Use @stream with realtime on Vercel failed

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What I did:

I am building a webpage with Graphql @stream to implement a ChatGPT completion API.
Everything works fine in the local environment.

Problem:

After I followed the Vercel instruction on the doc and deployed with Vercel, I got an error msg when calling the Graphql function:

{"errors":[{"message":"Unknown directive \"@stream\".","locations":[{"line":1,"column":100}]}]}

You can see it live here: https://cover-letter-helper-mpl2-hcywl2icw-aimbebergmailcoms-projects.vercel.app/home

Question:

Any hint on what may cause the problem?