Microsoft’s official C# SDK for implementing Model Context Protocol (MCP) servers and clients has reached its 1.0 milestone release. The update brings full support for the 2025-11-25 version of the MCP Specification, highlighted by enhanced authorization server discovery and icon metadata for tools, resources, and prompts.
MCP C# SDK 1.0 was unveiled March 5 and can be found on GitHub. The MCP C# SDK 1.0 release represents a major step forward for building MCP servers and clients in .NET, according to Microsoft. Developers can use the SDK to implement secure authorization flows, build rich tool experiences with sampling, or handle long-running operations, the company said.
With authorization server discovery in the 2025-11-25 MCP specification, servers have three ways 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, as before, via a URL in the resource metadata parameter of the WWW-Authentication header.
The 2025-11-25 specification also adds icon metadata to tools, resources, and prompts. This information is included in the response to tools/list, resources/list, and prompts/list requests. Implementation metadata (describing a client or server) also has been extended with icons and a website URL.
The 2025-11-25 specification features Client ID Metadata Documents (CIMDs) as an alternative to Dynamic Client Registration (DCR) for establishing client identity with an authorization server. CIMD now is the preferred method for client registration in MCP.
Another capability in the 2025-11-25 specification is that servers now can include tools in their sampling requests, which the large language model (LLM) may invoke to produce a response. This is one of the most powerful additions in the specification, Microsoft said.
For running requests over HTTP with polling, the 2025-11-25 specification improves the story for long-running requests. Previously, clients could disconnect and reconnect if the server provided an event ID in server-sent events, but few servers implemented this. Now, servers that open a server-sent event stream for a request begin with an empty event that includes an event ID and optionally a retry-after field. After sending this initial event, servers can close the stream at any time, since the client can reconnect using the event ID.
Finally, MCP C# SDK 1.0 introduces tasks, an experimental feature of the 2025-11-25 MCP specification that provides durable state tracking and deferred result retrieval for MCP requests.
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