Table of Contents

Model Context Protocol

It is possible to build applications that provide context to agent models using large language models (LLMs) using the Sisk.ModelContextProtocol package:

dotnet add package Sisk.ModelContextProtocol

This package exposes useful classes and methods for building MCP servers that work over Streamable HTTP.

Note

Before you start, note that this package is under development and may exhibit behaviors that do not conform to the specification. Read the package details to learn what is under development and what does not work yet.

Getting Started with MCP

The McpProvider class is the entry point for defining an MCP server. It is abstract and can be defined anywhere. Your Sisk application can have one or more MCP providers.

McpProvider mcp = new McpProvider(
    serverName: "math-server",
    serverTitle: "Mathematics server",
    serverVersion: new Version(1, 0));

mcp.Tools.Add(new McpTool(
    name: "math_sum",
    description: "Sums one or more numbers.",
    schema: JsonSchema.CreateObjectSchema(
        properties: new Dictionary<string, JsonSchema>()
        {
            { "numbers",
                JsonSchema.CreateArraySchema(
                    itemsSchema: JsonSchema.CreateNumberSchema(),
                    minItems: 1,
                    description: "The numbers to sum.")
            }
        },
        requiredProperties: ["numbers"]),
    executionHandler: async (McpToolContext context) =>
    {
        var numbers = context.Arguments["numbers"].GetJsonArray().ToArray<double>();
        var sum = numbers.Sum();
        return await Task.FromResult(McpToolResult.CreateText($"Sum result: {sum:N4}"));
    }));

If your application will provide only one MCP provider, you can use the builder's singleton:

static void Main(string[] args)
{
    using var host = HttpServer.CreateBuilder()
        .UseMcp(mcp =>
        {
            mcp.ServerName = "math-server";
            mcp.ServerTitle = "Mathematics server";

            mcp.Tools.Add(new McpTool(
                name: "math_sum",
                description: "Sums one or more numbers.",
                schema: JsonSchema.CreateObjectSchema(
                    properties: new Dictionary<string, JsonSchema>()
                    {
                        { "numbers",
                            JsonSchema.CreateArraySchema(
                                itemsSchema: JsonSchema.CreateNumberSchema(),
                                minItems: 1,
                                description: "The numbers to sum.")
                        }
                    },
                    requiredProperties: ["numbers"]),
                executionHandler: async (McpToolContext context) =>
                {
                    var numbers = context.Arguments["numbers"].GetJsonArray().ToArray<double>();
                    var sum = numbers.Sum();
                    return await Task.FromResult(McpToolResult.CreateText($"Sum result: {sum:N4}"));
                }));
        })
        .UseRouter(router =>
        {
            router.MapAny("/mcp", async (HttpRequest req) =>
            {
                await req.HandleMcpRequestAsync();
            });
        })
        .Build();

    host.Start();
}

Creating JSON Schemas for Functions

The [Sisk.ModelContextProtocol] library uses a fork of LightJson for JSON and JSON schema manipulation. This implementation provides a fluent JSON Schema builder for various objects:

  • JsonSchema.CreateObjectSchema
  • JsonSchema.CreateArraySchema
  • JsonSchema.CreateBooleanSchema
  • JsonSchema.CreateNumberSchema
  • JsonSchema.CreateStringSchema
  • JsonSchema.Empty

Example:

JsonSchema.CreateObjectSchema(
    properties: new Dictionary<string, JsonSchema>()
    {
        { "numbers",
            JsonSchema.CreateArraySchema(
                itemsSchema: JsonSchema.CreateNumberSchema(),
                minItems: 1,
                description: "The numbers to sum.")
        }
    },
    requiredProperties: ["numbers"]);

Produces the following schema:

{
  "type": "object",
  "properties": {
    "numbers": {
      "type": "array",
      "items": {
        "type": "number"
      },
      "minItems": 1,
      "description": "The numbers to sum."
    }
  },
  "required": ["numbers"]
}

Handling Function Calls

The function defined in the executionHandler parameter of McpTool provides a JsonObject containing the call arguments that can be read fluently:

mcp.Tools.Add(new McpTool(
    name: "browser_do_action",
    description: "Run an browser action, such as scrolling, refreshing or navigating.",
    schema: JsonSchema.CreateObjectSchema(
        properties: new Dictionary<string, JsonSchema>()
        {
            { "action_name",
                JsonSchema.CreateStringSchema(
                    enums: ["go_back", "refresh", "scroll_bottom", "scroll_top"],
                    description: "The action name.")
            },
            { "action_data",
                JsonSchema.CreateStringSchema(
                    description: "Action parameter."
                ) }
        },
        requiredProperties: ["action_name"]),
    executionHandler: async (McpToolContext context) =>
    {
        // read action name. will throw if null or not a explicit string
        string actionName = context.Arguments["action_name"].GetString();
        
        // action_data is defined as non-required, so it may be null here
        string? actionData = context.Arguments["action_data"].MaybeNull()?.GetString();
        
        // Handle the browser action based on the actionName
        return await Task.FromResult(
            McpToolResult.CreateText($"Performed browser action: {actionName}"));
    }));

Function Results

The McpToolResult object provides three methods for creating content for a tool response:

Additionally, it is possible to combine multiple different contents into a single JSON tool response:

mcp.Tools.Add(new McpTool(
    ...
    executionHandler: async (McpToolContext context) =>
    {
        // simulate real work

        byte[] browserScreenshot = await browser.ScreenshotAsync();
        
        return McpToolResult.Combine(
            McpToolResult.CreateText("Heres the screenshot of the browser:"),
            McpToolResult.CreateImage(browserScreenshot, "image/png")
        )
    }));

Continuing Work

The Model Context Protocol is a communication protocol for agent models and applications that provide content to them. It is a new protocol, so it is common for its specification to be constantly updated with deprecations, new features, and breaking changes.

It is crucial to understand the problems that the Model Context Protocol solves before starting to build agent applications.

Also read the specification of the Sisk.ModelContextProtocol package to understand its progress, status, and what can be done with it.

Edit this page