Comment by threeducks

I just tried it with GPT-5.1-Codex. The compression ratio is not amazing, so not sure if it really worked, but at least it ran without errors.

A few ideas how to make it work for you:

1. You gave a link to a PDF, but you did not describe how you provided the content of the PDF to the model. It might only have read the text with something like pdftotext, which for this PDF results in a garbled mess. It is safer to convert the pages to PNG (e.g. with pdftoppm) and let the model read it from the pages. A prompt like "Transcribe these pages as markdown." should be sufficient. If you can not see what the model did, there is a chance it made things up.

2. You used C++, but Python is much easier to write. You can tell the model to translate the code to C++ once it works in Python.

3. Tell the model to write unit tests to verify that the individual components work as intended.

4. Use Agent Mode and tell the model to print something and to judge whether the output is sensible, so it can debug the code.