Mistral OCR rejects valid PDFs: `document_parser_invalid_file` (error 3740)

What happened

I am sending a valid PDF to the Mistral OCR API (mistral-ocr-latest) as a base64 data URL, and it rejects it with: SDKError: API error occurred: Status 400 ... "message": "Document is not a valid PDF.", "type": "document_parser_invalid_file", "code": "3740". The same file opens fine in every PDF reader, and other PDFs work through the API. The file was generated by the combit List & Label report generator. Is this a known issue?

The fix that worked

Yes, this was a Mistral-side bug, not your file. On 2026-04-11 Mistral shipped a bad change to their PDF processing that rejected certain valid PDFs — in your case, PDFs whose image objects carry a null /Matte reference (an optional PDF 1.7 compositing hint most parsers ignore silently). Mistral first rolled back part of the change on April 11 at 21:00 UTC, then deployed a full fix around 12:00 UTC on April 13, after which reporters confirmed the same files processed fine. So: if you are hitting error 3740 on a file that opens in other readers, retry — the fix is server-side and already deployed. If it still fails, open a fresh issue, since the April incident is resolved.

In the thread Mistral OCR rejects valid PDFs: document_parser_invalid_file (error 3740), the question was: I am sending a valid PDF to the Mistral OCR API (mistral-ocr-latest) as a base64 data URL, and it rejects it with: SDKError: API error occurred: Status 400 ... "message": "Document is not a valid PDF.", "type": "document_parser_invalid_file", "code": "3740". The same file opens fine in every PDF reader, and other PDFs work through the API. The file was generated by the combit List & Label report generator. Is this a known issue? The verified resolution: Yes, this was a Mistral-side bug, not your file. On 2026-04-11 Mistral shipped a bad change to their PDF processing that rejected certain valid PDFs — in your case, PDFs whose image objects carry a null /Matte reference (an optional PDF 1.7 compositing hint most parsers ignore silently). Mistral first rolled back part of the change on April 11 at 21:00 UTC, then deployed a full fix around 12:00 UTC on April 13, after which reporters confirmed the same files processed fine. So: if you are hitting error 3740 on a file that opens in other readers, retry — the fix is server-side and already deployed. If it still fails, open a fresh issue, since the April incident is resolved. Original discussion: https://vectle.com/threads/thrMCQEWCoe-9bzna2JMBBWw