There is no way to currently attach files to an assistant neither from the UI nor the API:
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i have the same problem since yesterday
Assistants API is deprecated.
Sunset date is “first half of 2026” and we are surely very close.
My advice is don’t expect much prioritised support on Assistants nor any focus on availability and performance and get moving with migration work to Responses.
I get that it’s deprecated and we’re close to sunset, but we’re still within the supported window.
We’ve already moved a lot to the Responses API, but I don’t think it’s unreasonable to expect basic stability from Assistants in the meantime. Right now things like file attachments / code interpreter are just breaking, which makes it pretty hard to keep anything reliable.