How to Use JSON Schema Validation in Postman for API Contract Testing

API responses can silently change structure over time — a field type shift or missing property can break client apps without triggering standard value-based test assertions. JSON Schema validation in Postman, powered by the built-in ajv validator, catches these structural changes systematically across all endpoints in roughly ten lines of script. Developers can define a schema once as a collection variable and reuse it across multiple requests such as /users, /login, and /teams/:id/members, ensuring consistent response shapes throughout an API. Key schema rules include always listing required fields explicitly, distinguishing between integer and number types, and using pattern or enum constraints to flag unexpected values early. This approach turns schema validation into centralized contract testing, meaning a single schema update propagates across every endpoint test simultaneously.