The PMF Detection Kit. Eight operators on knowing you've got it.

Sean Ellis built the first growth team at Dropbox, then turned one survey question into the most-copied product-market-fit test in startups. Ask your active users a single thing: how would you feel if you could no longer use this product. Give them three options. Very disappointed, somewhat disappointed, or not disappointed.

The only number that matters is the share who say very disappointed, and the threshold Ellis drew is 40%. Above it, you hold a leading indicator of fit, earlier and faster than any retention curve will give you. Below it, you do not have a messaging problem, you have a must-have problem. The somewhat-disappointed crowd is the trap.

They like you and they will not miss you, and optimizing for them quietly buries the signal from the people who actually would.

Eric Migicovsky founded Pebble and now teaches YC founders how to talk to users, and his rule starts before the product exists: first understand if the problem is real. The questions that get you there are about the user's life, not your idea. How do you do this today. What is the hardest part.

When did it last go wrong. The pitfalls are the questions founders love most. Leading questions that fish for a yes, feature questions that ask would you use this, hypotheticals that invite a polite lie. The user's job is to describe the problem in their own words.

The moment you ask them to design your solution, you have stopped learning and started selling, and the signal goes dark.

Michael Margolis runs user research at GV and has watched founders burn months arguing about who the customer even is. His Bullseye Customer Sprint compresses that argument into a single day. Pick five bullseye customers, the people most likely to need you right now. Build three simple prototypes, rough enough to throw away without flinching.

Then interview, with the entire team in the room watching the same reactions at the same time. The output is not a persona deck. It is a shared, first-hand picture of who lights up and who shrugs, formed by everyone who will go build the thing. When the team watches the same person fail to care, the internal debate ends on its own.

Jake Knapp wrote the book on the Design Sprint at GV, then built the Foundation Sprint for the stage before you have anything to test. It runs two days through three phases. Basics, where you name the customer, the problem, and the competition without flinching. Differentiation, where you define the one or two things you will do that others cannot or will not.

Approach, where you map the paths you could actually take to build it. What you carry out is a founding hypothesis, written down, specific enough that the market can prove it wrong. Most founders haul that hypothesis around as a vague feeling for a year. The sprint forces it onto a single page in forty-eight hours, which is the only form a bet can be tested in.

Gustaf Alströmer scaled growth at Airbnb before becoming a YC partner, and the trap he watches founders fall into is their own certainty. On day one you believe you understand the problem, the customer, and the solution completely. You have to, he says. Overconfident foolishness is a key part of doing anything ambitious in life.

The error is staying there. The founders who find fit flip fast, from we have it figured out to maybe we do not know anything, usually only after they have burned the bridge behind them and quitting is off the table. What replaces the certainty is not a framework. It is attention.

The fastest way to learn what you need to know, about the problem and the solution both, is to actually care about your customers.

Immad Akhund started Mercury because the banking tools available to startups were, in his words, painful, and nobody with the chops to fix it had bothered. His counter to the minimum-viable-product instinct is the minimum delightful product. In a category like banking, viable is not enough, because viable is exactly what every incumbent already ships and nobody loves. So Mercury picked one customer profile, early-stage startups, and set a high bar for the first release.

Domestic and international wires, real support for immigrant founders, the specific things that would make a specific person switch and stay. The signal he was hunting was not tolerance. It was delight, from a narrow group, on purpose. Build the thing one type of customer finds delightful, not the thing everyone finds acceptable.

Elena Verna has run growth at Miro, Amplitude, and Lovable, the AI builder that reached 200 million dollars in ARR inside a single year. Her read on 2026 is blunt: the minimum viable product is finished as a concept. When anyone can ship viable in a weekend, viable is not a moat, and the bar moves to the minimum lovable product, where delight and emotional connection are the differentiators. Lovable's own playbook leans all the way in.

Launch new growth loops instead of optimizing old ones, give the product away generously, and build a community that does the adoption for you. The PMF question is no longer do they use it. It is do they love it enough to bring someone else through the door.

Shiv Rao spent five years in what he calls the desert before Abridge found product-market fit, on the way to a 5.3 billion dollar valuation. His story is the counterweight to the 40% test. Some categories, especially regulated ones like healthcare, do not hand you a fast leading indicator at all. The signal that keeps you alive is a different one.

It is founder-partner fit, the match between you and the specific investors who will fund conviction through years without a clean metric. Rao built deep relationships with backers who understood the arc and would not panic at a flat dashboard. Knowing you have PMF is one skill. Knowing you are in a category where PMF takes years, and choosing partners accordingly, is the skill that decides whether you are still around to detect it.