[BUG] Model emits fish-syntax to Bash tool when login shell is fish

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?

On a machine where the login shell is fish (/usr/bin/fish), Claude emitted fish-style command substitution to the Bash tool:

readlink -f (which claude)

The Bash tool executes /bin/bash, so this fails:

/bin/bash: eval: line 1: syntax error near unexpected token `('

What Should Happen?

The correct bash form is readlink -f "$(which claude)".

Commands sent to the Bash tool should always be POSIX/bash syntax, independent of the user's login shell.

Error Messages/Logs

Error: Exit code 2
     /bin/bash: eval: line 1: syntax error near unexpected token `('
     /bin/bash: eval: line 1: `readlink -f (which claude)'

Steps to Reproduce

Unsure, but here is a transcript:

❯ bundle check?

● Locating install first.

● Bash(readlink -f (which claude))
  ⎿  Error: Exit code 2
     /bin/bash: eval: line 1: syntax error near unexpected token `('
     /bin/bash: eval: line 1: `readlink -f (which claude)'

  Thought for 3s (ctrl+o to expand)

● Fish syntax leaked. Bash here.

● Bash(readlink -f "$(which claude)")
  ⎿  Interrupted · What should Claude do instead?

❯ fish syntax?

  Thought for 6s (ctrl+o to expand)

● Tool runs /bin/bash, not your fish login shell. My first call mistakenly used fish-style (which claude) → bash syntax error. Second call $(which claude) is correct bash. That's the one you rejected.

  Re-run the bash version?

  readlink -f "$(which claude)"

✻ Churned for 7s

❯ why you tried fish syntax?

  Thought for 5s (ctrl+o to expand)

● Mistake. Env block says Shell: /usr/bin/fish, so I pattern-matched to fish command-sub (...). Wrong — the Bash tool always executes via /bin/bash no matter your login shell. Should've used $(...) from the start. My error, not yours.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.191

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

Environment

• Claude Code 2.1.191
• Login shell: fish 4.x
• OS: Linux (Debian 13)