Stabilize Gemini visual fallback handling

backend/services/cse_test.go

@@ -134,6 +134,16 @@ func TestLowValueThumbnailDetection(t *testing.T) {
 	}
 }
 
+func TestGoogleVideoCollectorPrefersYouTubeDerivedThumbnail(t *testing.T) {
+	result := googleVideoCollector{}.Enrich(nil, SearchResult{
+		Link:         "https://www.youtube.com/watch?v=dQw4w9WgXcQ",
+		ThumbnailURL: "https://example.com/some-search-thumb.jpg",
+	})
+	if result.ThumbnailURL != "https://i.ytimg.com/vi/dQw4w9WgXcQ/hqdefault.jpg" {
+		t.Fatalf("expected derived youtube thumbnail, got %q", result.ThumbnailURL)
+	}
+}
+
 func TestGeminiCandidateLimitNeverExceedsCandidates(t *testing.T) {
 	if got := GeminiCandidateLimit(9); got != 9 {
 		t.Fatalf("expected Gemini limit to stay within candidate count, got %d", got)

backend/services/gemini.go

@@ -474,6 +474,7 @@ func newBrowserStyleImageRequest(imageURL, referer string) (*http.Request, error
 }
 
 func (g *GeminiService) fetchCandidateVisualInlineData(candidate SearchResult) (string, string, error) {
+	lastErr := fmt.Errorf("candidate has no thumbnail or preview video")
 	if candidate.PreviewVideoURL != "" && (candidate.Source == "Envato" || candidate.Source == "Artgrid") {
 		cacheKey := "frame\n" + candidate.PreviewVideoURL
 		if data, mimeType, ok := g.getCachedVisual(cacheKey); ok {
@@ -484,6 +485,7 @@ func (g *GeminiService) fetchCandidateVisualInlineData(candidate SearchResult) (
 			g.setCachedVisual(cacheKey, data, mimeType, 10*time.Minute)
 			return data, mimeType, nil
 		}
+		lastErr = err
 	}
 	if candidate.ThumbnailURL != "" {
 		if isLowValueThumbnail(candidate.ThumbnailURL) {
@@ -492,17 +494,31 @@ func (g *GeminiService) fetchCandidateVisualInlineData(candidate SearchResult) (
 				"source":       candidate.Source,
 				"thumbnailUrl": candidate.ThumbnailURL,
 			})
-			return "", "", fmt.Errorf("candidate thumbnail is low value")
+			lastErr = fmt.Errorf("candidate thumbnail is low value")
+		} else {
+			cacheKey := "image\n" + candidate.ThumbnailURL
+			if data, mimeType, ok := g.getCachedVisual(cacheKey); ok {
+				return data, mimeType, nil
+			}
+			data, mimeType, err := fetchImageAsInlineData(g.Client, candidate.ThumbnailURL, candidate.Link)
+			if err == nil {
+				g.setCachedVisual(cacheKey, data, mimeType, 10*time.Minute)
+				return data, mimeType, nil
+			}
+			lastErr = err
 		}
-		cacheKey := "image\n" + candidate.ThumbnailURL
+	}
+	if fallbackThumbnail := deriveThumbnail(candidate.Link); fallbackThumbnail != "" && fallbackThumbnail != candidate.ThumbnailURL {
+		cacheKey := "image\n" + fallbackThumbnail
 		if data, mimeType, ok := g.getCachedVisual(cacheKey); ok {
 			return data, mimeType, nil
 		}
-		data, mimeType, err := fetchImageAsInlineData(g.Client, candidate.ThumbnailURL, candidate.Link)
+		data, mimeType, err := fetchImageAsInlineData(g.Client, fallbackThumbnail, candidate.Link)
 		if err == nil {
 			g.setCachedVisual(cacheKey, data, mimeType, 10*time.Minute)
 			return data, mimeType, nil
 		}
+		lastErr = err
 	}
 	if candidate.PreviewVideoURL != "" {
 		cacheKey := "frame\n" + candidate.PreviewVideoURL
@@ -511,12 +527,13 @@ func (g *GeminiService) fetchCandidateVisualInlineData(candidate SearchResult) (
 		}
 		data, mimeType, err := extractFrameFromVideo(candidate.PreviewVideoURL)
 		if err != nil {
-			return "", "", err
+			lastErr = err
+		} else {
+			g.setCachedVisual(cacheKey, data, mimeType, 10*time.Minute)
+			return data, mimeType, nil
 		}
-		g.setCachedVisual(cacheKey, data, mimeType, 10*time.Minute)
-		return data, mimeType, nil
 	}
-	return "", "", fmt.Errorf("candidate has no thumbnail or preview video")
+	return "", "", lastErr
 }
 
 func extractFrameFromVideo(videoURL string) (string, string, error) {

backend/services/search_collectors.go

@@ -59,8 +59,8 @@ func (googleVideoCollector) Accept(result SearchResult) bool {
 	return isUsefulGoogleVideoResult(result)
 }
 func (googleVideoCollector) Enrich(searcher *SearchService, result SearchResult) SearchResult {
-	if result.ThumbnailURL == "" {
-		result.ThumbnailURL = deriveThumbnail(result.Link)
+	if derived := deriveThumbnail(result.Link); derived != "" {
+		result.ThumbnailURL = derived
 	}
 	result.Source = strings.TrimSpace(result.Source)
 	if result.Source == "" {