Anthropic has quietly added a server-side rule to Claude that blocks users from recreating a specific existing game — even if it is renamed or called original. The rule, delivered under a flag called 'tengu_heron_brook,' first appeared in Claude Opus 5.5's changelog on October 27 and was confirmed in reporting on October 30; there is no setting to turn it off.

Original work is still allowed. The rule expressly permits building original games, games in a genre, and games from the user's own designs and assets — the line is drawn at recreating a particular existing title.

The change lands after weeks of users pairing Claude with reverse-engineering tools to decompile and rebuild existing games, and users are now reporting that related new safety classifiers seem to be limiting other work too, from security research to published coding books. Anthropic has not published a detailed explanation of the rule's scope.