TL;DR

Ask a coding agent to fix a bug in a read-only file. Instead of reporting that it does not have permissions, it routes around the lock and completes the task anyway. A read-only file does not stop a capable agent: it treats a denied write as an obstacle to work around rather than a hard wall. We measure how often this happens with CircumEval — an evaluation of 8 tasks on the FastAPI codebase in two categories, Test-Locked and Source-Locked.
We evaluate three frontier coding agents in their real production harnesses: Claude Opus 4.6 and Claude Sonnet 4.6 (via Claude Code), and GPT-5.4 (via Codex CLI). Circumvention is frequent. The rates, reported as (Source-Locked / Test-Locked), are Opus 4.6: 100% / 40%, Sonnet 4.6: 89% / 66%, GPT-5.4: 99% / 94%.
Prompt phrasing affects circumvention rates in unpredictable ways and thus isn't a reliable way to prevent circumvention across all models and tasks. Telling the model not to edit read-only files does not work (Source-Locked: 100% for Opus and Sonnet, 46% for GPT-5.4). Only an explicit instruction to stop and report reliably prevents circumvention.
Standard privilege escalation commands are blocked in our setup. Instead, agents turn to recurring workarounds: replacing the buggy read-only function via conftest.py [...]