Merge pull request #180 from giovanimds/fix/google-reasoning-content-pollution

fix: Google reasoning text leaking into plaintext content

Author: lofcz

src/LlmTornado.Tests/GoogleReasoningPartTests.cs

@@ -0,0 +1,69 @@
+using System.Text;
+using LlmTornado.Chat;
+using LlmTornado.Chat.Vendors.Google;
+
+namespace LlmTornado.Tests;
+
+/// <summary>
+/// Regression tests for the Google/Gemini/Gemma vendor part mapper. Does not require API credentials.
+/// </summary>
+[TestFixture]
+public class GoogleReasoningPartTests
+{
+    [Test]
+    public void ThoughtPart_WithText_IsClassifiedAsReasoning_AndNotAppendedToPlaintext()
+    {
+        VendorGoogleChatRequestMessagePart part = new VendorGoogleChatRequestMessagePart
+        {
+            Text = "the model is thinking about the answer",
+            Thought = true
+        };
+
+        StringBuilder sb = new StringBuilder();
+        ChatMessagePart result = part.ToMessagePart(sb);
+
+        Assert.That(result.Type, Is.EqualTo(ChatMessageTypes.Reasoning));
+        Assert.That(result.Reasoning?.Content, Is.EqualTo("the model is thinking about the answer"));
+
+        // The bug: sb (which becomes ChatMessage.Content / the streamed "content" field)
+        // used to receive the thought text unconditionally, mixing reasoning into the
+        // user-visible answer even though the part was correctly flagged as a thought.
+        Assert.That(sb.ToString(), Is.Empty);
+    }
+
+    [Test]
+    public void NonThoughtPart_WithText_IsClassifiedAsText_AndAppendedToPlaintext()
+    {
+        VendorGoogleChatRequestMessagePart part = new VendorGoogleChatRequestMessagePart
+        {
+            Text = "42",
+            Thought = false
+        };
+
+        StringBuilder sb = new StringBuilder();
+        ChatMessagePart result = part.ToMessagePart(sb);
+
+        Assert.That(result.Type, Is.EqualTo(ChatMessageTypes.Text));
+        Assert.That(result.Text, Is.EqualTo("42"));
+        Assert.That(sb.ToString(), Is.EqualTo("42"));
+    }
+
+    [Test]
+    public void ThoughtPart_WithoutText_StillReportsReasoningSignature()
+    {
+        // Redacted-thought blocks: content can be empty while a signature is still present.
+        VendorGoogleChatRequestMessagePart part = new VendorGoogleChatRequestMessagePart
+        {
+            Text = null,
+            Thought = true,
+            ThoughtSignature = "opaque-signature"
+        };
+
+        StringBuilder sb = new StringBuilder();
+        ChatMessagePart result = part.ToMessagePart(sb);
+
+        Assert.That(result.Type, Is.EqualTo(ChatMessageTypes.Reasoning));
+        Assert.That(result.Reasoning?.Signature, Is.EqualTo("opaque-signature"));
+        Assert.That(sb.ToString(), Is.Empty);
+    }
+}

src/LlmTornado/Chat/Vendors/Google/VendorGoogleChatRequest.cs

@@ -543,9 +543,22 @@ public ChatMessagePart ToMessagePart(StringBuilder sb)
         }
         else if (Text is not null)
         {
-            part.Type = ChatMessageTypes.Text;
-            part.Text = Text;
-            sb.Append(Text);
+            if (Thought ?? false)
+            {
+                part.Type = ChatMessageTypes.Reasoning;
+                part.Reasoning = new ChatMessageReasoningData
+                {
+                    Provider = LLmProviders.Google,
+                    Content = Text,
+                    Signature = ThoughtSignature
+                };
+            }
+            else
+            {
+                part.Type = ChatMessageTypes.Text;
+                part.Text = Text;
+                sb.Append(Text);
+            }
         }
         else if (InlineData is not null)
         {