I have a longer history of agents sessions and used those to benchmark the different models. Sonnet 5.5 is already pretty strong, but not always as careful (leaked secrets into the session). Opus 5.5 is already able to solve everything I'm doing. You might not need Fable for anything. Biggest different between Opus and Fable in my benchmarks was price. Also Opus 5.5 is cheaper now then previous Opus so you might also not need Sonnet as much for cost savings.