Compatibility between Core SignalR server and Net 4 SignalR clients.

[tdhintz]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe the problem.

We have a Net 4 winform providing a bridge between browsers and Windows TWAIN scanners. It was migrated to Net Core and now the negotiate fails. Research suggests this is because there is no compatibility option between Core and Net 4 SignalR servers and clients. The impact is that it is not possible to update either the SignalR server or client without requiring a reinstall of the winform bridging component across the entire customer base. This is a huge logistical issue.

Describe the solution you'd like

Provide a Net 4 SignalR compatibility mode for Core, or describe how Core can be configured for compatibility.

Additional context

Trace:

Debug: 2025-01-03 09:22:25.757 1 candidate(s) found for the request path '/s/negotiate'
Debug: 2025-01-03 09:22:25.758 Request matched endpoint '/s/negotiate'
Debug: 2025-01-03 09:22:25.760 The maximum request body size has been set to 104857600.
Information: 2025-01-03 09:22:25.761 Executing endpoint '/s/negotiate'
Information: 2025-01-03 09:22:25.763 Executed endpoint '/s/negotiate'
Information: 2025-01-03 09:22:25.764 Request finished HTTP/2 GET https://localhost:59754/s/negotiate?clientProtocol=2.1&connectionData=%5B%7B%22name%22%3A%22s%22%7D%5D&_=1735912793367 - 405 0 text/plain 14.2070ms