diff --git a/src/convert-to-openrouter-chat-messages.test.ts b/src/convert-to-openrouter-chat-messages.test.ts
index 5499ad92..45febbce 100644
--- a/src/convert-to-openrouter-chat-messages.test.ts
+++ b/src/convert-to-openrouter-chat-messages.test.ts
@@ -184,8 +184,12 @@ describe('cache control', () => {
           {
             type: 'file',
             data: 'ZmlsZSBjb250ZW50',
-            filename: 'file.txt',
             mimeType: 'text/plain',
+            providerMetadata: {
+              openrouter: {
+                filename: 'file.txt',
+              },
+            },
           },
         ],
         providerMetadata: {
@@ -240,9 +244,13 @@ describe('cache control', () => {
           },
           {
             type: 'file',
-            filename: 'file.txt',
             data: 'ZmlsZSBjb250ZW50',
             mimeType: 'text/plain',
+            providerMetadata: {
+              openrouter: {
+                filename: 'file.txt',
+              },
+            },
             // No part-specific provider metadata
           },
         ],
diff --git a/src/convert-to-openrouter-chat-messages.ts b/src/convert-to-openrouter-chat-messages.ts
index 3c9d7ca7..4ce86dd4 100644
--- a/src/convert-to-openrouter-chat-messages.ts
+++ b/src/convert-to-openrouter-chat-messages.ts
@@ -87,7 +87,9 @@ export function convertToOpenRouterChatMessages(
                 return {
                   type: 'file' as const,
                   file: {
-                    filename: part.filename,
+                    filename: String(
+                      part.providerMetadata?.openrouter?.filename,
+                    ),
                     file_data:
                       part.data instanceof Uint8Array
                         ? `data:${part.mimeType};base64,${convertUint8ArrayToBase64(part.data)}`