Benchmarking Microsoft's MXC sandbox: git jumps from 10ms to 1.24s without a writable temp dir
lemire · x · 2026-10-13
Daniel Lemire benchmarked Microsoft Execution Containers (MXC) — a sandbox where a JSON policy file declares which files a program may read/write and whether it gets network access — for safely running agents like Claude Code or Grok locally.
Findings on a MacBook:
- Over 10ms startup overhead; worse, commands like git stall entirely when seeking a temp directory the sandbox doesn't allow.
- Without a writable temp dir: git --version goes from 10ms to 1.24s, git status from 31ms to 1.26s, python3 -c pass from 18ms to 1.37s.
- Workaround: set TMPDIR to an MXC-writable location. With the shim allowed to write a cache, overhead drops substantially (git status 31ms→55ms, python 18ms→53ms) but remains noticeable.
Verdict: a sound idea, but milliseconds add up and hurt quality of life.
Related event: Microsoft's Open-Source MXC Sandbox Slows git by 124x in Test(2 posts)→
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