Every public API comes with documentation. But some APIs go further — they ship an OpenAPI spec. It's a single JSON file that describes every endpoint, every parameter, and every possible response. Read it once and you know exactly how to use the API before writing a single line of code. Let's do that right now. What Is an OpenAPI Spec? It's a standardized JSON (or YAML) file that describes an API completely. Endpoints, required parameters, response shapes — all in one place. Tools like Swagger UI turn it into interactive documentation, but the raw file is what matters here. Step 1 — Get the Spec We're using the Swagger Petstore. It's a fake pet store API built purely for learning. Open this in your browser: https://petstore3.swagger.io/api/v3/openapi.json Save it locally. Don't try to memorize anything — just scan the structure. You'll notice three things that matter: paths — all available endpoints parameters — what each endpoint accepts responses — what comes back That's the whole mental model.…