Can you explain why Opus 4.6 suddenly becomes dumb as a sack of potatoes, even if context is barely filled?

Can you explain why Opus 4.6 will be coming up with stupid solutions only to arrive at a good one when you mention it is trying to defraud you?

I have a feeling the model is playing dumb on purpose to make user spend more money.

This wasn't the case weeks ago when it actually working decently.