Also, how important is protocol schema reliability? Without explicit Key attributes, the very best you could hope for is auto-assigned indexes, which would not be stable across (some) changes to your dto objects.

Hey, thanks for the response.
Yes, I'm talking about a Roslyn source generator. I have not explored any other generation tools for this yet.
I'm not sure about the second question, but I want to maximize my reliability on the data, I don't want to lost anything in the process.

I'm not a big fan of the classes generated by Grpc tools, and I have two use cases:
First, I simply want to transfer data received from Grpc to SignalR.

In the second case, I could have a bit of association on a repeated field (that I don't like. Why didn't they use basic IEnumerable, like everyone?), to match some data to create a pair of object, then send those pairs to SignalR, but I could do the association of values on my front, and have the same behavior as my first case.

Maybe I'm missing something, but receiving data from Grpc to send them in SignalR with MessagePack seems more complicated than I was expecting 😅

If a source generator produces the types that are missing the attributes, and you want a stable schema across evolutions of your DTOs, I'm afraid you have conflicting requirements and I'm not aware of anything that can help you.
Either use gRPC throughout, or maybe someone should author a proto->msgpack-csharp source generator.