Omit null mimeType from ResourceContents JSON serialization (#1325)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol.Core/Protocol/ResourceContents.cs

@@ -158,7 +158,10 @@ public abstract class ResourceContents
 
             writer.WriteStartObject();
             writer.WriteString("uri", value.Uri);
-            writer.WriteString("mimeType", value.MimeType);
+            if (value.MimeType is not null)
+            {
+                writer.WriteString("mimeType", value.MimeType);
+            }
 
             Debug.Assert(value is BlobResourceContents or TextResourceContents);
             if (value is BlobResourceContents blobResource)

tests/ModelContextProtocol.Tests/Protocol/ResourceContentsTests.cs

@@ -459,4 +459,34 @@ public static class ResourceContentsTests
         Assert.Equal(string.Empty, blobResource.Uri);
         Assert.Equal("YmxvYg==", System.Text.Encoding.UTF8.GetString(blobResource.Blob.ToArray()));
     }
+
+    [Fact]
+    public static void TextResourceContents_NullMimeType_OmittedFromJson()
+    {
+        var resource = new TextResourceContents
+        {
+            Uri = "file:///test.txt",
+            MimeType = null,
+            Text = "hello"
+        };
+
+        var json = JsonSerializer.Serialize<ResourceContents>(resource, McpJsonUtilities.DefaultOptions);
+
+        Assert.DoesNotContain("mimeType", json);
+    }
+
+    [Fact]
+    public static void BlobResourceContents_NullMimeType_OmittedFromJson()
+    {
+        var resource = new BlobResourceContents
+        {
+            Uri = "file:///test.bin",
+            MimeType = null,
+            Blob = new byte[] { 1, 2, 3 }
+        };
+
+        var json = JsonSerializer.Serialize<ResourceContents>(resource, McpJsonUtilities.DefaultOptions);
+
+        Assert.DoesNotContain("mimeType", json);
+    }
 }