A user of the .NET stack recently reported that the .NET stack is always sending the application Uri (server Uri) on the CreateSession service call and that the serverUri should be set to null instead as it is stated in the spec: https://reference.opcfoundation.org/Core/Part4/v104/docs/5.6.2.2

"This value is only specified if the EndpointDescription has a gatewayServerUri.
This value is the applicationUri from the EndpointDescription which is the applicationUri for the underlying Server. The type EndpointDescription is defined in 7.10."

The reporter claimed that he couldn't connect to a server when the field was set. However talking to the SDK vendor he claimed it should not be an issue, but it may be checked if the field is set.

Sending the serverUri had been tested for years in the IOP and in the field and I'm not aware of any issues.
Even worse, servers may check the field and changing it would cause IOP issues.
So I'm hesitating to change the code but it is not spec compliant.

If more SDK implement it this way, I would like to discuss if the text should be changed to allow to do it both ways as it seems to be 'de facto' standard.
Also gateways server uri will be deprecated.

Are there specific CTT tests that validate the behavior to allow for setting the application Uri?

see also https://github.com/OPCFoundation/UA-.NETStandard/pull/2193