For the past 2 weeks or so I've been doing the A/B test, sending identical prompts to Fable 5 and Opus 5 to test their ability to produce design documents for new feature work. I've consistently found that Opus 5 produces more complete, accurate and "imaginative" designs than Fable, often finding design issues or nearby bugs that Fable 5 misses. However, that creativity means Opus seems to hallucinate more, while Fable's design is clearly based on the actual existing code. Or as Opus put it: "I hedged — [Fable] checked."

By pitting them against each other I get much better design work, and then I've been happy to hand off the design file to Opus 5 for implementation. But some of the assumptions Opus 5 makes leaves me wary of relying on it too strongly. This might be fixable by prompting it to ground its answers.