Fix serialization of annotated content blocks in tool results

src/ModelContextProtocol.Core/Protocol/ContentBlock.cs

@@ -102,7 +102,14 @@ private static JsonSerializerOptions CreateNestedOptions(JsonSerializerOptions c
             // this custom converter. Keep caller contracts first, then supply the SDK metadata
             // (including nested collection types) using the same settings and converters.
             JsonSerializerOptions nestedOptions = new(callerOptions);
-            nestedOptions.TypeInfoResolverChain.Add(McpJsonUtilities.JsonContext.Default);
+
+            // When the caller is the SDK context itself (for example the server writing a
+            // CallToolResult), it is already the resolver and can't be added to its own chain.
+            if (!ReferenceEquals(nestedOptions.TypeInfoResolver, McpJsonUtilities.JsonContext.Default))
+            {
+                nestedOptions.TypeInfoResolverChain.Add(McpJsonUtilities.JsonContext.Default);
+            }
+
             nestedOptions.MakeReadOnly();
             return nestedOptions;
         }

tests/ModelContextProtocol.Tests/Server/McpServerToolAnnotatedContentTests.cs

@@ -0,0 +1,74 @@
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+
+namespace ModelContextProtocol.Tests.Server;
+
+public class McpServerToolAnnotatedContentTests : ClientServerTestBase
+{
+    public McpServerToolAnnotatedContentTests(ITestOutputHelper testOutputHelper)
+        : base(testOutputHelper)
+    {
+    }
+
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder)
+    {
+        mcpServerBuilder.WithTools<AnnotatedContentTools>();
+    }
+
+    [Fact]
+    public async Task CallTool_TextBlockWithAnnotations_RoundTrips()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var result = await client.CallToolAsync("annotated_text", cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.NotEqual(true, result.IsError);
+        var block = Assert.IsType<TextContentBlock>(Assert.Single(result.Content));
+        Assert.Equal("hello", block.Text);
+        Assert.NotNull(block.Annotations);
+        Assert.Equal([Role.User], block.Annotations.Audience);
+        Assert.Equal((float?)0.5, block.Annotations.Priority);
+    }
+
+    [Fact]
+    public async Task CallTool_ResourceLinkWithAnnotationsAndIcons_RoundTrips()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var result = await client.CallToolAsync("annotated_link", cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.NotEqual(true, result.IsError);
+        var block = Assert.IsType<ResourceLinkBlock>(Assert.Single(result.Content));
+        Assert.Equal("file:///test", block.Uri);
+        Assert.NotNull(block.Annotations);
+        Assert.Equal([Role.Assistant], block.Annotations.Audience);
+        Assert.Equal("https://example.com/icon.png", Assert.Single(Assert.IsAssignableFrom<IList<Icon>>(block.Icons)).Source);
+    }
+
+    [McpServerToolType]
+    public sealed class AnnotatedContentTools
+    {
+        [McpServerTool(Name = "annotated_text")]
+        public static CallToolResult AnnotatedText() => new()
+        {
+            Content = [new TextContentBlock { Text = "hello", Annotations = new() { Audience = [Role.User], Priority = 0.5f } }],
+        };
+
+        [McpServerTool(Name = "annotated_link")]
+        public static CallToolResult AnnotatedLink() => new()
+        {
+            Content =
+            [
+                new ResourceLinkBlock
+                {
+                    Uri = "file:///test",
+                    Name = "test",
+                    Annotations = new() { Audience = [Role.Assistant] },
+                    Icons = [new Icon { Source = "https://example.com/icon.png" }],
+                },
+            ],
+        };
+    }
+}