Having the same problem. I asked chatGPT4 and it said that the start and end index should relate to the characters extracted from (in my case) a pdf. So before I uploaded the pdf, I went through it page by page and recorded the start and end index for each page based on counting the characters on the page. This did not work… Later when I made a request to the assistant, it would always give back indexes that matched the first page. I would also ask a follow up question: ‘what page did this source come from?’ … that also didn’t work. The answers to that were never quite right. One more thing. The quotes that came back with the annotations seemed that they were sometimes not exact. I tried using the quotes to find the text within the pdf and I could sometimes, but not always.