MCP C# SDK 2.0 enables stateless HTTP, adds Multi Round-Trip Requests, adopts 2026-07-28 MCP revision

The MCP C# SDK's version 2.0 brings significant shifts to how HTTP interaction functions, making Streamable HTTP servers stateless by default and introducing Multi Round-Trip Requests (MRTR).

This stable release implements the 2026-07-28 revision of the Model Context Protocol (MCP) specification. It standardizes HTTP headers, allowing infrastructure to route MCP traffic efficiently.

The SDK team notes this specification revision is the largest since the protocol's debut, fundamentally rethinking MCP's operation over HTTP. The previous initialize handshake and the Mcp-Session-Id header are now obsolete.

Protocol version and capabilities are now carried with every request, meaning any server instance can handle any request without requiring sticky routing or shared session stores. HttpServerTransportOptions.Stateless defaults to true, though sessions can be enabled by setting it to false.

MRTR replaces the former channel servers for input. When a tool requires input, it throws an InputRequiredException containing one or more input requests, built using InputRequest.ForElicitation, ForSampling, or ForRootsList, alongside an opaque requestState.

Following this, the client gathers the responses and re-issues the call, a process McpClient handles automatically when handlers are registered.

Subsequent minor releases brought further refinement. Version 2.1 added an opt-in subscriptions/listen handler and a fallback to initialize when HTTP server/discover fails. Version 2.2 introduced HttpServerSessionMode, allowing clients using the 2025-11-25 and 2026-07-28 revisions to share one endpoint.

The SDK is available on NuGet, and documentation for versions 2.0, 2.1, 2.2, and 2.3 is accessible on GitHub. The team's next goal is establishing end-to-end authentication and authorisation.