API-first means the product is designed so every feature is accessible via API before — or simultaneously with — any UI. Developers can build on top of it programmatically, making it composable within other stacks and workflows. The API is the product, not an afterthought.
API-first products dominate developer infrastructure: Stripe for payments, Twilio for communications, Cloudflare for networking. The approach unlocks a second distribution channel — developer adoption — and a powerful moat, because switching requires unwinding embedded API calls across an entire codebase, not just a dashboard login.