modelcontextprotocol · stephentoub · Jun 16, 2025 · Jun 12, 2025 · Jun 13, 2025 · Jun 14, 2025
diff --git a/samples/EverythingServer/Program.cs b/samples/EverythingServer/Program.cs
@@ -86,9 +86,9 @@ await ctx.Server.SampleAsync([
         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 @@ await ctx.Server.SampleAsync([
             };
         }
 
-        if (@ref.Type == "ref/prompt")
+        if (@ref is PromptReference pr)
         {
             if (!exampleCompletions.TryGetValue(argument.Name, out IEnumerable<string>? value))
             {

diff --git a/samples/EverythingServer/Tools/AnnotatedMessageTool.cs b/samples/EverythingServer/Tools/AnnotatedMessageTool.cs
@@ -15,25 +15,22 @@ public enum MessageType
     }
 
     [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 static IEnumerable<Content> AnnotatedMessage(MessageType messageType, boo
 
         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 }

diff --git a/samples/EverythingServer/Tools/LongRunningTool.cs b/samples/EverythingServer/Tools/LongRunningTool.cs
@@ -15,7 +15,7 @@ public static async Task<string> LongRunningOperation(
         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++)

diff --git a/samples/EverythingServer/Tools/SampleLlmTool.cs b/samples/EverythingServer/Tools/SampleLlmTool.cs
@@ -17,7 +17,7 @@ public static async Task<string> SampleLLM(
         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 @@ private static CreateMessageRequestParams CreateRequestSamplingParams(string con
             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,

diff --git a/samples/QuickstartWeatherServer/Program.cs b/samples/QuickstartWeatherServer/Program.cs
@@ -17,7 +17,7 @@
 
 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;
 });

diff --git a/samples/TestServerWithHosting/Tools/SampleLlmTool.cs b/samples/TestServerWithHosting/Tools/SampleLlmTool.cs
@@ -20,7 +20,7 @@ public static async Task<string> SampleLLM(
         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 @@ private static CreateMessageRequestParams CreateRequestSamplingParams(string con
             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,

diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresUnreferencedCode.cs b/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

diff --git a/src/Common/Polyfills/System/Diagnostics/CodeAnalysis/StringSyntaxAttribute.cs b/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);
+}
diff --git a/src/ModelContextProtocol.Core/AIContentExtensions.cs b/src/ModelContextProtocol.Core/AIContentExtensions.cs
@@ -29,11 +29,13 @@ public static ChatMessage ToChatMessage(this PromptMessage promptMessage)
     {
         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] : [],
         };
     }
 
@@ -81,33 +83,33 @@ public static IList<PromptMessage> ToPromptMessages(this ChatMessage chatMessage
         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
+            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,
+        };
+
+        if (ac is not null)
         {
-            ac = new TextContent(content.Text);
+            ac.RawRepresentation = content;
         }
 
-        ac.RawRepresentation = content;
-
         return ac;
     }
 
@@ -135,8 +137,8 @@ public static AIContent ToAIContent(this ResourceContents content)
         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 AIContent ToAIContent(this ResourceContents content)
     /// 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 IList<AIContent> ToAIContents(this IEnumerable<Content> contents)
     /// </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 IList<AIContent> ToAIContents(this IEnumerable<ResourceContents> c
         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",
             }
         };
 }
diff --git a/src/ModelContextProtocol.Core/Client/McpClient.cs b/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 @@ public McpClient(IClientTransport clientTransport, McpClientOptions? options, IL
                     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 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)
                 }
 
                 // 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)
             {