Boris Cherny, the creator of Claude Code, has given an extremely high praise to Anthropic's newly released Claude Fable 5. He says Fable 5 is the biggest leap in model capability since Claude Opus 4.5 shipped in November 2025.
https://twitter.com/bcherny/status/2064431111154053187
After Opus 4.5, Cherny got so used to coding entirely in the terminal that he uninstalled his IDE. Now, after using Fable 5, he believes Claude has transformed from a mere code agent into a "thinking and design partner" for product building.
Cherny notes that when debugging with Fable 5, the new model solves problems in a remarkably systematic and precise way. It actively takes measurements and adds logs, and it checks whether a fix actually works before declaring success.
This verification behavior during debugging doesn't come from Claude Code's system prompts. It's a character trait the model exhibits on its own. Cherny calls it an unprecedented "big model flavor" — a self-emergent engineering rationality.