Junior interview question
You send a chat API a system message and a user message. What does the model actually receive, and why might the system prompt carry more weight?
Model answer
One token sequence. The chat template writes each role as marker tokens around its content; AI Engineering: system and user prompts are "processed the same way".
Scroll sideways to see the whole diagram
An API takes a list of messages, each with a role. This is the book's system and user prompt.
Text version
- An API takes a list of messages, each with a role. This is the book's system and user prompt.
- Before the model sees anything, a chat template turns the list into one sequence.
- Llama 3's template: begin-of-text, a header naming each role, a blank line (↵↵), the content, an end-of-turn marker.
- Each <|…|> marker is one special token. In a short chat, much of the sequence is the template's own.
- The sequence ends inside an assistant turn, so the most likely continuation is the assistant's reply.
Format: Meta's Llama 3 ChatFormat (github.com/meta-llama/llama3, llama/tokenizer.py, fetched 2026-10-21). No counts shown here; the next page counts them.
The system prompt can count for more only because it comes first, and because post-training may teach the model to favour it.
Practise the rest
5 Prompt anatomy questions, 492 in all, each with a model answer. Free.
More Prompt anatomy interview questions
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