diff --git a/packages/api/src/files/encode/document.spec.ts b/packages/api/src/files/encode/document.spec.ts
index 15c2e3b716..62bd974075 100644
--- a/packages/api/src/files/encode/document.spec.ts
+++ b/packages/api/src/files/encode/document.spec.ts
@@ -888,6 +888,60 @@ describe('encodeAndFormatDocuments - fileConfig integration', () => {
       expect(mockedGetFileStream).not.toHaveBeenCalled();
     });
 
+    it('should apply Claude document restrictions through an OpenAI-compatible gateway', async () => {
+      const req = createMockRequest(30) as ServerRequest;
+      const file = createMockDocFile(
+        1,
+        'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+        'report.xlsx',
+      );
+
+      const result = await encodeAndFormatDocuments(
+        req,
+        [file],
+        { provider: Providers.OPENAI, model: 'anthropic/claude-sonnet-4-6' },
+        mockStrategyFunctions,
+      );
+
+      expect(result.documents).toHaveLength(0);
+      expect(result.files).toHaveLength(0);
+      expect(mockedGetFileStream).not.toHaveBeenCalled();
+    });
+
+    it('should retain XLSX support for non-Claude OpenAI models', async () => {
+      const req = createMockRequest(30) as ServerRequest;
+      const file = createMockDocFile(
+        1,
+        'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+        'report.xlsx',
+      );
+      const mockContent = Buffer.from('xlsx-content').toString('base64');
+      mockedGetFileStream.mockResolvedValue({
+        file,
+        content: mockContent,
+        metadata: file,
+      });
+
+      const result = await encodeAndFormatDocuments(
+        req,
+        [file],
+        { provider: Providers.OPENAI, model: 'gpt-5.4' },
+        mockStrategyFunctions,
+      );
+
+      expect(result.documents).toMatchObject([
+        {
+          type: 'file',
+          file: {
+            filename: 'report.xlsx',
+            file_data: `data:${file.type};base64,${mockContent}`,
+          },
+        },
+      ]);
+      expect(result.files).toEqual([file]);
+      expect(mockedGetFileStream).toHaveBeenCalledTimes(1);
+    });
+
     it('should still encode supported Anthropic documents when mixed with unsupported ones', async () => {
       const req = createMockRequest(30) as ServerRequest;
       const docxFile = createMockDocFile(1, 'application/vnd.ms-excel', 'sheet.xls');
diff --git a/packages/api/src/files/encode/document.ts b/packages/api/src/files/encode/document.ts
index 7ef4b684e5..b225841258 100644
--- a/packages/api/src/files/encode/document.ts
+++ b/packages/api/src/files/encode/document.ts
@@ -106,15 +106,24 @@ function formatDocumentBlock(
 
 /**
  * Filters out files the provider's document path cannot send to the model.
- * Anthropic rejects non-PDF base64 documents with a 400 that recurs on every
- * retry, so unsupported types are skipped instead of bricking the conversation.
+ * Claude rejects non-PDF binary documents with a 400 that recurs on every retry,
+ * including when it is reached through an OpenAI-compatible gateway. Unsupported
+ * types are skipped instead of bricking the conversation.
  */
-function filterProviderDocumentFiles(provider: Providers, files: IMongoFile[]): IMongoFile[] {
+function filterProviderDocumentFiles(
+  provider: Providers,
+  files: IMongoFile[],
+  model?: string,
+): IMongoFile[] {
   if (provider === Providers.BEDROCK) {
     return files.filter((file) => isBedrockDocumentType(file.type));
   }
 
-  if (provider !== Providers.ANTHROPIC) {
+  const usesAnthropicDocumentCapabilities =
+    provider === Providers.ANTHROPIC ||
+    (isOpenAILikeProvider(provider) && model?.toLowerCase().includes('claude'));
+
+  if (!usesAnthropicDocumentCapabilities) {
     return files;
   }
 
@@ -130,7 +139,7 @@ function filterProviderDocumentFiles(provider: Providers, files: IMongoFile[]):
 
   if (skipped.length) {
     console.warn(
-      `Skipping attachment(s) unsupported by Anthropic document input: ${skipped.join(', ')}`,
+      `Skipping attachment(s) unsupported by Claude document input: ${skipped.join(', ')}`,
     );
   }
 
@@ -182,7 +191,7 @@ export async function encodeAndFormatDocuments(
     return result;
   }
 
-  const processableFiles = filterProviderDocumentFiles(provider, files);
+  const processableFiles = filterProviderDocumentFiles(provider, files, model);
 
   if (!processableFiles.length) {
     return result;