hello", pwd, git status
• Intermittent — works in some sessions, broken in others within the same day
• When broken, persists for the entire session (no recovery)

Root cause (diagnosed):

• Claude Code's shell snapshot mechanism injects set -o onecmd into $SHELLOPTS
• onecmd causes bash to exit after executing one command, which the tool interprets as failure
• Confirmed by comparing $SHELLOPTS in Claude Code (has onecmd) vs Codex in same environment (no onecmd)

Impact:

• All git operations, builds, typechecks, and terminal commands fail
• Skill auto-run commands (backtick blocks) abort entire skill load on first non-zero exit
• Led to ~1 hour of misdiagnosed "Windows/MINGW incompatibility" debugging, producing incorrect workarounds committed to codebase
• Workaround changes to /smart-commit skill were unnecessary — the commands were fine, the shell was broken

Related: #19057 (may be same issue — v2.1.30 changelog claimed a fix but it doesn't work on v2.1.45)

Repro:

1. Open Claude Code on Windows/MINGW
2. Run any Bash command: echo "hello"
3. Observe exit code 1, no output

What Should Happen?

Bash tool commands should execute normally and return their actual exit code and output. echo "hello" should return exit 0 with "hello" as output. pwd should return the current directory. All standard bash commands should work regardless of Claude Code's internal shell state snapshotting.

Error Messages/Logs

Every Bash tool call returns exit code 1 with empty output:

  > echo "hello"
  Exit code 1
  (no output)

  > pwd
  Exit code 1
  (no output)

  > git status
  Exit code 1
  (no output)

Diagnosed root cause: $SHELLOPTS contains "onecmd" which is injected by Claude Code's shell snapshot mechanism. Comparing same environment:

  Claude Code $SHELLOPTS: braceexpand:hashall:interactive-comments:onecmd
  Codex $SHELLOPTS:       braceexpand:hashall:interactive-comments
The "onecmd" option causes bash to exit after a single command, which the Bash tool interprets as failure.

Bug is intermittent across sessions — some sessions work, others are broken from the start with no recovery. When broken, ALL commands fail for the entire session.

Platform: Windows 10/11, MINGW64_NT-10.0-26200 3.6.4, Claude Code v2.1.45 (native installer)

Steps to Reproduce

1. Install Claude Code v2.1.45 via native Windows installer (not npm)
2. Open Claude Code in a Git Bash / MINGW64 terminal on Windows
3. Start a new conversation
4. Ask Claude to run any Bash command, e.g.: "run echo hello"
5. Observe: exit code 1, no output

Diagnosis step (run in a separate terminal, not through Claude Code):
echo $SHELLOPTS
# Expected: braceexpand:hashall:interactive-comments
# Actual (when bug is active): braceexpand:hashall:interactive-comments:onecmd

Key observations:

• Bug is session-level: if the first command fails, ALL commands fail for that session
• Starting a new conversation sometimes fixes it (new shell snapshot)
• The same terminal/environment works perfectly with Codex CLI
• No .bashrc or .bash_profile changes affect it — the onecmd injection happens inside Claude Code's shell snapshot mechanism
• Workarounds like renaming .bashrc, clearing BASH_ENV, and clearing snapshot cache do NOT fix it
• This is NOT a Windows/MINGW limitation — the same commands work in the same shell outside Claude Code

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.45

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

## System Details

• Terminal: Git Bash (MINGW64) — the default shell for Claude Code native Windows install
• OS: Windows 10, MINGW64_NT-10.0-26200 3.6.4-b9f03e96.x86_64
• Claude Code: v2.1.45 (native installer, not npm)
• Node: installed via pnpm
• Shell: /usr/bin/bash (MSYS2/MINGW64 bash)

Note: Git Bash is not listed in the terminal dropdown, but it's the shell
Claude Code defaults to on Windows native installs.