A custom GPT built with a carefully scoped system prompt often behaves exactly as intended, right up until a user uploads a document mid conversation and asks the bot to work with it. At that point, a whole category of instructions that governed the bot's behavior perfectly well in pure conversation suddenly stop applying cleanly, because the uploaded content introduces a new source of information that the original instructions never explicitly accounted for.
This is an easy gap to miss during development, because most testing happens through direct conversation, typing questions and checking responses, without necessarily stress testing what happens once a user brings their own file into the mix. The moment that happens, the bot has two sources of truth in front of it simultaneously, its original knowledge base or instructions, and whatever the uploaded document contains, and nothing in a typical system prompt written before file upload was considered explicitly tells the model how to weigh one against the other when they overlap or conflict.
A concrete version of this shows up constantly with document review or analysis style custom GPTs. The bot is scoped tightly to only discuss a specific domain, follow a specific tone, and stay within specific boundaries. A user uploads a document from outside that scope entirely and asks the bot to summarize or analyze it, and the bot, having no explicit instruction for this exact situation, either refuses in a way that feels unhelpful and rigid, since technically the request falls outside its intended purpose, or complies fully and abandons its scope boundaries entirely the moment a file enters the conversation, treating the upload as an implicit override of everything it was previously told to stick to.
Neither of those defaults is usually what the original design intended, and the gap exists specifically because file upload behavior needs its own explicit instruction layer, separate from the conversational scope rules that govern regular text exchanges. The system prompt has to address uploaded content as its own distinct case, specifying clearly whether uploaded files should be treated as falling under the same scope restrictions as regular conversation, or whether file analysis is meant to be a separate allowed capability with its own boundaries, rather than assuming the original scope language will simply extend itself sensibly to cover a scenario it was never written with in mind.
The other subtlety that trips up a lot of custom GPT builds is conflict handling once an uploaded document contains information that contradicts the bot's built in knowledge base. A user uploading their own version of a policy document, possibly outdated, possibly simply different from what the bot's official source material says, creates the same kind of authority question that shows up in multi document knowledge bases generally, except now one of the competing sources was handed directly to the model by the user in the moment, which tends to carry an outsized, almost automatic weight in how the model treats it, purely because it is the most recent and most directly provided piece of context. Left unaddressed, a bot will often default to trusting whatever the user just uploaded over its own more authoritative source material, simply because the uploaded content is fresher in the immediate context.
Fixing this means writing an explicit precedence rule specifically for this exact conflict, stating clearly whether the bot's own core knowledge base or an uploaded document should take priority when the two disagree, and instructing the model to flag the discrepancy directly to the user rather than silently picking one and presenting it as settled fact, something like the document you uploaded shows a different figure than what I have on file, want me to go with your document or confirm against our official source. That single instruction, addressing file upload as a distinct scenario with its own explicit rules rather than an extension of ordinary conversation, closes a gap that otherwise stays invisible until a real user brings in the exact kind of file nobody thought to test with.
Written by Mohammad Farhan Habib Faraz
Senior Prompt Engineer and Prompt Team Lead at PowerinAI
www.powerinai.com
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