The Video Assistant · Lesson 3 — The Briefing ← Course

The briefing — the system prompt.

Before a single ball is kicked, you can tell the VAR how to judge. That standing instruction is a system prompt.

Every time you chat with an AI there is a message you usually never see. It is set by whoever built the tool, before your conversation starts, and it tells the model who to be and how to answer — its job, its tone, the rules it follows, the shape its replies should take. That hidden, standing instruction is called the system prompt.

Before a match, the VAR is briefed by the captain on how that game will be run — what to watch for, how to handle things: “flag only clear and obvious errors”, “explain your decisions to the fourth official”, “keep your communication crisp”. The VAR keeps that briefing in mind on every incident. The incidents are identical — only the way they are handled changes.

The briefing is not part of any one question. It is sent ahead of all of them, and it rides along on every turn: the standing cost in the window that the last lesson measured, tokens spent before the conversation starts. One short instruction quietly shapes every reply that follows.

Same VAR, same incident — swap the briefing and the answer changes completely. That is what makes the system prompt the first thing a builder reaches for: often you do not need a different model or a cleverer question, just a clearer briefing.

Swap the briefing and replay the same incident. Watch the VAR's manner change — never the facts.

Same incident, four briefings. Pick one and see how the VAR answers.
the briefing · system prompt · set before play
+ ↓
the incident · sent up for review
↓
the VAR's call

One argument, set once.

In code the briefing is just a system instruction handed to the model alongside the question. Change that one string and every answer changes with it. 


      

    


    

      
One briefing, every incident.

      

        The briefing — the system prompt — is set once, before the conversation, and
        the model reads every answer through it. Same incident, same facts; change the
        briefing and you change the tone, the length, the format, even the role the
        VAR plays.
      

      

        It is the builder's first and biggest lever: before you reach for a different
        model or a cleverer question, change the instruction the model is standing on.
      

      

        Next: the specifics →
        
      

    


  


  

    

      ▸ Go deeper — getting a briefing right optional
      

        
Who writes the briefing

        

          The system prompt is set by the builder of the tool, not by the person
          typing questions into it. The model gives it higher standing than an
          ordinary message, because of how it was trained rather than any special
          mechanism, and treats it as the frame for the whole conversation —
          which is exactly why it is the right place to fix behaviour once, instead of
          repeating yourself in every message. It is not a separate rule engine — it is
          text in the window like everything else, and it teaches the model nothing that
          outlasts the conversation.
        

        
It is always in the window

        

          From the last lesson: the system prompt sits in the context window on every
          single turn. That makes it powerful — it is never forgotten halfway through a
          chat — but it is also a standing cost. A bloated briefing is tokens you spend
          before the conversation even begins, so good ones stay tight: a few clear
          rules, not an essay.
        

        
What a good briefing covers

        

          The dependable ingredients are a role (“you are a conservative VAR”), the rules
          to follow, the tone to take, the format to answer in, and what to refuse.
          Vague briefings give vague behaviour; specific ones give predictable answers.
          Most of what people call “prompt engineering” begins right here.
        

        
It steers, it does not lock

        

          A briefing strongly shapes how the model responds, but it is guidance, not an
          unbreakable rule — a determined user can still push against it. Making
          instructions genuinely stick is a harder problem the course comes back to
          later.