[Bug]: DeepSeek custom endpoint returns 400 when uploading PDF files ("file must have a file_id or file_data")

[Learn-Implement]

What happened?

Uploading a PDF file to a conversation using the DeepSeek custom endpoint (deepseek-flash) produces:

400 .messages[1]: file must have a file_id or file_data

This occurs on a brand-new conversation with no prior history. The PDF is small and well under the container's Nginx body limit (client_max_body_size 25M, confirmed inside the container). It is not a request-size issue.

Expected behaviour: LibreChat should either:

1. Use OCR to extract text from the PDF and send it as a text message, or
2. Clearly reject the upload with a user-facing message explaining DeepSeek does not support direct PDF uploads.

Actual behaviour: The PDF is forwarded as a file block in the chat completion request. DeepSeek's Files API only accepts image formats (JPEG, PNG, GIF, WebP) and requires either a file_id (from an upload to https://api.deepseek.com/files) or file_data (base64-encoded image). A PDF cannot satisfy either requirement, so DeepSeek returns 400.

Precedent: Discussion #10988 already documents that DeepSeek requires the OCR workaround for file uploads.

Workaround in use: Switching to Gemini 3.8 Flash for PDF uploads works because Google's API supports PDFs natively.

Version Information

LibreChat v0.8.7
Deployment: Self-hosted Docker Compose (local)
Image: registry.librechat.ai/danny-avila/librechat-dev:latest
Reverse proxy: None (direct port 3080)
Custom endpoints: DeepSeek V4 Pro, DeepSeek Flash, Gemini, Qwen
Redis: redis:7-alpine (docker), USE_REDIS_STREAMS=true

Steps to Reproduce

1. Start a new conversation with the DeepSeek custom endpoint (deepseek-flash selected).

2. Upload any PDF file using the attachment button.

3. Ask a question about the file.

4. Observe the error:
   400 .messages[1]: file must have a file_id or file_data

What browsers are you seeing the problem on?

Firefox, Chrome, Microsoft Edge

Relevant log output

2026-09-23 09:03:13 error: [api/server/controllers/agents/client.js #sendCompletion] Unhandled error type 400 .messages[1]: file must have a file_id or file_data

Screenshots

Code of Conduct

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[lia-by-librechat]

Thanks for the report. DeepSeek's documented file input is for images, not PDFs, so this does not look like an Nginx body-size issue. The file_id or file_data error alone does not establish that the PDF's base64 data was missing: LibreChat formats OpenAI-compatible files with nested file.file_data, while DeepSeek documents a different shape. Either way, a PDF should not be sent to DeepSeek as a native file part.

For new uploads on a build with unified file upload (not v0.8.7), route PDFs through text extraction for the custom endpoint:

fileConfig:
  endpoints:
    YourDeepSeekEndpointName: # match the endpoint's name in librechat.yaml
      legacyFileUploadUX: false
      defaultLLMDeliveryPath:
        overrides:
          'application/pdf': text

Please test a new PDF in a new conversation after upgrading. This setting does not repair PDFs already attached under the older routing. If the new test still fails, please provide the exact LibreChat image digest, whether this is a direct chat or saved agent, and a redacted outgoing content-part shape (keys only, no PDF bytes or credentials). Related vLLM/llama.cpp report: #16234.