This is because Cursor is not sending the full context even when you drag and drop things inside the chat box.

I started getting worse results from Cursor too. Then, Gemini 2.5 Pro dropped with 1M context, I repomixed my project, popped it into AIStudio, and asked it make me prompts I can feed into Cursor to fix the issues I have.

Gemini has the whole picture and the prompts it creates tell Cursor which items to change how.