Most modern APIs are documented using the OpenAPI Specification (you'll also hear "Swagger," its older name). It's a big structured document, often written in YAML or JSON, that describes every endpoint, every parameter, every possible response — machine-readable enough that tools can auto-generate documentation, client code, and test suites from it.

You do not need to write one. You need to be able to open one and answer three questions:

Question	Where to look in the spec
What can a caller do here?	The paths section — lists every endpoint and method
What do they have to send?	parameters and requestBody under each path
What will they get back, in every scenario?	responses — usually broken out by status code (200, 400, 404, etc.)

Here's the habit that separates PMs who influence API design from PMs who rubber-stamp it: read the responses section for every status code other than the success case. Engineers, understandably, design the happy path first and thoroughly. The error cases are where product judgment about "what does a confused developer need to hear" is most often missing entirely — and it's the cheapest possible fix, if you catch it before launch instead of after.

YOUR MOVE THIS WEEK

Ask for the OpenAPI spec (or equivalent doc) for one API you own. Open it. Find one endpoint. Read every response case listed for it, not just the success one. Write down anything that's missing, vague, or would confuse you as an outside developer.

APIs for Product Managers

Module 1Module 1: What an API Actually Is

Module 2Module 2: How APIs Work, Just Enough to Be Dangerous

Module 3Module 3: Designing APIs as a Product, Not a Feature

Module 4Module 4: The API Product Lifecycle

Module 5Module 5: Strategy, Security, and Working Cross-Functionally

Reading an API Spec (OpenAPI/Swagger) Without Panicking