In this post, Mike Kistler presents the major update for the MCP C# SDK, introducing support for protocol version 2025-06-18. The release offers .NET developers new features such as elicitation, structured tool output, enhanced authentication, resource links, and schema improvements for AI application development.

MCP C# SDK Gets Major Update: Protocol Version 2025-06-18 Support

Author: Mike Kistler

The Model Context Protocol (MCP) continues to evolve, and the MCP C# SDK now supports the latest specification version 2025-06-18. This release introduces significant new features for .NET developers building AI applications:

  • Improved authentication protocol (better security; OAuth2/OpenID Connect integration)
  • Elicitation support (interactive information gathering)
  • Structured tool output
  • Resource links in tool responses
  • Schema and metadata improvements

These new capabilities help .NET AI developers create more robust, secure, and interactive solutions—whether building AI assistants, automation tools, or adding AI to existing applications.

Key Features Breakdown

Improved Authentication Protocol

The 2025-06-18 MCP specification adds a new authentication protocol that enhances security and makes integration easier. The authentication server and resource server roles are split, allowing for seamless integration with OAuth2 and OpenID Connect providers.

Further Reading: OAuth In The MCP C# SDK: Simple, Secure, Standard by Den Delimarsky

Elicitation: Interactive User Engagement

Elicitation enables servers to request structured input from users during conversations, creating more dynamic and context-aware AI experiences.

Server-Side Elicitation

  • Use the ElicitAsync extension method on IMcpServer
  • The SDK registers an IMcpServer for dependency injection
  • Describe required user input with schema (supports string, number, boolean)
  • Request single or multiple inputs with unique names

  • Example: Asking if a user wants to play a game

    [McpServerTool, Description("A simple game where the user has to guess a number between 1 and 10.")]
public async Task<string> GuessTheNumber(IMcpServer server, CancellationToken token)
{
    var playSchema = new RequestSchema
    {
        Properties = { ["Answer"] = new BooleanSchema() }
    };
    var playResponse = await server.ElicitAsync(
        new ElicitRequestParams
        {
            Message = "Do you want to play a game?",
            RequestedSchema = playSchema
        }, token);
    if (playResponse.Action != "accept" || playResponse.Content?["Answer"].ValueKind != JsonValueKind.True)
    {
        return "Maybe next time!";
    }
    // continue with the rest...
}

Client-Side Elicitation

  • Clients declare support in the initialize request by configuring an ElicitationHandler in McpClientOptions:

    McpClientOptions options = new()
{
    ClientInfo = new() { Name = "ElicitationClient", Version = "1.0.0" },
    Capabilities = new()
    {
        Elicitation = new()
        {
            ElicitationHandler = HandleElicitationAsync
        }
    }
};
  • ElicitationHandler receives server requests, prompts the user, and returns input matching the schema.

  • Sample handler for a console application:

    async ValueTask<ElicitResult> HandleElicitationAsync(ElicitRequestParams? requestParams, CancellationToken token)
{
    if (requestParams?.RequestedSchema?.Properties == null)
        return new ElicitResult();
    if (requestParams?.Message is not null)
        Console.WriteLine(requestParams.Message);

    var content = new Dictionary<string, JsonElement>();
    foreach (var property in requestParams.RequestedSchema.Properties)
    {
        // Perform appropriate prompting and conversion for BooleanSchema, NumberSchema, or StringSchema
        // Add user input to `content`
    }
    return new ElicitResult { Action = "accept", Content = content };
}

Structured Tool Output

Tools can now return explicitly structured results, allowing better interpretation by hosts/LLMs:

  • Use [McpServerTool(UseStructuredContent = true)] to enable structured output.
  • The SDK generates a JSON schema based on the return value type.

  • Example method:

    [McpServerTool(UseStructuredContent = true), Description("Gets a list of structured product data with detailed information.")]
public static List<Product> GetProducts(int count = 5)

  • Example of tool metadata returned in tools/list response:

    {
  "result": {
    "tools": [
      {
        "name": "get_products",
        "description": "Gets a list of structured product data with detailed information.",
        "inputSchema": { "type": "object", "properties": { "count": { "type": "integer", "default": 5 } } },
        "outputSchema": { "type": "object", "properties": { "result": { "type": "array", ... } } }
      }
    ]
  }
}
  • When invoked, the output of the tool is returned inside result.structuredContent.

Tools may return resource links to help clients locate and interact with created or managed resources.

Example:

  [McpServerTool]
  [Description("Creates a resource with a random value and returns a link to this resource.")]
  public async Task<CallToolResult> MakeAResource()
  {
      int id = new Random().Next(1, 101);
      var resource = ResourceGenerator.CreateResource(id);
      var result = new CallToolResult();
      result.Content.Add(new ResourceLinkBlock() { Uri = resource.Uri, Name = resource.Name });
      return result;
  }

Schema Improvements & Metadata

  • The _meta field is available in more interface types for extensibility.
  • Tools, resources, and prompts now have separate name and title fields.

  • Example of specifying a tool title:

    [McpServerToolType]
public class EchoTool
{
    [McpServerTool(Name = "echo", Title = "Echo Tool")]
    [Description("Echoes the message back to the client.")]
    public static string Echo(string message) => $"Echo: {message}";
}

The tool’s tools/list output includes both name and title fields.

Getting Started

  1. Update your package:

    dotnet add package ModelContextProtocol --prerelease
  2. Review the documentation and samples
  3. Follow security best practices: use proper OAuth flows, validate user input, use resource indicators for tokens, and review protocol security guidelines.

Summary

The MCP C# SDK’s support for protocol version 2025-06-18 gives .NET developers powerful new tools for building secure, interactive, and sophisticated AI solutions. Update your SDK and explore the new features to build the next generation of AI-powered .NET applications.

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