Update content according to latest spec (#513)

* Update content according to latest spec

* Add Title to a bunch of types

* Add _meta to lots of types

* Address feedback, more cleanup, fix NAOT test failure

samples/EverythingServer/Program.cs

@@ -86,9 +86,9 @@ builder.Services
         var @ref = @params.Ref;
         var argument = @params.Argument;
 
-        if (@ref.Type == "ref/resource")
+        if (@ref is ResourceTemplateReference rtr)
         {
-            var resourceId = @ref.Uri?.Split("/").Last();
+            var resourceId = rtr.Uri?.Split("/").Last();
 
             if (resourceId is null)
             {
@@ -103,7 +103,7 @@ builder.Services
             };
         }
 
-        if (@ref.Type == "ref/prompt")
+        if (@ref is PromptReference pr)
         {
             if (!exampleCompletions.TryGetValue(argument.Name, out IEnumerable<string>? value))
             {

samples/EverythingServer/Tools/AnnotatedMessageTool.cs

@@ -15,25 +15,22 @@ public class AnnotatedMessageTool
     }
 
     [McpServerTool(Name = "annotatedMessage"), Description("Generates an annotated message")]
-    public static IEnumerable<Content> AnnotatedMessage(MessageType messageType, bool includeImage = true)
+    public static IEnumerable<ContentBlock> AnnotatedMessage(MessageType messageType, bool includeImage = true)
     {
-        List<Content> contents = messageType switch
+        List<ContentBlock> contents = messageType switch
         {
-            MessageType.Error => [new()
+            MessageType.Error => [new TextContentBlock
             {
-                Type = "text",
                 Text = "Error: Operation failed",
                 Annotations = new() { Audience = [Role.User, Role.Assistant], Priority = 1.0f }
             }],
-            MessageType.Success => [new()
+            MessageType.Success => [new TextContentBlock
             {
-                Type = "text",
                 Text = "Operation completed successfully",
                 Annotations = new() { Audience = [Role.User], Priority = 0.7f }
             }],
-            MessageType.Debug => [new()
+            MessageType.Debug => [new TextContentBlock
             {
-                Type = "text",
                 Text = "Debug: Cache hit ratio 0.95, latency 150ms",
                 Annotations = new() { Audience = [Role.Assistant], Priority = 0.3f }
             }],
@@ -42,9 +39,8 @@ public class AnnotatedMessageTool
 
         if (includeImage)
         {
-            contents.Add(new()
+            contents.Add(new ImageContentBlock()
             {
-                Type = "image",
                 Data = TinyImageTool.MCP_TINY_IMAGE.Split(",").Last(),
                 MimeType = "image/png",
                 Annotations = new() { Audience = [Role.User], Priority = 0.5f }

samples/EverythingServer/Tools/LongRunningTool.cs

@@ -15,7 +15,7 @@ public class LongRunningTool
         int duration = 10,
         int steps = 5)
     {
-        var progressToken = context.Params?.Meta?.ProgressToken;
+        var progressToken = context.Params?.ProgressToken;
         var stepDuration = duration / steps;
 
         for (int i = 1; i <= steps + 1; i++)

samples/EverythingServer/Tools/SampleLlmTool.cs

@@ -17,7 +17,7 @@ public class SampleLlmTool
         var samplingParams = CreateRequestSamplingParams(prompt ?? string.Empty, "sampleLLM", maxTokens);
         var sampleResult = await server.SampleAsync(samplingParams, cancellationToken);
 
-        return $"LLM sampling result: {sampleResult.Content.Text}";
+        return $"LLM sampling result: {(sampleResult.Content as TextContentBlock)?.Text}";
     }
 
     private static CreateMessageRequestParams CreateRequestSamplingParams(string context, string uri, int maxTokens = 100)
@@ -27,11 +27,7 @@ public class SampleLlmTool
             Messages = [new SamplingMessage()
                 {
                     Role = Role.User,
-                    Content = new Content()
-                    {
-                        Type = "text",
-                        Text = $"Resource {uri} context: {context}"
-                    }
+                    Content = new TextContentBlock { Text = $"Resource {uri} context: {context}" },
                 }],
             SystemPrompt = "You are a helpful test server.",
             MaxTokens = maxTokens,

samples/QuickstartWeatherServer/Program.cs

@@ -17,7 +17,7 @@ builder.Logging.AddConsole(options =>
 
 builder.Services.AddSingleton(_ =>
 {
-    var client = new HttpClient() { BaseAddress = new Uri("https://api.weather.gov") };
+    var client = new HttpClient { BaseAddress = new Uri("https://api.weather.gov") };
     client.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("weather-tool", "1.0"));
     return client;
 });

samples/TestServerWithHosting/Tools/SampleLlmTool.cs

@@ -20,7 +20,7 @@ public sealed class SampleLlmTool
         var samplingParams = CreateRequestSamplingParams(prompt ?? string.Empty, "sampleLLM", maxTokens);
         var sampleResult = await thisServer.SampleAsync(samplingParams, cancellationToken);
 
-        return $"LLM sampling result: {sampleResult.Content.Text}";
+        return $"LLM sampling result: {(sampleResult.Content as TextContentBlock)?.Text}";
     }
 
     private static CreateMessageRequestParams CreateRequestSamplingParams(string context, string uri, int maxTokens = 100)
@@ -30,11 +30,7 @@ public sealed class SampleLlmTool
             Messages = [new SamplingMessage()
                 {
                     Role = Role.User,
-                    Content = new Content()
-                    {
-                        Type = "text",
-                        Text = $"Resource {uri} context: {context}"
-                    }
+                    Content = new TextContentBlock { Text = $"Resource {uri} context: {context}" },
                 }],
             SystemPrompt = "You are a helpful test server.",
             MaxTokens = maxTokens,

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresUnreferencedCode.cs

@@ -5,7 +5,7 @@ namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
 /// Indicates that the specified method requires dynamic access to code that is not referenced
-/// statically, for example through <see cref="System.Reflection"/>.
+/// statically, for example through <see cref="Reflection"/>.
 /// </summary>
 /// <remarks>
 /// This allows tools to understand which methods are unsafe to call when removing unreferenced

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/StringSyntaxAttribute.cs

@@ -0,0 +1,68 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Diagnostics.CodeAnalysis;
+
+/// <summary>Specifies the syntax used in a string.</summary>
+[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
+internal sealed class StringSyntaxAttribute : Attribute
+{
+    /// <summary>Initializes the <see cref="StringSyntaxAttribute"/> with the identifier of the syntax used.</summary>
+    /// <param name="syntax">The syntax identifier.</param>
+    public StringSyntaxAttribute(string syntax)
+    {
+        Syntax = syntax;
+        Arguments = Array.Empty<object?>();
+    }
+
+    /// <summary>Initializes the <see cref="StringSyntaxAttribute"/> with the identifier of the syntax used.</summary>
+    /// <param name="syntax">The syntax identifier.</param>
+    /// <param name="arguments">Optional arguments associated with the specific syntax employed.</param>
+    public StringSyntaxAttribute(string syntax, params object?[] arguments)
+    {
+        Syntax = syntax;
+        Arguments = arguments;
+    }
+
+    /// <summary>Gets the identifier of the syntax used.</summary>
+    public string Syntax { get; }
+
+    /// <summary>Optional arguments associated with the specific syntax employed.</summary>
+    public object?[] Arguments { get; }
+
+    /// <summary>The syntax identifier for strings containing composite formats for string formatting.</summary>
+    public const string CompositeFormat = nameof(CompositeFormat);
+
+    /// <summary>The syntax identifier for strings containing date format specifiers.</summary>
+    public const string DateOnlyFormat = nameof(DateOnlyFormat);
+
+    /// <summary>The syntax identifier for strings containing date and time format specifiers.</summary>
+    public const string DateTimeFormat = nameof(DateTimeFormat);
+
+    /// <summary>The syntax identifier for strings containing <see cref="Enum"/> format specifiers.</summary>
+    public const string EnumFormat = nameof(EnumFormat);
+
+    /// <summary>The syntax identifier for strings containing <see cref="Guid"/> format specifiers.</summary>
+    public const string GuidFormat = nameof(GuidFormat);
+
+    /// <summary>The syntax identifier for strings containing JavaScript Object Notation (JSON).</summary>
+    public const string Json = nameof(Json);
+
+    /// <summary>The syntax identifier for strings containing numeric format specifiers.</summary>
+    public const string NumericFormat = nameof(NumericFormat);
+
+    /// <summary>The syntax identifier for strings containing regular expressions.</summary>
+    public const string Regex = nameof(Regex);
+
+    /// <summary>The syntax identifier for strings containing time format specifiers.</summary>
+    public const string TimeOnlyFormat = nameof(TimeOnlyFormat);
+
+    /// <summary>The syntax identifier for strings containing <see cref="TimeSpan"/> format specifiers.</summary>
+    public const string TimeSpanFormat = nameof(TimeSpanFormat);
+
+    /// <summary>The syntax identifier for strings containing URIs.</summary>
+    public const string Uri = nameof(Uri);
+
+    /// <summary>The syntax identifier for strings containing XML.</summary>
+    public const string Xml = nameof(Xml);
+}

src/ModelContextProtocol.Core/AIContentExtensions.cs

@@ -29,11 +29,13 @@ public static class AIContentExtensions
     {
         Throw.IfNull(promptMessage);
 
+        AIContent? content = ToAIContent(promptMessage.Content);
+
         return new()
         {
             RawRepresentation = promptMessage,
             Role = promptMessage.Role == Role.User ? ChatRole.User : ChatRole.Assistant,
-            Contents = [ToAIContent(promptMessage.Content)]
+            Contents = content is not null ? [content] : [],
         };
     }
 
@@ -81,32 +83,32 @@ public static class AIContentExtensions
         return messages;
     }
 
-    /// <summary>Creates a new <see cref="AIContent"/> from the content of a <see cref="Content"/>.</summary>
-    /// <param name="content">The <see cref="Content"/> to convert.</param>
-    /// <returns>The created <see cref="AIContent"/>.</returns>
+    /// <summary>Creates a new <see cref="AIContent"/> from the content of a <see cref="ContentBlock"/>.</summary>
+    /// <param name="content">The <see cref="ContentBlock"/> to convert.</param>
+    /// <returns>
+    /// The created <see cref="AIContent"/>. If the content can't be converted (such as when it's a resource link), <see langword="null"/> is returned.
+    /// </returns>
     /// <remarks>
     /// This method converts Model Context Protocol content types to the equivalent Microsoft.Extensions.AI 
     /// content types, enabling seamless integration between the protocol and AI client libraries.
     /// </remarks>
-    public static AIContent ToAIContent(this Content content)
+    public static AIContent? ToAIContent(this ContentBlock content)
     {
         Throw.IfNull(content);
 
-        AIContent ac;
-        if (content is { Type: "image" or "audio", MimeType: not null, Data: not null })
+        AIContent? ac = content switch
         {
-            ac = new DataContent(Convert.FromBase64String(content.Data), content.MimeType);
-        }
-        else if (content is { Type: "resource" } && content.Resource is { } resourceContents)
-        {
-            ac = resourceContents.ToAIContent();
-        }
-        else
-        {
-            ac = new TextContent(content.Text);
-        }
+            TextContentBlock textContent => new TextContent(textContent.Text),
+            ImageContentBlock imageContent => new DataContent(Convert.FromBase64String(imageContent.Data), imageContent.MimeType),
+            AudioContentBlock audioContent => new DataContent(Convert.FromBase64String(audioContent.Data), audioContent.MimeType),
+            EmbeddedResourceBlock resourceContent => resourceContent.Resource.ToAIContent(),
+            _ => null,
+        };
 
-        ac.RawRepresentation = content;
+        if (ac is not null)
+        {
+            ac.RawRepresentation = content;
+        }
 
         return ac;
     }
@@ -135,8 +137,8 @@ public static class AIContentExtensions
         return ac;
     }
 
-    /// <summary>Creates a list of <see cref="AIContent"/> from a sequence of <see cref="Content"/>.</summary>
-    /// <param name="contents">The <see cref="Content"/> instances to convert.</param>
+    /// <summary>Creates a list of <see cref="AIContent"/> from a sequence of <see cref="ContentBlock"/>.</summary>
+    /// <param name="contents">The <see cref="ContentBlock"/> instances to convert.</param>
     /// <returns>The created <see cref="AIContent"/> instances.</returns>
     /// <remarks>
     /// <para>
@@ -145,15 +147,15 @@ public static class AIContentExtensions
     /// when processing the contents of a message or response.
     /// </para>
     /// <para>
-    /// Each <see cref="Content"/> object is converted using <see cref="ToAIContent(Content)"/>,
+    /// Each <see cref="ContentBlock"/> object is converted using <see cref="ToAIContent(ContentBlock)"/>,
     /// preserving the type-specific conversion logic for text, images, audio, and resources.
     /// </para>
     /// </remarks>
-    public static IList<AIContent> ToAIContents(this IEnumerable<Content> contents)
+    public static IList<AIContent> ToAIContents(this IEnumerable<ContentBlock> contents)
     {
         Throw.IfNull(contents);
 
-        return [.. contents.Select(ToAIContent)];
+        return [.. contents.Select(ToAIContent).OfType<AIContent>()];
     }
 
     /// <summary>Creates a list of <see cref="AIContent"/> from a sequence of <see cref="ResourceContents"/>.</summary>
@@ -167,7 +169,7 @@ public static class AIContentExtensions
     /// </para>
     /// <para>
     /// Each <see cref="ResourceContents"/> object is converted using <see cref="ToAIContent(ResourceContents)"/>,
-    /// preserving the type-specific conversion logic: text resources become <see cref="TextContent"/> objects and
+    /// preserving the type-specific conversion logic: text resources become <see cref="TextContentBlock"/> objects and
     /// binary resources become <see cref="DataContent"/> objects.
     /// </para>
     /// </remarks>
@@ -178,29 +180,38 @@ public static class AIContentExtensions
         return [.. contents.Select(ToAIContent)];
     }
 
-    internal static Content ToContent(this AIContent content) =>
+    internal static ContentBlock ToContent(this AIContent content) =>
         content switch
         {
-            TextContent textContent => new()
+            TextContent textContent => new TextContentBlock
             {
                 Text = textContent.Text,
-                Type = "text",
             },
 
-            DataContent dataContent => new()
+            DataContent dataContent when dataContent.HasTopLevelMediaType("image") => new ImageContentBlock()
             {
                 Data = dataContent.Base64Data.ToString(),
                 MimeType = dataContent.MediaType,
-                Type =
-                    dataContent.HasTopLevelMediaType("image") ? "image" :
-                    dataContent.HasTopLevelMediaType("audio") ? "audio" :
-                    "resource",
             },
-            
-            _ => new()
+
+            DataContent dataContent when dataContent.HasTopLevelMediaType("audio") => new AudioContentBlock()
+            {
+                Data = dataContent.Base64Data.ToString(),
+                MimeType = dataContent.MediaType,
+            },
+
+            DataContent dataContent => new EmbeddedResourceBlock()
+            {
+                Resource = new BlobResourceContents()
+                {
+                    Blob = dataContent.Base64Data.ToString(),
+                    MimeType = dataContent.MediaType,
+                }
+            },
+
+            _ => new TextContentBlock
             {
                 Text = JsonSerializer.Serialize(content, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object))),
-                Type = "text",
             }
         };
 }

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -1,6 +1,5 @@
 using Microsoft.Extensions.Logging;
 using ModelContextProtocol.Protocol;
-using System.Diagnostics;
 using System.Text.Json;
 
 namespace ModelContextProtocol.Client;
@@ -58,7 +57,7 @@ internal sealed partial class McpClient : McpEndpoint, IMcpClient
                     RequestMethods.SamplingCreateMessage,
                     (request, _, cancellationToken) => samplingHandler(
                         request,
-                        request?.Meta?.ProgressToken is { } token ? new TokenProgress(this, token) : NullProgress.Instance,
+                        request?.ProgressToken is { } token ? new TokenProgress(this, token) : NullProgress.Instance,
                         cancellationToken),
                     McpJsonUtilities.JsonContext.Default.CreateMessageRequestParams,
                     McpJsonUtilities.JsonContext.Default.CreateMessageResult);
@@ -180,9 +179,12 @@ internal sealed partial class McpClient : McpEndpoint, IMcpClient
                 }
 
                 // Send initialized notification
-                await SendMessageAsync(
-                    new JsonRpcNotification { Method = NotificationMethods.InitializedNotification },
-                    initializationCts.Token).ConfigureAwait(false);
+                await this.SendNotificationAsync(
+                    NotificationMethods.InitializedNotification,
+                    new InitializedNotificationParams(),
+                    McpJsonUtilities.JsonContext.Default.InitializedNotificationParams,
+                    cancellationToken: initializationCts.Token).ConfigureAwait(false);
+
             }
             catch (OperationCanceledException oce) when (initializationCts.IsCancellationRequested && !cancellationToken.IsCancellationRequested)
             {

src/ModelContextProtocol.Core/Client/McpClientExtensions.cs

@@ -639,11 +639,6 @@ public static class McpClientExtensions
         Throw.IfNull(reference);
         Throw.IfNullOrWhiteSpace(argumentName);
 
-        if (!reference.Validate(out string? validationMessage))
-        {
-            throw new ArgumentException($"Invalid reference: {validationMessage}", nameof(reference));
-        }
-
         return client.SendRequestAsync(
             RequestMethods.CompletionComplete,
             new()
@@ -812,7 +807,7 @@ public static class McpClientExtensions
     /// </param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>
-    /// A task containing the <see cref="CallToolResponse"/> from the tool execution. The response includes
+    /// A task containing the <see cref="CallToolResult"/> from the tool execution. The response includes
     /// the tool's output content, which may be structured data, text, or an error message.
     /// </returns>
     /// <exception cref="ArgumentNullException"><paramref name="client"/> is <see langword="null"/>.</exception>
@@ -829,7 +824,7 @@ public static class McpClientExtensions
     ///     });
     /// </code>
     /// </example>
-    public static ValueTask<CallToolResponse> CallToolAsync(
+    public static ValueTask<CallToolResult> CallToolAsync(
         this IMcpClient client,
         string toolName,
         IReadOnlyDictionary<string, object?>? arguments = null,
@@ -855,10 +850,10 @@ public static class McpClientExtensions
                 Arguments = ToArgumentsDictionary(arguments, serializerOptions),
             },
             McpJsonUtilities.JsonContext.Default.CallToolRequestParams,
-            McpJsonUtilities.JsonContext.Default.CallToolResponse,
+            McpJsonUtilities.JsonContext.Default.CallToolResult,
             cancellationToken: cancellationToken);
 
-        static async ValueTask<CallToolResponse> SendRequestWithProgressAsync(
+        static async ValueTask<CallToolResult> SendRequestWithProgressAsync(
             IMcpClient client,
             string toolName,
             IReadOnlyDictionary<string, object?>? arguments,
@@ -871,7 +866,7 @@ public static class McpClientExtensions
             await using var _ = client.RegisterNotificationHandler(NotificationMethods.ProgressNotification,
                 (notification, cancellationToken) =>
                 {
-                    if (JsonSerializer.Deserialize(notification.Params, McpJsonUtilities.JsonContext.Default.ProgressNotification) is { } pn &&
+                    if (JsonSerializer.Deserialize(notification.Params, McpJsonUtilities.JsonContext.Default.ProgressNotificationParams) is { } pn &&
                         pn.ProgressToken == progressToken)
                     {
                         progress.Report(pn.Progress);
@@ -886,10 +881,10 @@ public static class McpClientExtensions
                 {
                     Name = toolName,
                     Arguments = ToArgumentsDictionary(arguments, serializerOptions),
-                    Meta = new() { ProgressToken = progressToken },
+                    ProgressToken = progressToken,
                 },
                 McpJsonUtilities.JsonContext.Default.CallToolRequestParams,
-                McpJsonUtilities.JsonContext.Default.CallToolResponse,
+                McpJsonUtilities.JsonContext.Default.CallToolResult,
                 cancellationToken: cancellationToken).ConfigureAwait(false);
         }
     }
@@ -924,29 +919,12 @@ public static class McpClientExtensions
             (options ??= new()).StopSequences = stopSequences.ToArray();
         }
 
-        List<ChatMessage> messages = [];
-        foreach (SamplingMessage sm in requestParams.Messages)
-        {
-            ChatMessage message = new()
-            {
-                Role = sm.Role == Role.User ? ChatRole.User : ChatRole.Assistant,
-            };
-
-            if (sm.Content is { Type: "text" })
-            {
-                message.Contents.Add(new TextContent(sm.Content.Text));
-            }
-            else if (sm.Content is { Type: "image" or "audio", MimeType: not null, Data: not null })
-            {
-                message.Contents.Add(new DataContent(Convert.FromBase64String(sm.Content.Data), sm.Content.MimeType));
-            }
-            else if (sm.Content is { Type: "resource", Resource: not null })
-            {
-                message.Contents.Add(sm.Content.Resource.ToAIContent());
-            }
-
-            messages.Add(message);
-        }
+        List<ChatMessage> messages =
+            (from sm in requestParams.Messages
+             let aiContent = sm.Content.ToAIContent()
+             where aiContent is not null
+             select new ChatMessage(sm.Role == Role.Assistant ? ChatRole.Assistant : ChatRole.User, [aiContent]))
+            .ToList();
 
         return (messages, options);
     }
@@ -965,32 +943,21 @@ public static class McpClientExtensions
 
         ChatMessage? lastMessage = chatResponse.Messages.LastOrDefault();
 
-        Content? content = null;
+        ContentBlock? content = null;
         if (lastMessage is not null)
         {
             foreach (var lmc in lastMessage.Contents)
             {
                 if (lmc is DataContent dc && (dc.HasTopLevelMediaType("image") || dc.HasTopLevelMediaType("audio")))
                 {
-                    content = new()
-                    {
-                        Type = dc.HasTopLevelMediaType("image") ? "image" : "audio",
-                        MimeType = dc.MediaType,
-                        Data = dc.Base64Data.ToString(),
-                    };
+                    content = dc.ToContent();
                 }
             }
         }
 
-        content ??= new()
-        {
-            Text = lastMessage?.Text ?? string.Empty,
-            Type = "text",
-        };
-
         return new()
         {
-            Content = content,
+            Content = content ?? new TextContentBlock { Text = lastMessage?.Text ?? string.Empty },
             Model = chatResponse.ModelId ?? "unknown",
             Role = lastMessage?.Role == ChatRole.User ? Role.User : Role.Assistant,
             StopReason = chatResponse.FinishReason == ChatFinishReason.Length ? "maxTokens" : "endTurn",
@@ -1024,7 +991,7 @@ public static class McpClientExtensions
             Throw.IfNull(requestParams);
 
             var (messages, options) = requestParams.ToChatClientArguments();
-            var progressToken = requestParams.Meta?.ProgressToken;
+            var progressToken = requestParams.ProgressToken;
 
             List<ChatResponseUpdate> updates = [];
             await foreach (var update in chatClient.GetStreamingResponseAsync(messages, options, cancellationToken).ConfigureAwait(false))
@@ -1107,7 +1074,7 @@ public static class McpClientExtensions
         SetLoggingLevel(client, McpServer.ToLoggingLevel(level), cancellationToken);
 
     /// <summary>Convers a dictionary with <see cref="object"/> values to a dictionary with <see cref="JsonElement"/> values.</summary>
-    private static IReadOnlyDictionary<string, JsonElement>? ToArgumentsDictionary(
+    private static Dictionary<string, JsonElement>? ToArgumentsDictionary(
         IReadOnlyDictionary<string, object?>? arguments, JsonSerializerOptions options)
     {
         var typeInfo = options.GetTypeInfo<object?>();

src/ModelContextProtocol.Core/Client/McpClientOptions.cs

@@ -9,7 +9,7 @@ namespace ModelContextProtocol.Client;
 /// These options are typically passed to <see cref="McpClientFactory.CreateAsync"/> when creating a client.
 /// They define client capabilities, protocol version, and other client-specific settings.
 /// </remarks>
-public class McpClientOptions
+public sealed class McpClientOptions
 {
     /// <summary>
     /// Gets or sets information about this client implementation, including its name and version.

src/ModelContextProtocol.Core/Client/McpClientPrompt.cs

@@ -44,6 +44,9 @@ public sealed class McpClientPrompt
     /// <summary>Gets the name of the prompt.</summary>
     public string Name => ProtocolPrompt.Name;
 
+    /// <summary>Gets the title of the prompt.</summary>
+    public string? Title => ProtocolPrompt.Title;
+
     /// <summary>Gets a description of the prompt.</summary>
     public string? Description => ProtocolPrompt.Description;
 

src/ModelContextProtocol.Core/Client/McpClientResource.cs

@@ -42,6 +42,9 @@ public sealed class McpClientResource
     /// <summary>Gets the name of the resource.</summary>
     public string Name => ProtocolResource.Name;
 
+    /// <summary>Gets the title of the resource.</summary>
+    public string? Title => ProtocolResource.Title;
+
     /// <summary>Gets a description of the resource.</summary>
     public string? Description => ProtocolResource.Description;
 

src/ModelContextProtocol.Core/Client/McpClientResourceTemplate.cs

@@ -42,6 +42,9 @@ public sealed class McpClientResourceTemplate
     /// <summary>Gets the name of the resource template.</summary>
     public string Name => ProtocolResourceTemplate.Name;
 
+    /// <summary>Gets the title of the resource template.</summary>
+    public string? Title => ProtocolResourceTemplate.Title;
+
     /// <summary>Gets a description of the resource template.</summary>
     public string? Description => ProtocolResourceTemplate.Description;
 

src/ModelContextProtocol.Core/Client/McpClientTool.cs

@@ -68,6 +68,9 @@ public sealed class McpClientTool : AIFunction
     /// <inheritdoc/>
     public override string Name => _name;
 
+    /// <summary>Gets the tool's title.</summary>
+    public string? Title => ProtocolTool.Title ?? ProtocolTool.Annotations?.Title;
+
     /// <inheritdoc/>
     public override string Description => _description;
 
@@ -84,8 +87,8 @@ public sealed class McpClientTool : AIFunction
     protected async override ValueTask<object?> InvokeCoreAsync(
         AIFunctionArguments arguments, CancellationToken cancellationToken)
     {
-        CallToolResponse result = await CallAsync(arguments, _progress, JsonSerializerOptions, cancellationToken).ConfigureAwait(false);
-        return JsonSerializer.SerializeToElement(result, McpJsonUtilities.JsonContext.Default.CallToolResponse);
+        CallToolResult result = await CallAsync(arguments, _progress, JsonSerializerOptions, cancellationToken).ConfigureAwait(false);
+        return JsonSerializer.SerializeToElement(result, McpJsonUtilities.JsonContext.Default.CallToolResult);
     }
 
     /// <summary>
@@ -104,14 +107,14 @@ public sealed class McpClientTool : AIFunction
     /// </param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>
-    /// A task containing the <see cref="CallToolResponse"/> from the tool execution. The response includes
+    /// A task containing the <see cref="CallToolResult"/> from the tool execution. The response includes
     /// the tool's output content, which may be structured data, text, or an error message.
     /// </returns>
     /// <remarks>
     /// The base <see cref="AIFunction.InvokeAsync"/> method is overridden to invoke this <see cref="CallAsync"/> method.
-    /// The only difference in behavior is <see cref="AIFunction.InvokeAsync"/> will serialize the resulting <see cref="CallToolResponse"/>"/>
-    /// such that the <see cref="object"/> returned is a <see cref="JsonElement"/> containing the serialized <see cref="CallToolResponse"/>.
-    /// This <see cref="CallToolResponse"/> method is intended to be called directly by user code, whereas the base <see cref="AIFunction.InvokeAsync"/>
+    /// The only difference in behavior is <see cref="AIFunction.InvokeAsync"/> will serialize the resulting <see cref="CallToolResult"/>"/>
+    /// such that the <see cref="object"/> returned is a <see cref="JsonElement"/> containing the serialized <see cref="CallToolResult"/>.
+    /// This <see cref="CallToolResult"/> method is intended to be called directly by user code, whereas the base <see cref="AIFunction.InvokeAsync"/>
     /// is intended to be used polymorphically via the base class, typically as part of an <see cref="IChatClient"/> operation.
     /// </remarks>
     /// <exception cref="McpException">The server could not find the requested tool, or the server encountered an error while processing the request.</exception>
@@ -124,7 +127,7 @@ public sealed class McpClientTool : AIFunction
     ///     });
     /// </code>
     /// </example>
-    public ValueTask<CallToolResponse> CallAsync(
+    public ValueTask<CallToolResult> CallAsync(
         IReadOnlyDictionary<string, object?>? arguments = null,
         IProgress<ProgressNotificationValue>? progress = null,
         JsonSerializerOptions? serializerOptions = null,
@@ -156,7 +159,7 @@ public sealed class McpClientTool : AIFunction
     /// </para>
     /// </remarks>
     public McpClientTool WithName(string name) =>
-        new McpClientTool(_client, ProtocolTool, JsonSerializerOptions, name, _description, _progress);
+        new(_client, ProtocolTool, JsonSerializerOptions, name, _description, _progress);
 
     /// <summary>
     /// Creates a new instance of the tool but modified to return the specified description from its <see cref="Description"/> property.
@@ -180,7 +183,7 @@ public sealed class McpClientTool : AIFunction
     /// </remarks>
     /// <returns>A new instance of <see cref="McpClientTool"/> with the provided description.</returns>
     public McpClientTool WithDescription(string description) =>
-        new McpClientTool(_client, ProtocolTool, JsonSerializerOptions, _name, description, _progress);
+        new(_client, ProtocolTool, JsonSerializerOptions, _name, description, _progress);
 
     /// <summary>
     /// Creates a new instance of the tool but modified to report progress via the specified <see cref="IProgress{T}"/>.

src/ModelContextProtocol.Core/Client/SseClientTransportOptions.cs

@@ -3,7 +3,7 @@ namespace ModelContextProtocol.Client;
 /// <summary>
 /// Provides options for configuring <see cref="SseClientTransport"/> instances.
 /// </summary>
-public class SseClientTransportOptions
+public sealed class SseClientTransportOptions
 {
     /// <summary>
     /// Gets or sets the base address of the server for SSE connections.
@@ -69,5 +69,5 @@ public class SseClientTransportOptions
     /// <remarks>
     /// Use this property to specify custom HTTP headers that should be sent with each request to the server.
     /// </remarks>
-    public Dictionary<string, string>? AdditionalHeaders { get; set; }
+    public IDictionary<string, string>? AdditionalHeaders { get; set; }
 }

src/ModelContextProtocol.Core/Client/StdioClientTransportOptions.cs

@@ -3,7 +3,7 @@ namespace ModelContextProtocol.Client;
 /// <summary>
 /// Provides options for configuring <see cref="StdioClientTransport"/> instances.
 /// </summary>
-public class StdioClientTransportOptions
+public sealed class StdioClientTransportOptions
 {
     /// <summary>
     /// Gets or sets the command to execute to start the server process.
@@ -53,7 +53,7 @@ public class StdioClientTransportOptions
     /// That includes removing the variables for any of this collection's entries with a null value.
     /// </para>
     /// </remarks>
-    public Dictionary<string, string?>? EnvironmentVariables { get; set; }
+    public IDictionary<string, string?>? EnvironmentVariables { get; set; }
 
     /// <summary>
     /// Gets or sets the timeout to wait for the server to shut down gracefully.

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -244,7 +244,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         }
     }
 
-    internal static void CopyAdditionalHeaders(HttpRequestHeaders headers, Dictionary<string, string>? additionalHeaders, string? sessionId = null)
+    internal static void CopyAdditionalHeaders(HttpRequestHeaders headers, IDictionary<string, string>? additionalHeaders, string? sessionId = null)
     {
         if (sessionId is not null)
         {

src/ModelContextProtocol.Core/Diagnostics.cs

@@ -13,7 +13,7 @@ internal static class Diagnostics
     internal static Meter Meter { get; } = new("Experimental.ModelContextProtocol");
 
     internal static Histogram<double> CreateDurationHistogram(string name, string description, bool longBuckets) =>
-        Diagnostics.Meter.CreateHistogram<double>(name, "s", description
+        Meter.CreateHistogram<double>(name, "s", description
 #if NET9_0_OR_GREATER
         , advice: longBuckets ? LongSecondsBucketBoundaries : ShortSecondsBucketBoundaries
 #endif