Microsoft’s C# SDK for Model Context Protocol Reaches Milestone Release, Fully Supporting 2025-11-25 MCP Specification.
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<div class="article-column__content">Microsoft has announced the 1.0 milestone release of its official C# SDK for building Model Context Protocol (MCP) servers and clients. This update introduces comprehensive support for the 2025-11-25 version of the MCP Specification, particularly notable for its enhanced authorization server discovery, including icon metadata for tools, resources, and prompts.
Unveiled on March 5, the MCP C# SDK 1.0 is available on GitHub. Microsoft emphasizes that this 1.0 release marks a significant stride in developing MCP servers and clients within the .NET ecosystem. The company states that developers can leverage the SDK to implement secure authorization flows, create advanced tool experiences with sampling features, and manage long-running operations.
With the authorization server discovery capabilities in the 2025-11-25 MCP specification, servers now have three distinct methods to expose the Protected Resource Metadata (PRM) document: via a “well-known” URL derived from the server’s MCP endpoint path, at the root well-known URL, and, consistent with previous versions, through a URL specified in the resource metadata parameter of the WWW-Authentication header.
Additionally, the 2025-11-25 specification incorporates icon metadata for tools, resources, and prompts. This information is now included within responses to tools/list, resources/list, and prompts/list requests. Furthermore, implementation metadata (which describes a client or server) has been expanded to include icons and a dedicated website URL.
A key enhancement in the 2025-11-25 specification is the introduction of Client ID Metadata Documents (CIMDs). These documents provide an alternative to Dynamic Client Registration (DCR) for authenticating client identities with an authorization server, with CIMD now being the recommended approach for client registration within MCP.
The 2025-11-25 specification also enables servers to incorporate tools directly into their sampling requests, allowing the large language model (LLM) to invoke these tools to generate a response. Microsoft considers this one of the most impactful new features in the specification.
For handling long-running requests over HTTP with polling, the 2025-11-25 specification significantly improves the process. Previously, clients could only disconnect and reconnect if servers explicitly provided an event ID in server-sent events, a practice not widely adopted. Now, servers that initiate a server-sent event stream for a request will begin by sending an empty event containing an event ID and an optional `retry-after` field. Following this initial event, servers can close the stream at any time, as clients can reliably reconnect using the provided event ID.
Finally, the MCP C# SDK 1.0 introduces “tasks,” an experimental feature from the 2025-11-25 MCP specification designed to provide durable state tracking and deferred result retrieval capabilities for MCP requests.
