diff --git a/app/(chat)/api/chat/_lib/image-media.ts b/app/(chat)/api/chat/_lib/image-media.ts
index 88532221a..ea19ca9e8 100644
--- a/app/(chat)/api/chat/_lib/image-media.ts
+++ b/app/(chat)/api/chat/_lib/image-media.ts
@@ -609,13 +609,20 @@ export async function convertMessagesWithImages(
         continue;
       }
 
+      const typedPart = part as {
+        type: string;
+        url?: string;
+        image?: string;
+        image_url?: { url?: string };
+        data?: string;
+      };
       const candidateImageUrl =
-        part.type === "file"
-          ? part.url || (part as { data?: string | undefined }).data
-          : part.type === "image"
-            ? part.image || part.url
-            : part.type === "image_url"
-              ? part.image_url?.url || part.url
+        typedPart.type === "file"
+          ? typedPart.url || typedPart.data
+          : typedPart.type === "image"
+            ? typedPart.image || typedPart.url
+            : typedPart.type === "image_url"
+              ? typedPart.image_url?.url || typedPart.url
               : undefined;
 
       if (typeof candidateImageUrl !== "string" || !candidateImageUrl.trim()) {
@@ -625,7 +632,7 @@ export async function convertMessagesWithImages(
       const normalizedImageUrl = candidateImageUrl.trim();
 
       // 兼容 file/image/image_url 三类输入；Gemini 内部会将图片交给 S3 后再请求生图网关。
-      if (part.type === "file" || part.type === "image" || part.type === "image_url") {
+      if (typedPart.type === "file" || typedPart.type === "image" || typedPart.type === "image_url") {
         content.push({
           type: "image_url",
           image_url: {