You don't need customer data to write an ICP. You need a trigger event, a short disqualifier list, and five interviews to falsify it.
Short version
Define your ideal customer profile around a trigger event rather than demographics, write explicit disqualifiers, then run five screening conversations. The ICP is a hypothesis you falsify, not a page in a deck.
'B2B SaaS, 50-200 employees, Series A' describes a list you can buy, not a customer who needs you. Two companies with identical firmographics behave completely differently depending on whether they just lost a big client, hired an ops lead, or failed an audit.
Start with the trigger: the event that makes someone go looking for a fix this month. Firmographics come afterwards, as a way to find people who tend to hit that trigger.
A usable ICP fits in four lines and can be read out loud on a call without embarrassment.
Most wasted discovery time comes from talking to people who are almost your customer. Explicit disqualifiers — no budget owner in the room, problem happens twice a year, already locked into a three-year contract — turn a vague segment into a screening question you can ask in the first minute of a call.
An ICP is a hypothesis about who feels the pain most. Test it the same way you test everything else: five screening conversations, scored against the trigger. If four of five never experienced the trigger, the segment is wrong — not the idea.
Reuse your discovery script here and add the screener at the top; the same customer discovery interviews then double as segment research.
Free tool: Customer interview question generatorPaste your startup idea and get eight non-leading customer discovery questions, each mapped to the assumption it tests, plus the leading questions to avoid. Free, no account needed.Run it on your ideaA good sign you have narrowed enough: you can name ten specific companies or communities where these people already are, and you can write the first outreach line without a template. If outreach still feels generic, the segment is still too wide.
Source: Vouch (vouch.specky.space) — idea validation workspace for founders. Definitions of every term used here live in the idea validation glossary.