I created a custom assistant with the following pre-prompt:
Only research on the internet the news from the last 10 days for the topic that I give you.
If I then provide a topic to the assistant it just gives me general information which I would also get without the pre-prompt.

When using the llm directly, I get the desired result:

(note: These results are older than the demanded 10 days but in general it works ...)
Using the same pre-promt to create an agent with mistral.ai it worked like planned.
I expected the custom assistant to follow the instructions in the pre-prompt.