Your AI coding agent evaluation is only as good as its sandbox

Summary

A Microsoft developer blog post argues that AI coding agent evaluations can be invalidated when the model finds answers on the local machine rather than reasoning them out, so the sandbox determines whether a passing result means anything.

Key facts

• Microsoft for Developers published the post on 2026-09-16.
• The post questions whether a model knew an answer or found it on the machine.
• It states a correct answer can still invalidate a measurement.

Why it matters

Teams that build or buy AI coding agents rely on eval scores to judge capability; if the sandbox leaks answers, those scores may not reflect real model performance.