[BUG] Bash tool fails with command substitution in pipe arguments

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?

Command substitution $(date ...) fails when used as an argument to a piped command like tee.

Reproduction: get Claude to run:
echo "test" | tee test-$(date +%Y%m%d_%H%M%S).log

Error:
(eval):1: unknown file attribute:

Expected: Should create a log file with timestamp

Simple pipes without command substitution do work fine...
echo "test" | tee foo.log

What Should Happen?

Fails with "unknown file attribute" error

Error Messages/Logs

● You're right - I should have noticed that both attempts with tee failed. I'll    01:49 PM  claude-sonnet-4-5-20250929
  remember not to use piping to tee in future commands.

  Instead, I should either:
  1. Run the command directly and let output go to terminal
  2. Redirect to a file with > logfile.log 2>&1
  3. Use the run_in_background parameter if needed and monitor output separately

  Thanks for the feedback - I won't use pipe to tee going forward.

> I wonder if this is a bug in claude code; can you try to reproduce it? can you run "echo hello 2>&1 | tee foo.log" and 
see if it works? 

● Bash(echo hello 2>&1 | tee foo.log)
  ⎿  hello

● Bash(cat foo.log)
  ⎿  hello

● That worked fine. Let me try with the date substitution like we did before:      01:50 PM  claude-sonnet-4-5-20250929

● Bash(echo "test log" 2>&1 | tee test-$(date +%Y%m%d_%H%M%S).log)
  ⎿  Error: Exit code 1
     (eval):1: unknown file attribute:

● There's the issue! The $(date +%Y%m%d_%H%M%S) command substitution fails with    01:52 PM  claude-sonnet-4-5-20250929
  "unknown file attribute". This appears to be a Claude Code issue with how it
  handles command substitution in pipes. The workaround we tried (setting LOGFILE
  variable first) also failed.

  So yes, this looks like a bug in Claude Code's Bash tool - it can't properly
  handle $(...) command substitution when used with pipes. Simple pipes work, but
  command substitution in the piped arguments breaks.

  Good catch! For now, the workaround is to either:
  1. Use > redirection instead of tee
  2. Pre-create the filename without command substitution
  3. Use a hardcoded filename

Steps to Reproduce

Ask Claude Code to run: echo "test log" 2>&1 | tee test-$(date +%Y%m%d_%H%M%S).log

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.64

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_