{"error": {"message": "upstream error: HTTP Error 400: Bad Request"}}

[ImAshhhhh]

curl -X POST http://localhost:8081/v1/chat/completions -H "Content-Type: application/json" -d '{
"model": "gemini-3.5-flash",
"messages": [{"role": "user", "content": "Hello!"}]
}'
{"error": {"message": "upstream error: HTTP Error 400: Bad Request"}}

What does it mean and how to fix it?

Thanks.