Assistant “Instruction” takes the place of (and is) a system role message placed at the start of a conversation (still a system message internally). It is the assistant behavior, and you also have a field additional_instructions that can be appended to that text on a per-run basis.
When you create a thread, there is no assistant associated with the thread ID. You can choose whatever assistant ID to run the thread against each time you start a run. You can choose if an expert in Rust programming with new documentation or Cleopatra answers the same thread (and an adorable squirrel cartoon Assistant will quickly drop out to answer your programming questions anyway, as the model is only good at acting like ChatGPT).
You should just be able to select an assistant, and if the previous conversation isn’t too confusing, get that new behavior with new tools - in theory. The thread maintains tool calls and returns that may no longer be available or representative of the new Assistant, and other things internally that would make such a mid-conversation switch difficult.
GPT-4o is just terrible about following procedural step-by-step instructions beyond a chat-like initial input - especially with the input context length that can grow beyond the size of original GPT-4’s context window with just one data return from Assistants’ file search, with documents that are distraction. You can use the ranker threshold so that fewer chunks are returned, so that any second question doesn’t have instructions lost in 16000 other tokens.
You can see the internal instructions for file search tools simply are not oriented to documents being part of a built-in knowledge. You’ll have to basically give overriding instructions, now knowing the tool name, and also tell the AI Assistant the contents of the vector store and the requirement to use it, so it can make informed searches.
The model is also the reason the whole ChatGPT GPT idea went to “give up on it” quality as soon as OpenAI switched to that model. It is the 1% computational cost solution to needing to shut off ChatGPT Plus signups for over a month after release of assistants and GPTs. You still have a few variants of gpt-4-turbo to experiment with.