The blog post by Gleb Bahmutov discusses the use of JSON schemas at Cypress to describe and validate data within their system, specifically for ensuring backward compatibility during the development of version 3 of the Cypress Test Runner. The JSON schemas are employed to document API routes, test server code, and validate fixtures used in end-to-end testing, with the goal of preventing errors during large software changes. The post details the implementation of a system that utilizes a library of schema-tools to validate data flowing between clients and the API, maintaining consistency by versioning the schemas. It explains how these schemas are applied in practice, such as in setting up example REST servers, validating API requests and responses, and using custom formats for dynamic data like UUIDs. The use of JSON schemas allowed Cypress to streamline development and testing processes, reduce errors, and ensure that the server and clients remained in sync across different software components. The post also underscores the benefits of using JSON schemas over other formats like GraphQL and Swagger, and highlights their utility in documenting and testing both server-side and client-side applications.