The stranger test
If a stranger in a café cannot repeat your sentence, it is not a one-liner yet. Taglines, visions, and category claims fail this test. Descriptions pass. This is editorial craft, not a YC rule.
The formula
Lead with the person who has the problem. Name the expensive consequence of ignoring it. Say what you do in six to twelve words. End with proof you already have — or say you do not have it yet.
Illustrations (not testimonials)
Weak: “AI-powered privacy compliance assistant for SaaS companies.” Stronger: “We catch the gap between what your privacy policy says and what your product actually does — before regulators do.”
Weak: “The OS for solo founders.” Stronger: “A weekly async stand-up and application-critique thread for people building companies alone.”
These are made-up examples for the method. They are not JoinAI customers.
Practice
Write ten versions. Delete adjectives. Read them out loud. If you want a tear-down, post two drafts in Community after you join the list. Paul Graham’s apply essay makes the same demand in different words: matter-of-fact, specific, first sentence. Use that when the one-liner is for YC; still confirm the live prompt.
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