Normalize CallToolResult.StructuredContent to JsonElement? (#1357)

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>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/CallToolResult.cs

@@ -1,5 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
-using System.Text.Json.Nodes;
+using System.Text.Json;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;
@@ -41,7 +41,7 @@ public sealed class CallToolResult : Result
     /// Gets or sets an optional JSON object representing the structured result of the tool call.
     /// </summary>
     [JsonPropertyName("structuredContent")]
-    public JsonNode? StructuredContent { get; set; }
+    public JsonElement? StructuredContent { get; set; }
 
     /// <summary>
     /// Gets or sets a value that indicates whether the tool call was unsuccessful.

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -266,7 +266,7 @@ public override async ValueTask<CallToolResult> InvokeAsync(
         object? result;
         result = await AIFunction.InvokeAsync(arguments, cancellationToken).ConfigureAwait(false);
 
-        JsonNode? structuredContent = CreateStructuredResponse(result);
+        JsonElement? structuredContent = CreateStructuredResponse(result);
         return result switch
         {
             AIContent aiContent => new()
@@ -529,33 +529,37 @@ typeProperty.ValueKind is not JsonValueKind.String ||
         return outputSchema;
     }
 
-    private JsonNode? CreateStructuredResponse(object? aiFunctionResult)
+    private JsonElement? CreateStructuredResponse(object? aiFunctionResult)
     {
         if (ProtocolTool.OutputSchema is null)
         {
             // Only provide structured responses if the tool has an output schema defined.
             return null;
         }
 
-        JsonNode? nodeResult = aiFunctionResult switch
+        JsonElement? elementResult = aiFunctionResult switch
         {
-            JsonNode node => node,
-            JsonElement jsonElement => JsonSerializer.SerializeToNode(jsonElement, McpJsonUtilities.JsonContext.Default.JsonElement),
-            _ => JsonSerializer.SerializeToNode(aiFunctionResult, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object))),
+            JsonElement jsonElement => jsonElement,
+            JsonNode node => JsonSerializer.SerializeToElement(node, McpJsonUtilities.JsonContext.Default.JsonNode),
+            null => null,
+            _ => JsonSerializer.SerializeToElement(aiFunctionResult, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object))),
         };
 
         if (_structuredOutputRequiresWrapping)
         {
-            return new JsonObject
+            JsonNode? resultNode = elementResult is { } je
+                ? JsonSerializer.SerializeToNode(je, McpJsonUtilities.JsonContext.Default.JsonElement)
+                : null;
+            return JsonSerializer.SerializeToElement(new JsonObject
             {
-                ["result"] = nodeResult
-            };
+                ["result"] = resultNode
+            }, McpJsonUtilities.JsonContext.Default.JsonObject);
         }
 
-        return nodeResult;
+        return elementResult;
     }
 
-    private static CallToolResult ConvertAIContentEnumerableToCallToolResult(IEnumerable<AIContent> contentItems, JsonNode? structuredContent)
+    private static CallToolResult ConvertAIContentEnumerableToCallToolResult(IEnumerable<AIContent> contentItems, JsonElement? structuredContent)
     {
         List<ContentBlock> contentList = [];
         bool allErrorContent = true;

tests/ModelContextProtocol.Tests/Client/McpClientToolTests.cs

@@ -122,7 +122,7 @@ public static CallToolResult StructuredContentTool() =>
             new()
             {
                 Content = [new TextContentBlock { Text = "Regular content" }],
-                StructuredContent = JsonNode.Parse("{\"key\":\"value\"}")
+                StructuredContent = JsonElement.Parse("{\"key\":\"value\"}")
             };
 
         // Tool that returns CallToolResult with Meta

tests/ModelContextProtocol.Tests/Protocol/CallToolResultTests.cs

@@ -12,7 +12,7 @@ public static void CallToolResult_SerializationRoundTrip_PreservesAllProperties(
         var original = new CallToolResult
         {
             Content = [new TextContentBlock { Text = "Result text" }],
-            StructuredContent = JsonNode.Parse("""{"temperature":72}"""),
+            StructuredContent = JsonElement.Parse("""{"temperature":72}"""),
             IsError = false,
             Task = new McpTask
             {
@@ -32,7 +32,7 @@ public static void CallToolResult_SerializationRoundTrip_PreservesAllProperties(
         var textBlock = Assert.IsType<TextContentBlock>(deserialized.Content[0]);
         Assert.Equal("Result text", textBlock.Text);
         Assert.NotNull(deserialized.StructuredContent);
-        Assert.Equal(72, deserialized.StructuredContent["temperature"]!.GetValue<int>());
+        Assert.Equal(72, deserialized.StructuredContent.Value.GetProperty("temperature").GetInt32());
         Assert.False(deserialized.IsError);
         Assert.NotNull(deserialized.Task);
         Assert.Equal("task-1", deserialized.Task.TaskId);

tests/ModelContextProtocol.Tests/Protocol/UnknownPropertiesTests.cs

@@ -252,7 +252,7 @@ public void CallToolResult_WithStructuredContentAtCorrectLevel_PreservesProperty
         // Assert
         Assert.NotNull(deserialized);
         Assert.NotNull(deserialized.StructuredContent);
-        Assert.Equal("correctly placed here", deserialized.StructuredContent["result"]?.ToString());
-        Assert.Equal(42, (int?)deserialized.StructuredContent["value"]);
+        Assert.Equal("correctly placed here", deserialized.StructuredContent.Value.GetProperty("result").GetString());
+        Assert.Equal(42, deserialized.StructuredContent.Value.GetProperty("value").GetInt32());
     }
 }

tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs

@@ -654,11 +654,11 @@ public object InstanceMethod()
         }
     }
 
-    private static void AssertMatchesJsonSchema(JsonElement schemaDoc, JsonNode? value)
+    private static void AssertMatchesJsonSchema(JsonElement schemaDoc, JsonElement? value)
     {
         JsonSchema schema = JsonSerializer.Deserialize(schemaDoc, JsonContext2.Default.JsonSchema)!;
         EvaluationOptions options = new() { OutputFormat = OutputFormat.List };
-        EvaluationResults results = schema.Evaluate(JsonSerializer.SerializeToElement(value, JsonContext2.Default.JsonNode), options);
+        EvaluationResults results = schema.Evaluate(value!.Value, options);
         if (!results.IsValid)
         {
             IEnumerable<string> errors = (results.Details ?? [])
@@ -670,7 +670,7 @@ Instance JSON document does not match the specified schema.
                 Schema:
                 {JsonSerializer.Serialize(schema)}
                 Instance:
-                {value?.ToJsonString() ?? "null"}
+                {value?.ToString() ?? "null"}
                 Errors:
                 {string.Join(Environment.NewLine, errors)}
                 """);