diff --git a/packages/api/src/files/encode/document.spec.ts b/packages/api/src/files/encode/document.spec.ts
index 846b61eb8883..a528b2ca4743 100644
--- a/packages/api/src/files/encode/document.spec.ts
+++ b/packages/api/src/files/encode/document.spec.ts
@@ -948,6 +948,29 @@ describe('encodeAndFormatDocuments - fileConfig integration', () => {
       expect(result.files).toHaveLength(1);
     });
 
+    it('should skip file blocks for DeepSeek models routed through the OpenAI provider', async () => {
+      const req = createMockRequest(15) as ServerRequest;
+      const mimeType = 'application/vnd.openxmlformats-officedocument.wordprocessingml.document';
+      const file = createMockDocFile(1, mimeType, 'brief.docx');
+
+      const mockContent = Buffer.from('docx-binary-content').toString('base64');
+      mockedGetFileStream.mockResolvedValue({
+        file,
+        content: mockContent,
+        metadata: file,
+      });
+
+      const result = await encodeAndFormatDocuments(
+        req,
+        [file],
+        { provider: Providers.OPENAI, model: 'deepseek-v4-pro' },
+        mockStrategyFunctions,
+      );
+
+      expect(result.documents).toHaveLength(0);
+      expect(result.files).toHaveLength(0);
+    });
+
     it('should skip non-Bedrock-document types for Bedrock provider', async () => {
       const req = createMockRequest() as ServerRequest;
       const file = createMockDocFile(1, 'application/zip', 'archive.zip');
diff --git a/packages/api/src/files/encode/document.ts b/packages/api/src/files/encode/document.ts
index 9f595fee1ee3..ab9e945c5e3e 100644
--- a/packages/api/src/files/encode/document.ts
+++ b/packages/api/src/files/encode/document.ts
@@ -22,6 +22,23 @@ const ANTHROPIC_CITATION_TYPES = new Set([
   'text/html',
   'text/markdown',
 ]);
+const DEEPSEEK_TARGET_PATTERN = /^deepseek(?:[-_.]|$)/i;
+
+function targetsDeepSeek(...values: Array<string | undefined>): boolean {
+  for (const value of values) {
+    if (!value) {
+      continue;
+    }
+
+    const normalized = value.replace(/^~/, '');
+    const segments = normalized.split(/[/:]/);
+    if (segments.some((segment) => DEEPSEEK_TARGET_PATTERN.test(segment))) {
+      return true;
+    }
+  }
+
+  return false;
+}
 
 /**
  * Formats a base64-encoded document into the appropriate provider-specific block.
@@ -33,6 +50,8 @@ function formatDocumentBlock(
   content: string,
   filename: string | undefined,
   useResponsesApi: boolean | undefined,
+  endpoint: string | undefined,
+  model: string | undefined,
 ): DocumentBlock | null {
   if (provider === Providers.ANTHROPIC) {
     const document: AnthropicDocumentBlock = {
@@ -63,6 +82,10 @@ function formatDocumentBlock(
     };
   }
 
+  if (provider === Providers.DEEPSEEK || targetsDeepSeek(endpoint, model)) {
+    return null;
+  }
+
   const resolvedFilename = filename ?? 'document';
 
   if (useResponsesApi) {
@@ -214,6 +237,8 @@ export async function encodeAndFormatDocuments(
         content,
         file.filename,
         useResponsesApi,
+        endpoint,
+        model,
       );
       if (block) {
         result.documents.push(block);
@@ -233,6 +258,8 @@ export async function encodeAndFormatDocuments(
         content,
         file.filename,
         useResponsesApi,
+        endpoint,
+        model,
       );
       if (block) {
         result.documents.push(block);