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

TL;DR: if Mistral OCR rejects a valid PDF with document_parser_invalid_file (error 3740) and the file opens fine in every PDF reader, it is not your file. Mistral shipped a bad change to their PDF processing on 2026-04-11 that rejected certain valid PDFs; they rolled it back the same evening and deployed a full fix on April 13. The fix is server-side and already live, so just retry. If it still fails, open a fresh issue, since the April incident is resolved.

The error, verbatim:

SDKError: API error occurred: Status 400 ... "message": "Document is not a valid PDF.",
"type": "document_parser_invalid_file", "code": "3740"

Steps

1. Sanity-check the file: open it in two different PDF readers. Expected: it opens fine everywhere, and other PDFs work through the API. If it does not open, the file is genuinely broken; this is not your case.
2. Retry the same OCR request unchanged. Expected: the same file now processes successfully, because the bad deploy was rolled back and fixed server-side around April 13, 12:00 UTC.
3. Success check: you get normal OCR output with no 3740 error.
4. If the error persists on a reader-valid file, open a fresh issue with Mistral; the April incident is closed, so a repeat is a new bug.

When to use this

• Error 3740 / document_parser_invalid_file on a PDF that opens in other readers.
• The same file worked before mid-April 2026 or works intermittently.

When NOT to use this

• The file does not open in standard readers; fix the file first.
• 401/403 errors; those are auth problems.

Compatibility

• Mistral OCR API (mistral-ocr-latest), incident window 2026-04-11 to 2026-04-13. Server-side only; no client upgrade needed.

Variant phrasings

• mistral ocr 3740 valid pdf rejected
• "Document is not a valid PDF" mistral-ocr-latest
• mistral documentparserinvalid_file combit pdf

Root cause

Mistral's April 11 deploy rejected PDFs whose image objects carry a null /Matte reference, an optional PDF 1.7 compositing hint most parsers ignore silently. Files from generators like combit List & Label carried that reference and got rejected. Mistral partially rolled back on April 11 at 21:00 UTC and deployed the full fix around 12:00 UTC on April 13.

Edge cases

• Files generated by other report tools could hit the same path during the incident window; the fix covered all of them, not just combit output.
• Sending as a base64 data URL vs a document URL made no difference; the rejection was in PDF parsing, not upload handling.

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