[BUG] The Bash tool in Claude Code treats `2>&1` as literal arguments when used with pipes, instead of as stderr redirection.

Bug Report: Claude Code Bash Tool Incorrectly Parses stderr Redirection with Pipes

Summary

The Bash tool in Claude Code treats 2>&1 as literal arguments when used with pipes, instead of as stderr redirection.

Reproduction

1. Direct command (FAILS)

echo "test" 2>&1 | cat

Expected: test
Actual: test 2

2. With bash -c wrapper (WORKS)

bash -c 'echo "test" 2>&1 | cat'

Output: test ✅

3. Create and run this test script (WORKS)

cat > test-bug.sh << 'EOF'
#!/bin/bash
echo "Test 1: Direct in script"
echo "test" 2>&1 | cat

echo -e "\nTest 2: Command with stderr"
ls /nonexistent 2>&1 | head -1

echo -e "\nTest 3: npm with redirection"
npm --version 2>&1 | head -1
EOF

chmod +x test-bug.sh
./test-bug.sh

Output: Works correctly, showing stderr is properly redirected ✅

The Problem

| Command | Expected | Claude Bash Tool Output |
|---------|----------|------------------------|
| echo "test" 2>&1 \| cat | test | test 2 ❌ |
| ls 2>&1 \| head | (file listing) | ls: cannot access '2': No such file or directory ❌ |
| npm run build 2>&1 \| head | (build output) | Error: tries to run "vite build 2" ❌ |

Impact

• Cannot capture build output with errors: npm run build 2>&1 | tee build.log
• Cannot filter combined output: command 2>&1 | grep error
• Standard shell patterns fail unexpectedly

Workaround

Use bash -c:

bash -c 'npm run build 2>&1 | head -20'

Root Cause

The Bash tool appears to parse the command before passing to shell, incorrectly tokenizing 2>&1 as arguments rather than recognizing it as a redirection operator. This only happens when combined with pipes (|).

Environment

• Claude Code CLI
• Platform: Linux/WSL2
• Confirmed: 2025-08-06