Opus: Yeah, it can be a little fuzzy at 600-700k, I prefer it 400K and under in general. But, I've had usable sessions to 850k.

It really depends on the session, and what you are doing how far you can push it.

Not really, on the project I'm working on right now most of my longer prompts seems to consume somewhere between 300k and 400k tokens (input + output) and I find that both Opus 4.8 and Fable 5 don't seem to get lost with that much context. At least the results still look good to me.

I don't think I've ever pushed past 500k though, it may well fall apart at the higher end of what is officially supported.

I see it more past 300-350k these days, but probably there is a smaller decline even past 250k.