I had to add to the instructions for it to search the knowledge files 2000 characters at a time, and to search for keywords and not exact phrases, which is really the only thing I could find online about developing one. It also needs to have the code interpreter enabled afaik and it seems to have issues with zip files as well but can extract and search them sometimes, though it seems to vary the technique and sometimes fail. I can confirm that it can search multiple files as I uploaded a mailing list archive and it would return results from multiple files in it.

I've moved to combining all my data into single files, but sometimes it also seems to have issues with them as well even if they are under the upload size limit, I assume that is due to how many characters are in them, and it will just brick the whole GPT until the offending file is removed.

The part I have issues with is having it actually use the data, it will quote/summarize data it found in the knowledge base and return where it found it if it can, but I can never make it do more than that. Ideally I want it to contextualize the data it finds in the knowledge files and prompt itself or factor it into a response, but anytime it accesses the knowledge base I get nothing more than a paraphrased response of what it found and why it may be applicable to my prompt.