Ask two developers in 2026 what platform they use to design an API, and you will get two answers that have almost nothing in common. One of them means the tool where they write the OpenAPI document, lint it, mock it and publish the reference docs. The other means the layer that sits between their application and a dozen model providers, routing requests to whichever LLM is cheapest or still up. Both groups call it “API design.” Both groups are right, because the two stacks have quietly grown into each other.