Enhance content validation in CallToolResult processing to ensure "type" discriminator presence and normalize text-only items

Author: halllo

dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs

@@ -605,12 +605,52 @@ private static string NormalizeToCallToolResult(string raw, JsonSerializerOption
             using var doc = JsonDocument.Parse(raw);
             var root = doc.RootElement;
 
-            // Already a CallToolResult-like object with a content array whose items have "type".
+            // CallToolResult-like object with a content array.
+            // Validate that every item carries a "type" discriminator before returning as-is;
+            // the MCP SDK may strip the discriminator when mapping TextContentBlock →
+            // Microsoft.Extensions.AI.TextContent, producing {"content":[{"text":"..."}]}.
             if (root.ValueKind == JsonValueKind.Object &&
                 root.TryGetProperty("content", out var existing) &&
                 existing.ValueKind == JsonValueKind.Array)
             {
-                return raw;
+                bool needsNormalization = false;
+                foreach (var item in existing.EnumerateArray())
+                {
+                    if (item.ValueKind != JsonValueKind.Object || !item.TryGetProperty("type", out _))
+                    {
+                        needsNormalization = true;
+                        break;
+                    }
+                }
+
+                if (!needsNormalization)
+                {
+                    return raw;
+                }
+
+                // Rebuild the content array, injecting "type":"text" for any text-only items.
+                var sb = new StringBuilder("{\"content\":[");
+                bool first = true;
+                foreach (var item in existing.EnumerateArray())
+                {
+                    if (!first) { sb.Append(','); }
+                    first = false;
+
+                    if (item.ValueKind == JsonValueKind.Object &&
+                        !item.TryGetProperty("type", out _) &&
+                        item.TryGetProperty("text", out var itemTextProp) &&
+                        itemTextProp.ValueKind == JsonValueKind.String)
+                    {
+                        string encodedText = JsonSerializer.Serialize(itemTextProp.GetString(), options.GetTypeInfo(typeof(string)));
+                        sb.Append($"{{\"type\":\"text\",\"text\":{encodedText}}}");
+                    }
+                    else
+                    {
+                        sb.Append(item.GetRawText());
+                    }
+                }
+                sb.Append("]}");
+                return sb.ToString();
             }
 
             // Single object with a "text" property (Microsoft.Extensions.AI TextContent — type stripped).