Sonnet 5.5 built a working clone of Proof, our open-source document editor, from scratch. It did it at low effort. @kieranklaassen uses this as a test for coding models. Before Sonnet 5.5, only three models he’d tested had managed it: • Fable 5 • GPT-6 Astra • Opus 5.5 Opus 5 couldn’t do it. Sonnet 5.5 joined the group without needing a higher reasoning setting. The result wasn’t as strong as Opus 5.5’s, and a working clone isn’t a claim that it’s ready for production. Kieran still reaches…