diff --git a/backend/internal/service/gemini_chat_completions_compat_service.go b/backend/internal/service/gemini_chat_completions_compat_service.go
index 595bb0bb..5ea02df5 100644
--- a/backend/internal/service/gemini_chat_completions_compat_service.go
+++ b/backend/internal/service/gemini_chat_completions_compat_service.go
@@ -266,7 +266,8 @@ func (s *GeminiMessagesCompatService) forwardClaudeBodyAsChatCompletions(
 	}
 
 	imageCount := 0
-	imageSize := s.extractImageSize(claudeBody)
+	imageInputSize := s.extractImageInputSize(claudeBody)
+	imageSize := normalizeOpenAIImageSizeTier(imageInputSize)
 	if isImageGenerationModel(originalModel) {
 		imageCount = 1
 	}
@@ -282,6 +283,7 @@ func (s *GeminiMessagesCompatService) forwardClaudeBodyAsChatCompletions(
 		ReasoningEffort:  reasoningEffort,
 		ImageCount:       imageCount,
 		ImageSize:        imageSize,
+		ImageInputSize:   imageInputSize,
 		ClientDisconnect: false,
 	}, nil
 }