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Add support for MCP prompts
- Add prompt registry class and methods. - Add prompt message, prompt parameter and roles classes. - Extract common methods to RegistryBase class. - Adjust McpToolRegistry accordingly. - Add tests for simple prompts. - Update MCP client test to list available prompts.
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‎nanoFramework.WebServer.Mcp/McpPromptMessage.cs

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// Licensed to the .NET Foundation under one or more agreements.
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// The .NET Foundation licenses this file to you under the MIT license.
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namespace nanoFramework.WebServer.Mcp
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{
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/// <summary>
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/// Represents a message within the Model Context Protocol (MCP) system, used for communication between clients and AI models.
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/// </summary>
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/// <remarks>
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/// <para>
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/// A <see cref="PromptMessage"/> encapsulates content sent to or received from AI models in the Model Context Protocol.
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/// Each message has a specific role (<see cref="Role.User"/> or <see cref="Role.Assistant"/>) and contains content which can be text.
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/// </para>
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/// <para>
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/// It serves as a core data structure in the MCP message exchange flow, particularly in prompt formation and model responses.
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/// </para>
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/// </remarks>
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public sealed class PromptMessage
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{
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/// <summary>
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/// Gets or sets the text content of the message.
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/// </summary>
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public string Text { get; set; }
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/// <summary>
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/// Gets or sets the role of the message sender, specifying whether it's from a "user" or an "assistant".
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/// </summary>
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/// <remarks>
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/// In the Model Context Protocol, each message must have a clear role assignment to maintain
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/// the conversation flow. User messages represent queries or inputs from users, while assistant
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/// messages represent responses generated by AI models.
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/// </remarks>
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public Role Role { get; set; } = Role.User;
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/// <summary>
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/// Initializes a new instance of the <see cref="PromptMessage"/> class with this prompt text.
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/// </summary>
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/// <param name="text">The text content of the message.</param>
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public PromptMessage(string text)
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{
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Text = text;
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}
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/// <inheritdoc/>
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public override string ToString()
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{
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return $"{{\"role\":\"{RoleToString(Role)}\",\"content\":{{\"type\": \"text\",\"text\":\"{Text}\"}}}}";
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}
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private static string RoleToString(Role role)
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{
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return role == Role.User ? "user" : "assistant";
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}
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}
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}

‎nanoFramework.WebServer.Mcp/McpPromptParameter.cs

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// Licensed to the .NET Foundation under one or more agreements.
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// The .NET Foundation licenses this file to you under the MIT license.
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using System;
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namespace nanoFramework.WebServer.Mcp
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{
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/// <summary>
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///
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/// </summary>
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/// <remarks>
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/// These have to be added to the methods implementing the MCP prompts in the same order as the parameters.
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/// </remarks>
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[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
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public sealed class McpPromptParameterAttribute : Attribute
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{
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/// <summary>
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/// Gets or sets the name of the prompt parameter.
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/// </summary>
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public string Name { get; set; }
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/// <summary>
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/// Gets or sets the description of the prompt parameter.
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/// </summary>
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/// <remarks>
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/// The description is optional.
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/// </remarks>
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public string Description { get; set; }
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/// <summary>
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/// Gets or sets a value indicating whether the prompt parameter is required.
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/// </summary>
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public bool Required { get; set; }
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///// <summary>
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///// Initializes a new instance of the <see cref="McpPromptParameterAttribute"/> class.
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///// </summary>
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//public McpPromptParameterAttribute()
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//{
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//}
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/// <summary>
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/// Initializes a new instance of the <see cref="McpPromptParameterAttribute"/> class with the specified name and description.
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/// </summary>
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/// <param name="name">The name of the prompt parameter.</param>
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/// <param name="description">The description of the prompt parameter.</param>
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public McpPromptParameterAttribute(string name, string description)
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{
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Name = name;
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Description = description;
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}
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/// <summary>
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/// Initializes a new instance of the <see cref="McpPromptParameterAttribute"/> class with the specified name, description, and required status.
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/// </summary>
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/// <param name="name">The name of the prompt parameter.</param>
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/// <param name="description">The description of the prompt parameter.</param>
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/// <param name="required">Indicates whether the prompt parameter is required.</param>
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public McpPromptParameterAttribute(string name, string description, bool required)
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{
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Name = name;
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Description = description;
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Required = required;
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}
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/// <inheritdoc/>
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public override string ToString()
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{
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return $"{{\"name\":\"{Name}\",\"description\":\"{Description}\",\"required\":{Required.ToString().ToLower()}}}";
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}
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}
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}

‎nanoFramework.WebServer.Mcp/McpPromptRegistry.cs

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// Licensed to the .NET Foundation under one or more agreements.
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// The .NET Foundation licenses this file to you under the MIT license.
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using System;
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using System.Collections;
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using System.Diagnostics;
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using System.Reflection;
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using System.Text;
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namespace nanoFramework.WebServer.Mcp
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{
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/// <summary>
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/// Registry for Model Context Protocol (MCP) prompts, allowing discovery and invocation of prompts defined with the <see cref="McpServerPromptAttribute"/>.
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/// </summary>
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public class McpPromptRegistry : RegistryBase
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{
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private static readonly Hashtable promtps = new Hashtable();
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private static bool isInitialized = false;
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/// <summary>
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/// Discovers MCP prompts by scanning the provided types for methods decorated with the <see cref="McpServerPromptAttribute"/>.
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/// This method should be called once to populate the tool registry.
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/// </summary>
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/// <param name="typesWithPrompts">An array of types to scan for MCP prompts.</param>
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public static void DiscoverPrompts(Type[] typesWithPrompts)
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{
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if (isInitialized)
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{
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// prompts already discovered
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return;
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}
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foreach (Type mcpPrompt in typesWithPrompts)
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{
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MethodInfo[] methods = mcpPrompt.GetMethods();
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foreach (MethodInfo method in methods)
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{
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try
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{
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object[] allAttribute = method.GetCustomAttributes(false);
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foreach (object attrib in allAttribute)
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{
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if (attrib.GetType() != typeof(McpServerPromptAttribute))
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{
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continue;
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}
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McpServerPromptAttribute attribute = (McpServerPromptAttribute)attrib;
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if (attribute != null)
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{
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// validate if the method returns an array of PromptMessage
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if (method.ReturnType != typeof(PromptMessage[]) && !method.ReturnType.IsArray)
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{
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throw new Exception($"Method {method.Name} does not return an array of PromptMessage.");
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}
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promtps.Add(attribute.Name, new PromptMetadata
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{
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Name = attribute.Name,
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Description = attribute.Description,
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Arguments = ComposeArgumentsAsJson(allAttribute),
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Method = method
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});
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}
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}
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}
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catch (Exception)
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{
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continue;
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}
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}
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}
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isInitialized = true;
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}
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private static string ComposeArgumentsAsJson(object[] attributes)
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{
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StringBuilder sb = new StringBuilder();
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bool isFirst = true;
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foreach (object attrib in attributes)
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{
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if (attrib is not McpPromptParameterAttribute)
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{
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continue;
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}
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McpPromptParameterAttribute parameterNameAttribute = (McpPromptParameterAttribute)attrib;
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if (parameterNameAttribute != null)
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{
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sb.Append(isFirst ? "" : ",");
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sb.Append("{");
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sb.Append($"\"name\": \"{parameterNameAttribute.Name}\"");
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sb.Append(string.IsNullOrEmpty(parameterNameAttribute.Description) ? "" : $",\"description\": \"{parameterNameAttribute.Description}\"");
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sb.Append($",\"required\": {parameterNameAttribute.Required.ToString().ToLower()}");
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sb.Append("}");
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isFirst = false;
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}
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}
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return sb.Length > 0 ? sb.ToString() : string.Empty;
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}
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/// <summary>
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/// Gets the metadata of all registered MCP prompts in JSON format.
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/// This method should be called after <see cref="DiscoverPrompts"/> to retrieve the prompt metadata.
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/// </summary>
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/// <returns>A JSON string containing the metadata of all registered prompts.</returns>
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/// <exception cref="Exception">Thrown if there is an error building the prompts list.</exception>
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public static string GetPromptMetadataJson()
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{
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try
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{
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StringBuilder sb = new StringBuilder();
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sb.Append("\"prompts\":[");
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foreach (PromptMetadata prompt in promtps.Values)
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{
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sb.Append(prompt.ToString());
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sb.Append(",");
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}
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sb.Remove(sb.Length - 1, 1);
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sb.Append("],\"nextCursor\":null");
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return sb.ToString();
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}
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catch (Exception)
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{
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throw new Exception("Impossible to build prompts list.");
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}
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}
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/// <summary>
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/// Gets the description of a registered MCP prompt by its name.
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/// </summary>
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/// <param name="promptName">The name of the prompt to invoke.</param>
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/// <returns>A string containing the description of the prompt.</returns>
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/// <exception cref="Exception">Thrown when the specified prompt is not found in the registry.</exception>
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public static string GetPromptDescription(string promptName)
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{
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if (promtps.Contains(promptName))
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{
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PromptMetadata promptMetadata = (PromptMetadata)promtps[promptName];
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return promptMetadata.Description;
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}
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throw new Exception("Prompt not found");
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}
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/// <summary>
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/// Invokes a registered MCP prompt by name and returns the serialized result.
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/// </summary>
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/// <param name="promptName">The name of the prompt to invoke.</param>
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/// <param name="arguments">The arguments to pass to the tool.</param>
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/// <returns>A JSON string containing the serialized result of the prompt invocation.</returns>
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/// <exception cref="Exception">Thrown when the specified prompt is not found in the registry.</exception>
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public static string InvokePrompt(string promptName, Hashtable arguments)
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{
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if (promtps.Contains(promptName))
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{
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PromptMetadata promptMetadata = (PromptMetadata)promtps[promptName];
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MethodInfo method = promptMetadata.Method;
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Debug.WriteLine($"Prompt name: {promptName}, method: {method.Name}");
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object[] methodParams = null;
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if (arguments is not null && arguments.Count > 0)
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{
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methodParams = new object[arguments.Count];
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arguments.Values.CopyTo(methodParams, 0);
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}
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PromptMessage[] result = (PromptMessage[])method.Invoke(null, methodParams);
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// serialize the result to JSON using a speedy approach with a StringBuilder
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StringBuilder sb = new StringBuilder();
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// start building the JSON response
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sb.Append($"{{\"description\":\"{GetPromptDescription(promptName)}\",\"messages\":[");
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// iterate through the result array and append each message
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for (int i = 0; i < result.Length; i++)
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{
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sb.Append(result[i]);
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if (i < result.Length - 1)
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{
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sb.Append(",");
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}
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}
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// close the messages array and the main object
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sb.Append("]}");
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// done here, return the JSON string
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return sb.ToString();
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}
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throw new Exception("Prompt not found");
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}
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}
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}

‎nanoFramework.WebServer.Mcp/McpServerController.cs

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sb.Append($",\"result\":{{\"content\":[{{\"type\":\"text\",\"text\":{result}}}]}}}}");
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}
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else if (request["method"].ToString() == "prompts/list")
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{
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string promptListJson = McpPromptRegistry.GetPromptMetadataJson();
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sb.Append($",\"result\":{{{promptListJson}}}}}");
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}
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else if (request["method"].ToString() == "prompts/get")
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{
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string promptName = ((Hashtable)request["params"])["name"].ToString();
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Hashtable arguments = ((Hashtable)request["params"])["arguments"] == null ? null : (Hashtable)((Hashtable)request["params"])["arguments"];
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string result = McpPromptRegistry.InvokePrompt(promptName, arguments);
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sb.Append($",\"result\":{result}}}}}");
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}
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else
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{
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sb.Append($",\"error\":{{\"code\":-32601,\"message\":\"Method not found\"}}}}");
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}
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Debug.WriteLine();
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Debug.WriteLine($"Response: {sb.ToString()}");
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Debug.WriteLine();
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WebServer.OutPutStream(e.Context.Response, sb.ToString());
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return;
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}

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