Can someone else place trades on my account through AI without my knowledge?

No. That is not possible. 

Every trade placed through ArihantPlus MCP requires you to explicitly instruct the AI in your chat session. The AI does not act on its own, it does not have an autopilot mode, and it cannot initiate any action without a direct instruction from you in that moment. 

Think of it this way, the AI is like a very capable assistant sitting across the table from you. It can only do what you tell it to do, when you tell it to do it. It won't place a trade while you're asleep, it won't act on something you said three days ago, and it certainly won't take instructions from anyone else. 

On top of that, your ArihantPlus login credentials are never shared with the AI. Access is tied to your session, resets every 24 hours, and can be revoked by you at any time. So even if someone had access to your AI chat, they would still need your ArihantPlus credentials to have authenticated the connection in the first place.