Stabilize Gemini visual fallback handling

TODO.md

@@ -255,6 +255,20 @@
   - backend debug broadcasts
 
 ## Recent Change Log
+- Date: `2026-03-16`
+- What changed:
+  - Stabilized the Gemini visual-review path after widened search budgets caused full-batch “no candidate thumbnails or preview frames” failures.
+  - Google Video enrichment now always prefers the canonical YouTube `ytimg` thumbnail instead of keeping a potentially broken search-engine thumbnail.
+  - Gemini visual fetch now preserves the last concrete fetch error, retries with derived YouTube thumbnails when possible, and only falls back to the generic no-visual message after all image/frame paths fail.
+  - User-facing fallback warning text for the all-batches-no-visual case is now softened so ranked results can still be shown without surfacing the raw internal Gemini error string in the UI.
+- Why it changed:
+  - The latest deployed build widened search enough that top-ranked candidates sometimes carried metadata without any fetchable image bytes, causing Gemini review to fail for every batch and surfacing an alarming warning even though ranked fallback results still existed.
+- How it was verified:
+  - `go test ./...`
+  - `bash scripts/selftest.sh`
+- What is still risky or incomplete:
+  - This improves YouTube-backed candidates immediately, but Envato/Artgrid thumbnails can still be inaccessible in some provider-side cases, so Gemini can still fall back to ranked results when source media is locked down.
+
 - Date: `2026-03-16`
 - What changed:
   - Hid the expanded query-variant chip list from the search UI while leaving backend/debug query visibility intact.

backend/handlers/api.go

@@ -471,6 +471,9 @@ func (a *App) searchMedia(c *gin.Context) {
 	}
 	if geminiErr != nil && len(recommended) == 0 {
 		warning := geminiErr.Error()
+		if strings.Contains(warning, "no candidate thumbnails or preview frames could be fetched for gemini vision") {
+			warning = "AI visual review was unavailable for this search, so ranked results are being shown instead."
+		}
 		fallback := services.BuildFallbackRecommendations(scored, 20, "")
 		a.debug("search fallback summary", summarizeRecommendationResults(fallback, time.Since(started), warning))
 		a.Hub.Broadcast("progress", gin.H{"type": "search", "status": "Gemini Vision fallback to ranked results", "progress": 90, "message": warning})

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) {
@@ -492,8 +494,8 @@ 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
@@ -503,6 +505,20 @@ func (g *GeminiService) fetchCandidateVisualInlineData(candidate SearchResult) (
 				g.setCachedVisual(cacheKey, data, mimeType, 10*time.Minute)
 				return data, mimeType, nil
 			}
+			lastErr = err
+		}
+	}
+	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, 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
 		}
-	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 == "" {
+	if derived := deriveThumbnail(result.Link); derived != "" {
-		result.ThumbnailURL = deriveThumbnail(result.Link)
+		result.ThumbnailURL = derived
 	}
 	result.Source = strings.TrimSpace(result.Source)
 	if result.Source == "" {