Key point
One habit
Treat a prompt like a function. It takes input, returns output, and should pass repeatable checks before you trust it in a demo.
You do not need a huge test suite. For a hackathon project, 8 to 15 real cases are enough to catch many bad prompt changes.
What to test
Test the task your user actually needs. If your app helps students turn messy notes into a study plan, test messy notes from real college life, not only clean samples.
Each test case needs input, the expected behaviour, and a simple pass check. The pass check can be exact, like valid JSON. It can also be human-rated, like “mentions deadline and splits work by day”.