Try

   {"messages":[
      {"role": "user",
       "content": "hi"}
   ]}

but also an explicitly empty system message:

   {"messages":[
      {"role": "system",
       "content": ""}
      {"role": "user",
       "content": "hi"}
   ]}

and finally

   {"messages":[
      {"role": "system",
       "content": "x"}
      {"role": "user",
       "content": "hi"}
   ]}

Comparing OpenRouter’s tokensPrompt with nativeTokensPrompt can tell you if it came from the provider