Why do we have CorrelationIdOperationFilter? app.UseCorrelationId() already sets a X-Correlation-ID header.

[VictorioBerra]

As per the docs, also I tested this myself and got this: X-Correlation-ID: 0HLF2MM0MDN26:00000007.

The TraceIdentifier on the HttpContext will be used for new requests and additionally set a header on the response.

Poor wording but it does in fact set the header.

[RehanSaeed]

All CorrelationIdOperationFilter does is let you set the X-Correlation-ID from Swagger by providing a text box and some nice documentation.

The whole point of X-Correlation-ID is to send a unique GUID from your client to your server (if you control both), then include the X-Correlation-ID in any logs (I need to add this as a feature). This gives you the ability to correlate logs from your client to your server.