[BUG] Environment variables expand to empty string when command contains pipe

[BUG] Environment variables expand to empty string when command contains pipe

What's Wrong?

Environment variables like $HOME expand to an empty string when the Bash tool command contains a pipe (|). This occurs even when the variable is used before the pipe in the command.

What Should Happen?

Environment variables should expand correctly regardless of whether the command contains pipes. echo "$HOME" | cat should output the same value as echo "$HOME".

Reproduction Steps

1. Run a simple command without a pipe:

``bash
echo "HOME=$HOME"
`
**Result:** HOME=/Users/username` ✓

1. Run the same variable expansion with a pipe:

``bash
echo "$HOME" | cat
``
Result: (empty output) ✗

1. Use the variable in a cat argument with a pipe:

``bash
cat "$HOME/.bashrc" 2>&1 | head -1
`
**Result:** cat: /.bashrc: No such file or directory (note the missing /Users/username` prefix)

1. Same command without a pipe works:

``bash
head -1 "$HOME/.bashrc"
``
Result: (file contents displayed correctly) ✓

Error Messages/Logs

No error messages - the variable silently expands to an empty string.

Is this a regression?

Unknown - I have not tested previous versions.

Environment

• Claude Code version: 2.1.12
• API: Anthropic (direct)
• Model: claude-opus-4-5-20250101
• OS: macOS 26.1 (Darwin 25.1.0)
• Terminal: Terminal.app with zsh

Additional Context

This bug was discovered while building a skill that reads JSON files from ~/Library/Application Support/. The workaround is to have Python handle path expansion using os.path.expanduser('~') instead of relying on shell variable expansion.

This may be related to issue #2859 (pipe characters in quoted args being misinterpreted), as both suggest the Bash tool does unusual preprocessing when pipes are present.

Workaround

Instead of:

cat "$HOME/path/to/file.json" | python3 -c "..."

Use:

python3 -c "
import os
path = os.path.expanduser('~/path/to/file.json')
with open(path) as f:
    # process file
"