Why does GPT-5 ignore my prompts?

API
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Hello. I spent a few months working with o1 to write an advanced document transformation program. o1 handles all the prompting like a champ. However, when I put those exact same prompts into GPT-5 it fails miserably, even on high reasoning effort. What’s even worse, GPT-5 completely ignores my instructions. I am trying to modify my o1 prompt to work with GPT-5, but the changes that I make just get ignored.

What is going on here, and how do I fix it?

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Welcome to the forum.

Yes, gpt-5 prompting is a bit different from previous models.

Have a look at these articles, they might help you a bit:

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I’ve found the GPT-5 prompt optimizer helpful for a few things.

https://platform.openai.com/chat/edit?prompt=

You will likely need to adapt the prompts, though.

Were the old ones reasoning too?

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Depending on your prompt, the verbosity parameter (default medium) may be problematic.

For example, we translate entire documents and have to set verbosity to low.

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Basically, your old o1 prompt isn’t clicking with GPT-5 because the two models think in different ways.

  • o1 is great at reading between the lines — it reasons deeply and fills in gaps.

  • GPT-5 is more literal — it wants clear, structured, no-nonsense instructions.

So when you reuse your o1 prompt, GPT-5 either ignores parts of it or gets confused.

Here’s how to fix it:

  1. Put your must-follow rules in the system message, not the user prompt.

  2. Use structured outputs (like JSON mode or function calling) instead of hoping it formats things right.

  3. Set temperature = 0 so it stays consistent.

  4. Give it a tiny example of what you want.

  5. Don’t rely on hidden “reasoning” — spell out the steps you expect.

In short: make your prompt more explicit, structured, and strict, and GPT-5 will follow along just fine.

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