[BUG] Sandbox mode breaks zsh heredocs - TMPPREFIX not set to sandbox-allowed path

Description

When sandbox mode is enabled on macOS with zsh, heredocs fail with:

(eval):1: can't create temp file for here document: operation not permitted

Root Cause

Claude Code sets TMPDIR=/tmp/claude for sandbox mode, but zsh uses TMPPREFIX (not TMPDIR) for its internal temporary files, including heredocs.

Per the official zsh documentation:

TMPPREFIX: A pathname prefix which the shell will use for all temporary files. The default is '/tmp/zsh'.

When a heredoc is processed, zsh creates temp files like /tmp/zshXXXXXX, which is outside the sandbox-allowed /tmp/claude/ directory, causing the permission error.

Evidence

# In sandbox mode:
$ echo "TMPPREFIX: $TMPPREFIX"
TMPPREFIX: /tmp/zsh

$ echo "TMPDIR: $TMPDIR"
TMPDIR: /tmp/claude

$ cat << 'EOF'
test
EOF
# ERROR: can't create temp file for here document: operation not permitted

Proposed Fix

Add TMPPREFIX to the sandbox environment setup alongside TMPDIR:

export TMPDIR=/tmp/claude
export TMPPREFIX=/tmp/claude/zsh  # <-- Missing, needed for zsh

This aligns with the recommended fix for similar TMPPREFIX issues: export TMPPREFIX="$TMPDIR/zsh"

Workaround

Users can add this to their settings.json:

{
  "env": {
    "TMPPREFIX": "/tmp/claude/zsh"
  }
}

Environment

• OS: macOS (Darwin 24.3.0)
• Shell: zsh
• Claude Code: Latest version with sandbox enabled

Related Issues

• #13583 - Sandbox bypass security issue (symptoms include heredoc failures)
• #11480 - Sandbox cwd tracking writes to blocked /tmp/ path

References

• zsh Parameters Documentation - Official TMPPREFIX documentation
• zsh heredoc temp file explanation - Detailed writeup of this issue class