I am on the mid-plan (paid) and using the web interface and in the last weeks the quality of Claude's output quality decreased dramatically. The model used is Sonnet 4.5 and my workflow is copy/pasting code and inspecting the diffs.

It changes code it shouldn't change, even makes syntax errors (like a character different than the one provided in a source file).

I know these LLMs change all the time, and this is just an anecdote, but I am interested what others think.