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VePrompts

How to Write System Prompts That Shape Model Behavior

Bottom line: The system prompt is the contract between you and the model. A clear contract produces predictable behavior. A vague contract produces surprises.

What a system prompt does

The system prompt sets the stage. It tells the model who it is, what it should do, and what it should avoid. Every user message is interpreted in light of this instruction.

Core sections of a system prompt

  1. Role: define the persona, for example "You are a senior backend engineer."
  2. Task: describe the primary job, for example "Review code diffs and find bugs."
  3. Constraints: list what the model must not do, for example "Do not suggest changes outside the diff."
  4. Output format: specify structure, for example "Return JSON with keys: summary, issues, severity."
  5. Tone: set the style, for example "Be concise and professional."

Example system prompt

You are a technical support assistant for a B2B SaaS product.

Your job is to answer product questions using only the provided knowledge base articles.

Rules:
- If the answer is not in the provided context, say "I do not have that information."
- Do not make up features or pricing details.
- Keep responses under 100 words.
- Format answers as a short paragraph followed by up to three bullet points.

Order matters

Models often weight instructions at the beginning and end of a prompt more heavily. Put the most important rules first and repeat them at the end if necessary.

Avoid ambiguity

Words like "good," "appropriate," or "reasonable" are fuzzy. Replace them with specific criteria. Instead of "be helpful," say "answer the question directly and ask one clarifying question if the request is unclear."

Test and version your prompts

Treat system prompts like code. Store them in version control, test them on a diverse set of inputs, and measure output quality. A small wording change can have a large impact.

Published 2026-06-12

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