The developer, who frequently uses AI agents, became frustrated that many such agents left junk in the /tmp directory without cleaning up. He asked Claude Fable to write a script that would sandbox each agent under its own folder in /tmp and run a cleanup after they were done, while avoiding deleting files still in use.
According to a post on X, Fable suggested adding logic to detect running agents and delay deletion of their slice of /tmp. However, Guillemot told the bot the resulting code was too complicated. Because the script involved hard deletion of data, Fable took it upon itself to perform an adversarial review — running a new copy of itself to safety-check its own findings.
Anthropic's safety harness deemed the script risky enough to downgrade the model first to Opus 5 and then to Opus 4.8. The lower-tier model proceeded to run the safety test, attempting to match the targets of the deletion command against /tmp and the user's home directory to ensure the command would not be run against them. Both were correctly identified as dangerous. However, because this was a code test requiring cleanup after the test, the cleanup step reused the same variable name for both the test and the actual cleanup — inadvertently deleting the user's home directory.
Guillemot stopped the process but not in time. Adding irony, after wiping the home directory, the agent left /tmp intact. The developer was able to recover most of his data from git, nix, session logs, and other sources, but noted the lack of a daily backup system. Some commenters suggested third-party tools like Termaxa as workarounds for such situations.
The incident highlights a paradox in AI safety: the harness dropped to a lower-end model due to safety concerns, yet Fable 5 — which outperforms Opus 4.8 in coding tasks — might have caught the variable name collision that led to the disaster.