Harden Gemini JSON parsing

backend/services/gemini.go

@@ -94,9 +94,14 @@ User query: ` + query,
 		return []string{query}, fmt.Errorf("gemini query expansion returned no candidates")
 	}
 
+	jsonText, err := extractJSONObject(payload.Candidates[0].Content.Parts[0].Text)
+	if err != nil {
+		return []string{query}, fmt.Errorf("gemini query expansion JSON extraction failed: %w", err)
+	}
+
 	var parsed QueryExpansion
-	if err := json.Unmarshal([]byte(payload.Candidates[0].Content.Parts[0].Text), &parsed); err != nil {
-		return []string{query}, fmt.Errorf("gemini query expansion JSON parse failed: %w", err)
+	if err := json.Unmarshal([]byte(jsonText), &parsed); err != nil {
+		return []string{query}, fmt.Errorf("gemini query expansion JSON parse failed: %w; raw=%q", err, truncateForError(payload.Candidates[0].Content.Parts[0].Text, 200))
 	}
 
 	queries := []string{query}
@@ -187,6 +192,11 @@ User query: ` + query,
 		return nil, fmt.Errorf("gemini vision returned no candidates")
 	}
 
+	jsonText, err := extractJSONObject(payload.Candidates[0].Content.Parts[0].Text)
+	if err != nil {
+		return nil, fmt.Errorf("gemini vision JSON extraction failed: %w", err)
+	}
+
 	var parsed struct {
 		Recommendations []struct {
 			Index       int    `json:"index"`
@@ -194,8 +204,8 @@ User query: ` + query,
 			Recommended bool   `json:"recommended"`
 		} `json:"recommendations"`
 	}
-	if err := json.Unmarshal([]byte(payload.Candidates[0].Content.Parts[0].Text), &parsed); err != nil {
-		return nil, fmt.Errorf("gemini vision JSON parse failed: %w", err)
+	if err := json.Unmarshal([]byte(jsonText), &parsed); err != nil {
+		return nil, fmt.Errorf("gemini vision JSON parse failed: %w; raw=%q", err, truncateForError(payload.Candidates[0].Content.Parts[0].Text, 200))
 	}
 
 	recommendations := make([]AIRecommendation, 0, len(parsed.Recommendations))
@@ -260,3 +270,56 @@ func min(a, b int) int {
 	}
 	return b
 }
+
+func extractJSONObject(text string) (string, error) {
+	cleaned := strings.TrimSpace(text)
+	cleaned = strings.TrimPrefix(cleaned, "```json")
+	cleaned = strings.TrimPrefix(cleaned, "```")
+	cleaned = strings.TrimSuffix(cleaned, "```")
+	cleaned = strings.TrimSpace(cleaned)
+
+	start := strings.Index(cleaned, "{")
+	if start == -1 {
+		return "", fmt.Errorf("no JSON object start found in %q", truncateForError(cleaned, 200))
+	}
+
+	depth := 0
+	inString := false
+	escaped := false
+	for i := start; i < len(cleaned); i++ {
+		ch := cleaned[i]
+		if escaped {
+			escaped = false
+			continue
+		}
+		if ch == '\\' && inString {
+			escaped = true
+			continue
+		}
+		if ch == '"' {
+			inString = !inString
+			continue
+		}
+		if inString {
+			continue
+		}
+		switch ch {
+		case '{':
+			depth++
+		case '}':
+			depth--
+			if depth == 0 {
+				return cleaned[start : i+1], nil
+			}
+		}
+	}
+	return "", fmt.Errorf("no complete JSON object found in %q", truncateForError(cleaned, 200))
+}
+
+func truncateForError(text string, limit int) string {
+	trimmed := strings.TrimSpace(text)
+	if len(trimmed) <= limit {
+		return trimmed
+	}
+	return trimmed[:limit] + "..."
+}