Microsoft ships MCP C# SDK 2.0 with stateless HTTP by default

On November 5, 2026, InfoQ reported that Microsoft has officially released MCP C# SDK 2.0, which implements the July 28, 2026 revision of the MCP specification. In this release, the Streamable HTTP server is stateless by default. The initialize handshake and the Mcp-Session-Id request header have been removed, while protocol version and capability declarations are now sent with each request, so any server instance can handle a request without sticky routing or shared session storage. HttpServerTransportOptions.Stateless defaults to true, and servers that need session capability can set it to false. The new version also introduces Multi-Round Turn Requests (MRTR) to replace the channel where the server asks the client for input mid-call. Clients first probe server/discover and fall back to the legacy handshake when an older server is detected, so older clients can automatically downgrade. Configurations that only apply to stateful mode and legacy SSE endpoints now trigger explicit warning codes, including MCP9006, MCP9004 and MCP9005. The C# SDK is co-maintained by Microsoft and built on ASP.NET Core. According to the report, the SDK team called this specification revision the largest since the protocol was released, because it redesigns how MCP runs over HTTP. The same report also notes that versions 2.1 through 2.3 were released between August and October. Public information about this event currently comes from this single report, and the matter stands at the point where SDK 2.0 has been released and its compatibility behavior and warning codes have been explained.