With the MCP C# SDK, which now supports protocol specification version 2025-06-18, developers get a new authentication protocol that enhances security and flexibility for AI applications, according to Microsoft. The new protocol separates roles of the authentication and resource servers, enabling easier integration with existing OAuth 2.0 and OpenID connect providers.
Developers using the updated SDK also gain access to MCP’s new elicitation feature, which allows servers to request additional information during interaction. Elicitation enables more dynamic and interactive AI experiences, making it easier to gather necessary context before executing tasks, Microsoft said. Elicitation is an optional feature. Clients can declare support for it as part of their initialize request. In the MCP C# SDK, this is done by configuring an ElicitationHandler in the McpClientOptions.
Finally, the 2025-06-18 spec also supports structured tool output. Prior to this support, tool results were allowed to contain structured data but the LLM had to perform parsing and interpretation without any guidance from the tool itself. Now, tools can return structured content that is explicitly defined. This allows AI models to better understand and process the output, according to Microsoft. The MCP C# SDK enables tools to specify that their output is structured via the UseStructuredContent parameter of the McpServerTool attribute.
