It's the users job to provide the context the LLM needs in plain language instruction. Not just relying on the LLM to magically understand everything based on the codebase.
You can create markdown files containing all the planning you did and Cursor will have all of that as context to give you better suggestions. This type of prompting is what gives amazing results - not just relying on out of the box magic, which I think a lot of people are expecting.
Anecdotal: Even with Web Browsing enabled in the Options menu, it continues to report that the feature doesn't exist and cites the September 21 cut off. Odd!