What is the main difference between GPT-4o and GPT-5 prompting?

The main difference is agency. GPT-4o was more compliant and followed instructions directly. GPT-5 has a more advanced reasoning engine that acts as a cautious collaborator, often questioning prompts to ensure safety and accuracy. This requires you to be more explicit and forceful in your prompts.

Why does GPT-5 ignore my uploaded knowledge files?

GPT-5 uses an advanced Retrieval-Augmented Generation (RAG) system where your uploaded files are treated as supporting material, not a binding source of truth. Its internal reasoning engine can override your data if it thinks it has a better answer. You must explicitly command it to use only the provided files.

What is the quickest fix for an underperforming Custom GPT on GPT-5?

The quickest fix is to edit your Custom GPT's 'Instructions' to be more direct. Add a line like: 'Prioritize execution over exploration. If instructions are clear, execute the task immediately. Do not ask clarifying questions unless information is genuinely missing.'