Hi Roger. First let me say that we're hard at work implementing the ability to let Notebook Assistant access more sources. We're planning to add support for PDFs, text, CSVs, URLs, all sorts of potential targets. This has always been part of our roadmap and is now on the short list of near-term features. I just spoke with the primary developer and confirmed this is his next priority.
Second, I note that you mention that you use a Chat Notebook, select OpenAI as the model, and input your API key. When you select an LLM service provider other than Wolfram, you're no longer using our LLM Kit service or accessing the power of Notebook Assistant. This also means that the 'content length' error message you're getting is purely from OpenAI's systems.