diff --git a/src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs b/src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs
index 664ede5a..2f9286af 100644
--- a/src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs
+++ b/src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs
@@ -257,10 +257,13 @@ public override async ValueTask<CallToolResult> InvokeAsync(
                 Content = [],
                 StructuredContent = structuredContent,
             },
-            
+
             string text => new()
             {
-                Content = [new TextContentBlock { Text = text }],
+                // If there is structuredOutput we must return that in stringified form,
+                // as required for backward compatibility by the MCP specification.
+                Content = [new TextContentBlock { Text = structuredContent == null ? text :
+                 JsonSerializer.Serialize(structuredContent, AIFunction.JsonSerializerOptions.GetTypeInfo(typeof(object)))}],
                 StructuredContent = structuredContent,
             },