[BUG] $TMPDIR resolves to different paths between sandboxed and non-sandboxed Bash invocations

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?

$TMPDIR resolves to different directories depending on whether a Bash command runs sandboxed or not:

• Sandboxed Bash: $TMPDIR = /tmp/claude-501
• Non-sandboxed Bash: $TMPDIR = /var/folders/sg/.../T/ (macOS default)

Sandbox mode is applied per-command based on the excludedCommands configuration. This means within the same session, one command may run sandboxed (seeing /tmp/claude-501) while another runs unsandboxed (seeing /var/folders/.../T/). Files written to $TMPDIR by a sandboxed command are invisible to an unsandboxed command, and vice versa.

This is compounded by the file tools (Read/Write/Edit), which don't evaluate $TMPDIR at all — the model decides which path to use based on what it previously observed. If the model saw /tmp/claude-501 from an earlier sandboxed Bash invocation, it will pass that literal path to Read/Write even when Bash has since switched to the host $TMPDIR, or vice versa.

The system prompt instructs the model to "always use the $TMPDIR environment variable" for temporary files, but this guidance breaks down when the variable points to different locations across invocations.

What Should Happen?

$TMPDIR should resolve to the same directory across all Bash invocations in a session, regardless of sandbox state. File tools should also have a reliable way to reference the same temp directory.

Steps to Reproduce

1. Start a Claude Code session on macOS with sandbox enabled
2. Run echo $TMPDIR via a sandboxed Bash command — returns /tmp/claude-501
3. Run echo $TMPDIR via a non-sandboxed Bash command (e.g. one matching excludedCommands) — returns /var/folders/.../T/
4. Write a file in one context: echo hello > $TMPDIR/test.txt
5. Try to read it from the other context — file not found because $TMPDIR resolves to a different directory

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.109

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_