Codex on Windows? Turn on MXC, the new recommended sandbox that needs no admin setup

#What changed

OpenAI's Codex Windows docs now list three sandbox implementations, and MXC (Microsoft Execution Containers) is the recommended one on compatible devices. It isolates commands with native Windows process isolation, without the administrator-approved setup, extra Windows accounts, or firewall rules that the older elevated sandbox needs. elevated is now the preferred legacy fallback, and unelevated is a last resort with weaker network isolation and no denied read paths.

#Who is affected

Anyone running Codex on Windows 11. The desktop app already prefers MXC automatically for consumer accounts when the device supports it. Standalone CLI users and enterprise deployments have to opt in.

#What to do now

1. Check support first. From your project folder, with Codex CLI 0.162.0 or later, run codex -c windows.sandbox=mxc sandbox --include-managed-config --permission-profile :workspace -- cmd.exe /d /c echo MXC_OK and look for MXC_OK with exit code 0. This doesn't change your saved setting.
2. Opt in by adding prefer_mxc = true under [features] in config.toml. Codex then uses MXC when the device and policy allow it and falls back to the legacy flow otherwise.
3. In mixed fleets, keep windows.sandbox set to elevated or unelevated as the fallback. Admins can enforce features.prefer_mxc = true through requirements.toml, or block MXC with allow_mxc = false under [windows].
4. Test dev-server workflows: when the foreground command exits, leftover child processes stop, so detached servers may die. Managed networking also needs allow_local_binding = true.

#What is not confirmed

Microsoft is still rolling MXC capabilities out to Windows 11 devices, so a supported build doesn't guarantee your machine has them yet; the test command is the real check. We haven't measured any speed or reliability difference versus the elevated sandbox. Spotted via @ayyazdev on X.