Per-run additional instructions vs appending instructions to user messages

In my app, users are greeted with some personalized content from the assistant. I’m wondering which would be more effective: adding information about the user in the additional_instructions field of the run creation request body or simply appending it to the initial user message that I send automatically to kick off this greeting message (this first message is always hidden to the user).

Is there any reason one should work much better than the other? Do they do substantially different things under the hood?

I’m tempted to just append the info to the start message for simplicity.

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I would just add it at the start of the message for simplicity.

I’m not too familiar with additional_instructions, but I can’t imaging they’d be any more effective than the user message.

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That’s my gut feeling as well, though I’d love to know if OpenAI has shed any light on what additional_instructions actually does.

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