20 yoe, application/systems stuff, and I always run models on xhigh or max effort level.

Fable has been more intelligent, with better taste and defaults (e.g. make impossible states impossible without being told, build for testability), and considers/solves things that Opus did not.

My workflow is to run Claude in planning mode first to spit out a plan file and then review->revise cycle it with Codex or other agents.

One big tell is that Opus will say that it can't find any more revision advice for a plan file, yet Fable will find more issues but also smart pivots into better solutions. This is probably the best test since it's not based on vibes.