Bash tool ignores login shell from /etc/passwd, auto-detects zsh from .zshrc

Bug Description

The Bash tool overrides the SHELL environment variable and uses zsh instead of the user's actual login shell (fish), apparently because it finds a .zshrc file.

Environment

• OS: WSL2 (Ubuntu) on Windows
• Login shell (from /etc/passwd): /usr/bin/fish
• Claude Code version: latest CLI

Steps to Reproduce

1. Set login shell to fish: chsh -s /usr/bin/fish
2. Have a .zshrc file present (e.g. from previously using zsh)
3. Launch Claude Code from fish
4. Use the Bash tool to run any command

Expected Behavior

The Bash tool should respect the login shell from /etc/passwd (/usr/bin/fish) or at minimum the SHELL environment variable inherited from the parent process.

The parent Claude process correctly has SHELL=/usr/bin/fish (confirmed via /proc/$PPID/environ), but the spawned shell process has SHELL=/usr/bin/zsh.

Actual Behavior

The Bash tool sets SHELL=/usr/bin/zsh and launches zsh, which sources .zshrc and loads oh-my-zsh with all its plugins. This causes:

• Unexpected aliases (e.g. tmux plugin alias overrides tmux command)
• Slower shell startup due to oh-my-zsh initialization
• Side effects from zsh plugins (e.g. ZSH_TMUX_AUTOSTART=true trying to create tmux sessions)

Evidence

# Parent process (claude) has correct SHELL:
cat /proc/$PPID/environ | tr '\0' '\n' | grep SHELL
# Output: SHELL=/usr/bin/fish

# But Bash tool runs zsh:
echo "$0 / $SHELL"
# Output: /usr/bin/zsh / /usr/bin/zsh

# Login shell is fish:
getent passwd $USER | cut -d: -f7
# Output: /usr/bin/fish

Suggested Fix

Shell detection priority should be:

1. SHELL environment variable inherited from parent process
2. Login shell from /etc/passwd (getent passwd $USER)
3. Fall back to bash

The presence of .zshrc or .bashrc files should NOT override the user's configured shell.

Workaround

Renamed .zshrc so Claude Code doesn't detect zsh.