Newer Anthropic models do worse with custom edit tools. Use the native format.

Willison documented on July 4 that Opus 4.8 and Sonnet 5 can perform worse than older versions when driving bespoke file-edit tools, because they're increasingly trained and optimized for Claude Code's native editor format (Simon Willison). This is a real trap if you're building your own agent. The intuitive move is to invent a clean edit-tool schema that fits your app. Don't. Mirror the native diff/replace format the model was actually tuned on, or you'll watch your agent regress every time a newer, supposedly better model ships. Better model, worse tool, is a genuinely counterintuitive failure mode worth remembering.