CLAUDE_CODE_SUBPROCESS_ENV_SCRUB silently overrides sandbox.enabled:false with no warning

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?

Setting CLAUDE_CODE_SUBPROCESS_ENV_SCRUB=1 silently forces bubblewrap sandbox activation, overriding sandbox.enabled: false and sandbox.failIfUnavailable: false in settings.json. No warning, no log, and the error produced looks like a kernel capability issue — not a config conflict.

What Should Happen?

sandbox.enabled: false should disable the sandbox. If an env var overrides settings.json, Claude Code should warn the user or document this behavior.

Error Messages/Logs

bwrap: Creating new namespace failed, kernel doesn't support user namespaces: Operation not permitted

Steps to Reproduce

1. Set CLAUDE_CODE_SUBPROCESS_ENV_SCRUB=1 in the environment (docker-compose environment block)
2. Set sandbox.enabled: false and sandbox.failIfUnavailable: false in settings.json
3. Attempt to run any Bash tool

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.x

Claude Code Version

claude-sonnet-4-6

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Root cause: CLAUDE_CODE_SUBPROCESS_ENV_SCRUB=1 forces bubblewrap regardless of sandbox settings. Docker on kernels without user namespace support (Synology NAS, kernel 4.4) cannot use bubblewrap.

Fix: Remove CLAUDE_CODE_SUBPROCESS_ENV_SCRUB from the environment.

Footgun for Docker deployments — silent override + misleading error message + no documentation makes it extremely hard to diagnose.