WIP: Reduce copies, eliminate a lot of UTF-16 transcoding

src/ModelContextProtocol.AspNetCore/StreamableHttpSession.cs

@@ -1,4 +1,5 @@
-using ModelContextProtocol.Server;
+using ModelContextProtocol.Core;
+using ModelContextProtocol.Server;
 using System.Diagnostics;
 using System.Security.Claims;
 
@@ -16,7 +17,7 @@ internal sealed class StreamableHttpSession(
     private readonly object _stateLock = new();
 
     private int _getRequestStarted;
-    private readonly CancellationTokenSource _disposeCts = new();
+    private CancellationTokenSource _disposeCts = new();
 
     public string Id => sessionId;
     public StreamableHttpServerTransport Transport => transport;
@@ -124,7 +125,8 @@ public async ValueTask DisposeAsync()
             {
                 sessionManager.DecrementIdleSessionCount();
             }
-            _disposeCts.Dispose();
+
+            CanceledTokenSource.Defuse(ref _disposeCts);
         }
     }
 

src/ModelContextProtocol.Core/AIContentExtensions.cs

@@ -1,9 +1,8 @@
 using Microsoft.Extensions.AI;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
-#if !NET
+using System.Buffers.Text;
 using System.Runtime.InteropServices;
-#endif
 using System.Text.Json;
 using System.Text.Json.Nodes;
 
@@ -263,10 +262,12 @@ public static IList<PromptMessage> ToPromptMessages(this ChatMessage chatMessage
         AIContent? ac = content switch
         {
             TextContentBlock textContent => new TextContent(textContent.Text),
+
+            Utf8TextContentBlock utf8TextContent => new TextContent(utf8TextContent.Text),
 
-            ImageContentBlock imageContent => new DataContent(Convert.FromBase64String(imageContent.Data), imageContent.MimeType),
+            ImageContentBlock imageContent => new DataContent(imageContent.DecodedData, imageContent.MimeType),
 
-            AudioContentBlock audioContent => new DataContent(Convert.FromBase64String(audioContent.Data), audioContent.MimeType),
+            AudioContentBlock audioContent => new DataContent(audioContent.DecodedData, audioContent.MimeType),
 
             EmbeddedResourceBlock resourceContent => resourceContent.Resource.ToAIContent(),
 
@@ -275,7 +276,9 @@ public static IList<PromptMessage> ToPromptMessages(this ChatMessage chatMessage
 
             ToolResultContentBlock toolResult => new FunctionResultContent(
                 toolResult.ToolUseId,
-                toolResult.Content.Count == 1 ? toolResult.Content[0].ToAIContent() : toolResult.Content.Select(c => c.ToAIContent()).OfType<AIContent>().ToList())
+                toolResult.StructuredContent is JsonElement structured ? structured :
+                toolResult.Content.Count == 1 ? toolResult.Content[0].ToAIContent() :
+                toolResult.Content.Select(c => c.ToAIContent()).OfType<AIContent>().ToList())
             {
                 Exception = toolResult.IsError is true ? new() : null,
             },
@@ -307,7 +310,7 @@ public static AIContent ToAIContent(this ResourceContents content)
 
         AIContent ac = content switch
         {
-            BlobResourceContents blobResource => new DataContent(Convert.FromBase64String(blobResource.Blob), blobResource.MimeType ?? "application/octet-stream"),
+            BlobResourceContents blobResource => new DataContent(blobResource.DecodedData, blobResource.MimeType ?? "application/octet-stream"),
             TextResourceContents textResource => new TextContent(textResource.Text),
             _ => throw new NotSupportedException($"Resource type '{content.GetType().Name}' is not supported.")
         };
@@ -380,21 +383,25 @@ public static ContentBlock ToContentBlock(this AIContent content)
 
             DataContent dataContent when dataContent.HasTopLevelMediaType("image") => new ImageContentBlock
             {
-                Data = dataContent.Base64Data.ToString(),
+                Data = MemoryMarshal.TryGetArray(dataContent.Base64Data, out ArraySegment<char> segment)
+                    ? new string(segment.Array!, segment.Offset, segment.Count)
+                    : new string(dataContent.Base64Data.ToArray()),
                 MimeType = dataContent.MediaType,
             },
 
             DataContent dataContent when dataContent.HasTopLevelMediaType("audio") => new AudioContentBlock
             {
-                Data = dataContent.Base64Data.ToString(),
+                Data = MemoryMarshal.TryGetArray(dataContent.Base64Data, out ArraySegment<char> segment)
+                    ? new string(segment.Array!, segment.Offset, segment.Count)
+                    : new string(dataContent.Base64Data.ToArray()),
                 MimeType = dataContent.MediaType,
             },
 
             DataContent dataContent => new EmbeddedResourceBlock
             {
                 Resource = new BlobResourceContents
                 {
-                    Blob = dataContent.Base64Data.ToString(),
+                    DecodedData = dataContent.Data,
                     MimeType = dataContent.MediaType,
                     Uri = string.Empty,
                 }
@@ -414,21 +421,51 @@ public static ContentBlock ToContentBlock(this AIContent content)
                 Content =
                     resultContent.Result is AIContent c ? [c.ToContentBlock()] :
                     resultContent.Result is IEnumerable<AIContent> ec ? [.. ec.Select(c => c.ToContentBlock())] :
-                    [new TextContentBlock { Text = JsonSerializer.Serialize(content, McpJsonUtilities.DefaultOptions.GetTypeInfo<object>()) }],
-                StructuredContent = resultContent.Result is JsonElement je ? je : null,
+                    [new TextContentBlock { Text = "" }],
+                StructuredContent =
+                    resultContent.Result is JsonElement je ? je :
+                    resultContent.Result is null ? null :
+                    JsonSerializer.SerializeToElement(resultContent.Result, McpJsonUtilities.DefaultOptions.GetTypeInfo<object>()),
             },
 
-            _ => new TextContentBlock
+            _ => CreateJsonResourceContentBlock(content)
+        };
+
+        static ContentBlock CreateJsonResourceContentBlock(AIContent content)
+        {
+            byte[] jsonUtf8 = JsonSerializer.SerializeToUtf8Bytes(content, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object)));
+
+#if NET
+            int maxLength = Base64.GetMaxEncodedToUtf8Length(jsonUtf8.Length);
+#else
+            int maxLength = ((jsonUtf8.Length + 2) / 3) * 4;
+#endif
+
+            byte[] base64 = new byte[maxLength];
+            if (Base64.EncodeToUtf8(jsonUtf8, base64, out _, out int bytesWritten) != System.Buffers.OperationStatus.Done)
             {
-                Text = JsonSerializer.Serialize(content, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object))),
+                throw new InvalidOperationException("Failed to base64-encode JSON payload.");
             }
-        };
+
+            ReadOnlyMemory<byte> blob = base64.AsMemory(0, bytesWritten);
+
+            return new EmbeddedResourceBlock
+            {
+                Resource = new BlobResourceContents
+                {
+                    Uri = string.Empty,
+                    MimeType = "application/json",
+                    BlobUtf8 = blob,
+                },
+            };
+        }
 
         contentBlock.Meta = content.AdditionalProperties?.ToJsonObject();
 
         return contentBlock;
     }
 
+
     private sealed class ToolAIFunctionDeclaration(Tool tool) : AIFunctionDeclaration
     {
         public override string Name => tool.Name;

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -6,6 +6,7 @@
 using System.Buffers.Text;
 #endif
 using System.Diagnostics.CodeAnalysis;
+using ModelContextProtocol.Internal;
 using System.Net.Http.Headers;
 using System.Security.Cryptography;
 using System.Text;
@@ -581,8 +582,9 @@ private async Task PerformDynamicClientRegistrationAsync(
             Scope = GetScopeParameter(protectedResourceMetadata),
         };
 
-        var requestJson = JsonSerializer.Serialize(registrationRequest, McpJsonUtilities.JsonContext.Default.DynamicClientRegistrationRequest);
-        using var requestContent = new StringContent(requestJson, Encoding.UTF8, "application/json");
+        using var requestContent = new JsonTypeInfoHttpContent<DynamicClientRegistrationRequest>(
+            registrationRequest,
+            McpJsonUtilities.JsonContext.Default.DynamicClientRegistrationRequest);
 
         using var request = new HttpRequestMessage(HttpMethod.Post, authServerMetadata.RegistrationEndpoint)
         {

src/ModelContextProtocol.Core/CanceledTokenSource.cs

@@ -0,0 +1,44 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace ModelContextProtocol.Core;
+
+/// <summary>
+/// A <see cref="CancellationTokenSource"/> that is already canceled.
+/// Disposal is a no-op.
+/// </summary>
+public sealed class CanceledTokenSource : CancellationTokenSource
+{
+    /// <summary>
+    /// Gets a singleton instance of a canceled token source.
+    /// </summary>
+    public static readonly CanceledTokenSource Instance = new();
+
+    private CanceledTokenSource()
+        => Cancel();
+
+    /// <inheritdoc/>
+    protected override void Dispose(bool disposing)
+    {
+        // No-op
+    }
+
+    /// <summary>
+    /// Defuses the given <see cref="CancellationTokenSource"/> by optionally canceling it
+    /// and replacing it with the singleton canceled instance.
+    /// The original token source is left for garbage collection and finalization provided
+    /// there are no other references to it outstanding if <paramref name="dispose"/> is false.
+    /// </summary>
+    /// <param name="cts"> The token source to pseudo-dispose. May be null.</param>
+    /// <param name="cancel"> Whether to cancel the token source before pseudo-disposing it.</param>
+    /// <param name="dispose"> Whether to call Dispose on the token source.</param>
+    [SuppressMessage("Design", "CA1062:Validate arguments of public methods")]
+    public static void Defuse(ref CancellationTokenSource cts, bool cancel = true, bool dispose = false)
+    {
+        // don't null check; allow replacing null, allow throw on attempt to call Cancel
+        var orig = cts;
+        if (cancel) orig.Cancel();
+        Interlocked.Exchange(ref cts, Instance);
+        // presume the GC will finalize and dispose the original CTS as needed
+        if (dispose) orig.Dispose();
+    }
+}

src/ModelContextProtocol.Core/Client/McpClientTool.cs

@@ -142,8 +142,27 @@ result.StructuredContent is null &&
                 case 1 when result.Content[0].ToAIContent() is { } aiContent:
                     return aiContent;
 
-                case > 1 when result.Content.Select(c => c.ToAIContent()).ToArray() is { } aiContents && aiContents.All(static c => c is not null):
-                    return aiContents;
+                case > 1:
+                    AIContent[] aiContents = new AIContent[result.Content.Count];
+                    bool allConverted = true;
+
+                    for (int i = 0; i < aiContents.Length; i++)
+                    {
+                        if (result.Content[i].ToAIContent() is not { } c)
+                        {
+                            allConverted = false;
+                            break;
+                        }
+
+                        aiContents[i] = c;
+                    }
+
+                    if (allConverted)
+                    {
+                        return aiContents;
+                    }
+
+                    break;
             }
         }
 

src/ModelContextProtocol.Core/Client/McpHttpClient.cs

@@ -32,10 +32,9 @@ internal virtual async Task<HttpResponseMessage> SendAsync(HttpRequestMessage re
 #if NET
         return JsonContent.Create(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
 #else
-        return new StringContent(
-            JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage),
-            Encoding.UTF8,
-            "application/json"
+        return new ModelContextProtocol.Internal.JsonTypeInfoHttpContent<JsonRpcMessage>(
+            message,
+            McpJsonUtilities.JsonContext.Default.JsonRpcMessage
         );
 #endif
     }

src/ModelContextProtocol.Core/Client/SseClientSessionTransport.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using ModelContextProtocol.Core;
 using ModelContextProtocol.Protocol;
 using System.Diagnostics;
 using System.Net.Http.Headers;
@@ -18,7 +19,7 @@ internal sealed partial class SseClientSessionTransport : TransportBase
     private readonly HttpClientTransportOptions _options;
     private readonly Uri _sseEndpoint;
     private Uri? _messageEndpoint;
-    private readonly CancellationTokenSource _connectionCts;
+    private CancellationTokenSource _connectionCts;
     private Task? _receiveTask;
     private readonly ILogger _logger;
     private readonly TaskCompletionSource<bool> _connectionEstablished;
@@ -114,7 +115,7 @@ private async Task CloseAsync()
             }
             finally
             {
-                _connectionCts.Dispose();
+                CanceledTokenSource.Defuse(ref _connectionCts, dispose: true);
             }
         }
         finally

src/ModelContextProtocol.Core/Client/StdioClientSessionTransport.cs

@@ -7,7 +7,7 @@ namespace ModelContextProtocol.Client;
 /// <summary>Provides the client side of a stdio-based session transport.</summary>
 internal sealed class StdioClientSessionTransport(
     StdioClientTransportOptions options, Process process, string endpointName, Queue<string> stderrRollingLog, ILoggerFactory? loggerFactory) :
-    StreamClientSessionTransport(process.StandardInput.BaseStream, process.StandardOutput.BaseStream, encoding: null, endpointName, loggerFactory)
+    StreamClientSessionTransport(process.StandardInput.BaseStream, process.StandardOutput.BaseStream, endpointName, loggerFactory)
 {
     private readonly StdioClientTransportOptions _options = options;
     private readonly Process _process = process;