Skip empty Gemini thought parts instead of emitting empty reasoning

provider/geminiprovider/agent.go

@@ -395,17 +395,22 @@ func buildResponsePart(part *genai.Part, contents []message.Content) ([]message.
 	if part.Thought {
 		// Thinking model: emit TextReasoningContent. Encode ThoughtSignature as
 		// base64 in ProtectedData so it can be passed back in multi-turn requests.
-		protectedData := ""
-		if len(part.ThoughtSignature) > 0 {
-			protectedData = base64.StdEncoding.EncodeToString(part.ThoughtSignature)
+		// Skip a thought part that carries neither thinking text nor a signature,
+		// consistent with how empty text parts are skipped below; such a part
+		// would otherwise add an empty, information-free reasoning content.
+		if part.Text != "" || len(part.ThoughtSignature) > 0 {
+			protectedData := ""
+			if len(part.ThoughtSignature) > 0 {
+				protectedData = base64.StdEncoding.EncodeToString(part.ThoughtSignature)
+			}
+			contents = append(contents, &message.TextReasoningContent{
+				Text:          part.Text,
+				ProtectedData: protectedData,
+				ContentHeader: message.ContentHeader{
+					RawRepresentation: part,
+				},
+			})
 		}
-		contents = append(contents, &message.TextReasoningContent{
-			Text:          part.Text,
-			ProtectedData: protectedData,
-			ContentHeader: message.ContentHeader{
-				RawRepresentation: part,
-			},
-		})
 	} else if part.Text != "" {
 		contents = append(contents, &message.TextContent{
 			Text: part.Text,

provider/geminiprovider/agent_test.go

@@ -763,6 +763,55 @@ func TestFunctionResultMissingCallID(t *testing.T) {
 	}
 }
 
+// TestResponseWithEmptyThoughtPartOmitted verifies that a thought part carrying
+// neither text nor a signature does not produce an empty TextReasoningContent.
+func TestResponseWithEmptyThoughtPartOmitted(t *testing.T) {
+	resp := map[string]any{
+		"candidates": []any{
+			map[string]any{
+				"content": map[string]any{
+					"role": "model",
+					"parts": []any{
+						map[string]any{"thought": true}, // empty thought: no text, no signature
+						map[string]any{"text": "The answer is 42."},
+					},
+				},
+				"finishReason": "STOP",
+			},
+		},
+		"usageMetadata": map[string]any{"promptTokenCount": 10, "candidatesTokenCount": 15, "totalTokenCount": 25},
+	}
+	respBody, _ := json.Marshal(resp)
+
+	server := httptest.NewServer(captureAndRespond(t, make(chan []byte, 1), "application/json", string(respBody)))
+	defer server.Close()
+
+	a := newTestClient(t, server)
+
+	result, err := a.RunText(t.Context(), "hi").Collect()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	var foundText string
+	for _, msg := range result.Messages {
+		for _, c := range msg.Contents {
+			switch content := c.(type) {
+			case *message.TextReasoningContent:
+				t.Errorf("expected no reasoning content for an empty thought part, got Text=%q ProtectedData=%q", content.Text, content.ProtectedData)
+			case *message.TextContent:
+				foundText += content.Text
+			}
+		}
+	}
+	// The normal (non-thought) text part must still be emitted; asserting it
+	// guards against a regression that drops all parts rather than just the
+	// empty thought.
+	if foundText != "The answer is 42." {
+		t.Errorf("text content = %q, want %q", foundText, "The answer is 42.")
+	}
+}
+
 // TestResponseWithThinkingContent verifies that response parts with thought=true
 // are translated into TextReasoningContent.
 func TestResponseWithThinkingContent(t *testing.T) {
@@ -1592,7 +1641,8 @@ func TestGenerateContentConfigOption(t *testing.T) {
 	temp := float32(0.5)
 	topP := float32(0.9)
 	topK := float32(20)
-	_, err := a.RunText(t.Context(), "Test",
+	_, err := a.RunText(
+		t.Context(), "Test",
 		geminiprovider.GenerateContentConfig(genai.GenerateContentConfig{
 			Temperature:     &temp,
 			TopP:            &topP,