In what format are messages exchanged between a client app and a cognitive services resource when using a REST API?

Messages exchanged between a client application and a cognitive services resource when using a REST API are typically formatted in JSON. JavaScript Object Notation (JSON) is widely used in RESTful services due to its lightweight nature and ease of readability, both for humans and machines. JSON allows for structured data representation, making it simpler for developers to parse and generate the data being sent to and from the server.

This choice is highly aligned with modern web standards where APIs prioritize efficiency and speed; JSON is less verbose than formats like XML, which leads to faster transmission and reduces the amount of bandwidth required for communication between the client and server. Additionally, most programming languages support JSON natively, which enhances its usability across various platforms and applications.

Other formats listed, such as XML, HTML, and CSV, either are less efficient for data exchange or do not conform to the structured requirements of REST APIs. XML is more verbose, HTML is designed for web content rather than data exchange, and CSV is primarily used for tabular data, which doesn't fit the needs of a structured message exchange in most REST API scenarios. Thus, JSON is the preferred and standard format for these interactions.