I Tested Every Way .cursorrules Can Break (Most of Them Don't)

People treat .cursorrules like they're fragile. One wrong character and everything stops working, right? I spent an evening trying to break them. Malformed YAML, massive files, conflicting instructions, encoding weirdness. I wanted to find the edge cases that silently fail, the ones where your rules look fine but Cursor ignores them. Turns out Cursor is way more forgiving than you'd expect. Most things that "should" break... don't. How I tested Every test followed the same pattern: Create a .cursorrules file with the specific edge case Give Cursor a prompt that would reveal whether the rule loaded Check the output for compliance All tests ran through the Cursor CLI ( cursor agent ), which loads both .cursorrules and .cursor/rules/ files the same way the GUI does. Test 1: Huge files The worry: "My rules file is too long, Cursor probably truncates it." I tested with a 121KB file. The rules at the bottom were followed just as well as the ones at the top. Verdict: File size isn't your prob