MCP C# SDK 2.0 makes HTTP stateless by default, adds multi round-trip requests

Microsoft released the stable MCP C# SDK 2.0, implementing the July 28, 2026 revision of the Model Context Protocol spec. Streamable HTTP servers are now stateless by default, HTTP headers are standardized for routing, and Multi Round-Trip Requests replace server-initiated input for interactive tools.

• The initialize handshake and Mcp-Session-Id header are removed; version and capabilities travel with every request
• HttpServerTransportOptions.Stateless defaults to true; servers needing sessions set it to false
• MRTR replaces server-to-client requests via InputRequiredException and requestState
• Packages target .NET 8, 9, 10 and .NET Standard 2.0, available on NuGet