My Cursor rules are 900 lines now and the agent seems worse, not better

Yes, delete. Always-on rules are read before every request and compete with your actual code for attention. 900 lines of rules is a lot of noise with a few signals in it.

What I do:

1. Always on: under ~50 lines. Stack, package manager, the 5–10 things it keeps getting wrong, what not to touch.
2. Glob rules for area-specific stuff (app/api/** → API conventions). Loaded only when relevant.
3. Delete rules for mistakes it no longer makes. Many of your 900 lines were fixes for one bad day.
4. Contradictions: search for "never" and "always" and read those side by side. That's where the conflicts usually are.

Test: ask a fresh chat to do a task that used to fail, see if it still fails without the rule.

Slightly different view: I don't think length alone is the issue, it's vagueness. "Write clean code" or "follow best practices" is 10 wasted lines. "Use lib/db.ts for all queries, never import supabase directly in components" is one useful line.

Go through the file and delete every rule you couldn't check with a grep.

Not a developer, but what worked for me: at the end of a session I ask the agent to suggest ONE rule based on what went wrong, and I only add it if the same mistake happened twice. My rules stay short that way.