Have you ever designed the use of non-standard HTTP headers? In what cases?

Yes, I have had to design the use of non-standard HTTP headers in cases where standard ones were insufficient for transmitting specific information between the client and the server.

Examples:

• Transmitting API version via the X-API-Version header to support multiple service versions.
• Using X-Request-ID for tracing requests in distributed systems.
• Headers for custom authentication or metadata transfer, such as X-Custom-Auth.

This helps to delineate logic and avoid conflicts with existing standard headers, as well as improve the readability and maintainability of the interaction protocol.