Fix NotSupportedException when returning IEnumerable<ContentBlock> (#675)

src/ModelContextProtocol.Core/McpJsonUtilities.cs

@@ -146,6 +146,7 @@ public static partial class McpJsonUtilities
     [JsonSerializable(typeof(AudioContentBlock))]
     [JsonSerializable(typeof(EmbeddedResourceBlock))]
     [JsonSerializable(typeof(ResourceLinkBlock))]
+    [JsonSerializable(typeof(IEnumerable<ContentBlock>))]
     [JsonSerializable(typeof(PromptReference))]
     [JsonSerializable(typeof(ResourceTemplateReference))]
     [JsonSerializable(typeof(BlobResourceContents))]

tests/ModelContextProtocol.Tests/McpJsonUtilitiesTests.cs

@@ -1,6 +1,7 @@
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Text.Json.Serialization.Metadata;
+using ModelContextProtocol.Protocol;
 
 namespace ModelContextProtocol.Tests;
 
@@ -43,6 +44,38 @@ public static class McpJsonUtilitiesTests
         }
     }
 
+    [Fact]
+    public static void DefaultOptions_CanSerializeIEnumerableOfContentBlock()
+    {
+        var options = McpJsonUtilities.DefaultOptions;
+        
+        // Create an IEnumerable<ContentBlock> with different content types
+        IEnumerable<ContentBlock> contentBlocks = new List<ContentBlock>
+        {
+            new TextContentBlock { Text = "Hello World" },
+            new TextContentBlock { Text = "Test message" }
+        };
+
+        // Should not throw NotSupportedException
+        string json = JsonSerializer.Serialize(contentBlocks, options);
+        
+        Assert.NotNull(json);
+        Assert.Contains("Hello World", json);
+        Assert.Contains("Test message", json);
+        Assert.Contains("\"type\":\"text\"", json);
+        
+        // Should also be able to deserialize back
+        var deserialized = JsonSerializer.Deserialize<IEnumerable<ContentBlock>>(json, options);
+        Assert.NotNull(deserialized);
+        var deserializedList = deserialized.ToList();
+        Assert.Equal(2, deserializedList.Count);
+        Assert.All(deserializedList, cb => Assert.Equal("text", cb.Type));
+        
+        var textBlocks = deserializedList.Cast<TextContentBlock>().ToArray();
+        Assert.Equal("Hello World", textBlocks[0].Text);
+        Assert.Equal("Test message", textBlocks[1].Text);
+    }
+
     public enum EnumWithoutAnnotation { A = 1, B = 2, C = 3 }
 
     [JsonConverter(typeof(JsonStringEnumConverter<EnumWithAnnotation>))]