This is very anecdotal evidence but I have to rant about it... I tried 5.6 Terra (high) earlier today to fix a bug with a slow page. It just... removed the part of the page that was slow, and made it a client side request (still slow, but not blocking SSR I guess). I tried with Sonnet 5 and it correctly found the issue where an unhandled case was continuing to retry and failing. I am always telling people how the frontier models are SO much more capable than what they may think but this one thing today had me scratching my head at why it would ever do that. It was the first time I experienced the "great, I removed the failing test case" kind of issue.
I really really like Fable for software engineering cases; add an alert and write a runbook, go pull metrics for this incident, write me a TUI that does blank, etc. It is astoundingly good and I am very picky. It costs A LOT of money though. I am not sure how I get away with my Fable usage.