diff --git a/js/plugins/compat-oai/src/model.ts b/js/plugins/compat-oai/src/model.ts
index d2ce7cce99..b3567fb51e 100644
--- a/js/plugins/compat-oai/src/model.ts
+++ b/js/plugins/compat-oai/src/model.ts
@@ -177,7 +177,9 @@ export function toOpenAITextAndMedia(
     }
 
     // Check if this is an image type
-    if (isImageContentType(contentType)) {
+    // If no contentType is provided, preserve legacy behavior by treating the media
+    // as an image URL (e.g. signed URLs or remote images without metadata)
+    if (!contentType || isImageContentType(contentType)) {
       return {
         type: 'image_url',
         image_url: {
diff --git a/js/plugins/compat-oai/tests/compat_oai_test.ts b/js/plugins/compat-oai/tests/compat_oai_test.ts
index 93844e3fba..295d76a9d1 100644
--- a/js/plugins/compat-oai/tests/compat_oai_test.ts
+++ b/js/plugins/compat-oai/tests/compat_oai_test.ts
@@ -171,6 +171,21 @@ describe('toOpenAiTextAndMedia', () => {
     });
   });
 
+  it('should transform image from signed URLs', () => {
+    const part: Part = {
+      media: {
+        url: 'https://storage.googleapis.com/bucket/image.png?X-Goog-Signature=fake',
+      },
+    };
+    expect(toOpenAITextAndMedia(part, 'low')).toStrictEqual({
+      type: 'image_url',
+      image_url: {
+        url: 'https://storage.googleapis.com/bucket/image.png?X-Goog-Signature=fake',
+        detail: 'low',
+      },
+    });
+  });
+
   it('should throw error for file URLs (non-base64 PDFs)', () => {
     const part: Part = {
       media: {