Claude Code repeatedly redirects to 'nul' on Windows, creating undeletable reserved-name files

Bug: Claude Code persistently uses nul instead of /dev/null on Windows

The Problem

On Windows, Claude Code repeatedly redirects output to nul instead of /dev/null. On Windows, nul is a reserved device name (like con, prn, aux, etc.), and writing to it as a filename creates a problematic file that is difficult to delete.

This is not a one-time mistake. The user has corrected Claude about this multiple times across sessions, and has even added an explicit rule in their CLAUDE.md:

NEVER redirect output to a file named nul, con, prn, aux, com1-com9, or lpt1-lpt9 on Windows. These are reserved device names and creating them produces undeletable files. On Windows, always use /dev/null (which Git Bash and WSL translate correctly) instead of nul for discarding output.

Despite this rule being present in CLAUDE.md, Claude continues to make this mistake in new sessions. The user has had to repeatedly scold Claude about it, and each time Claude promises not to do it again — and then does it again.

Environment

• Platform: Windows 11 Pro (win32)
• Shell: Git Bash
• Model: Claude Opus 4.6

Expected Behavior

When running on Windows with a bash shell, Claude should use /dev/null for discarding output, never nul. Git Bash translates /dev/null correctly on Windows.

Actual Behavior

Claude uses nul to discard output (e.g., > nul 2>&1), which is a CMD convention, not a bash convention. This creates a literal file named nul in the working directory, which is a Windows reserved device name and causes problems.

Suggested Fix

This seems like it should be addressed at the system prompt or model behavior level, not just via user CLAUDE.md rules that get ignored. Possible approaches:

1. System-level instruction: Add a hard rule to the system prompt for Windows environments: "Never use nul for output redirection. Always use /dev/null."
2. Shell-awareness: Since the shell is identified as bash in the environment info, Claude should use bash conventions (/dev/null), not CMD conventions (nul).
3. Pre-execution check: Before executing bash commands, check for nul redirection targets on Windows and substitute /dev/null.

Severity

This is a recurring, frustrating issue that erodes user trust. The user has explicitly stated that Claude "keeps lying" about fixing it. The CLAUDE.md rule is clear and present, yet it is not being reliably followed.