Make LoggingMessageNotificationParams.Data required per MCP spec

The MCP schema defines `data` as required in LoggingMessageNotification params.
Change from `JsonElement?` (nullable, optional) to `required JsonElement`
(non-nullable, required) to match the spec.

JsonElement remains the correct type (not JsonObject) because the spec
defines data as typeless (any JSON-serializable value), not restricted
to JSON objects.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/LoggingMessageNotificationParams.cs

@@ -45,8 +45,9 @@ public sealed class LoggingMessageNotificationParams : NotificationParams
     public string? Logger { get; set; }
 
     /// <summary>
-    /// Gets or sets the data to be logged, such as a string message.
+    /// Gets or sets the data to be logged, such as a string message or an object.
+    /// Any JSON serializable type is allowed here.
     /// </summary>
     [JsonPropertyName("data")]
-    public JsonElement? Data { get; set; }
+    public required JsonElement Data { get; set; }
 }

tests/ModelContextProtocol.Tests/Client/McpClientTests.cs

@@ -539,11 +539,9 @@ public async Task AsClientLoggerProvider_MessagesSentToClient()
             {
                 var m = await channel.Reader.ReadAsync(TestContext.Current.CancellationToken);
                 Assert.NotNull(m);
-                Assert.NotNull(m.Data);
-
                 Assert.Equal("TestLogger", m.Logger);
 
-                string? s = JsonSerializer.Deserialize<string>(m.Data.Value, McpJsonUtilities.DefaultOptions);
+                string? s = JsonSerializer.Deserialize<string>(m.Data, McpJsonUtilities.DefaultOptions);
                 Assert.NotNull(s);
 
                 if (s.Contains("Information"))

tests/ModelContextProtocol.Tests/Protocol/LoggingMessageNotificationParamsTests.cs

@@ -23,8 +23,7 @@ public static void LoggingMessageNotificationParams_SerializationRoundTrip_Prese
         Assert.NotNull(deserialized);
         Assert.Equal(LoggingLevel.Warning, deserialized.Level);
         Assert.Equal("MyApp.Services", deserialized.Logger);
-        Assert.NotNull(deserialized.Data);
-        Assert.Equal("Something went wrong", deserialized.Data.Value.GetString());
+        Assert.Equal("Something went wrong", deserialized.Data.GetString());
         Assert.NotNull(deserialized.Meta);
         Assert.Equal("value", (string)deserialized.Meta["key"]!);
     }
@@ -34,7 +33,8 @@ public static void LoggingMessageNotificationParams_SerializationRoundTrip_WithM
     {
         var original = new LoggingMessageNotificationParams
         {
-            Level = LoggingLevel.Error
+            Level = LoggingLevel.Error,
+            Data = JsonDocument.Parse("\"error occurred\"").RootElement.Clone(),
         };
 
         string json = JsonSerializer.Serialize(original, McpJsonUtilities.DefaultOptions);
@@ -43,7 +43,7 @@ public static void LoggingMessageNotificationParams_SerializationRoundTrip_WithM
         Assert.NotNull(deserialized);
         Assert.Equal(LoggingLevel.Error, deserialized.Level);
         Assert.Null(deserialized.Logger);
-        Assert.Null(deserialized.Data);
+        Assert.Equal("error occurred", deserialized.Data.GetString());
         Assert.Null(deserialized.Meta);
     }
 }