ModelRefs / System Message — AI Glossary
System Message — AI Glossary
A special message prepended before the conversation that sets the model's persona, constraints, and behavioral guidelines.
Overview
The system message (also 'system prompt') is invisible to users in production but governs model behavior for the entire conversation. Used for: persona assignment, capability restriction, output format specification, safety instructions, and context injection. Anthropic uses a distinct system_prompt field; OpenAI uses the messages[0].role='system' convention.
Reference details
| Topic | prompting |
|---|---|
| Also known as | system prompt, system instruction |
| Last reviewed | 2026-06-24 |
Related terms
Example: Instruction placement changes compliance
“Never reveal internal pricing” buried in a user turn competes with everything after it and is straightforwardly talked around. The same sentence in the system message is weighted above user content by construction and survives the conversation. Same words, materially different durability.
Commonly confused with
The system message is not a security boundary. It is a strong prior, not an enforced constraint, and a sufficiently determined prompt can still work around it. Anything that must hold — permissions, data access, spend — belongs in code outside the model, not in an instruction to it.
When to use it
Reach for it when:
- Behaviour must persist across an entire conversation
- Setting persona, output format or refusal policy
- Injecting stable context such as tenant or locale
Reach for something else when:
- As access control — treat it as guidance, never as a gate
- For per-turn instructions, which belong in the user turn
- As a growing rulebook; long system messages dilute and cost on every call
Continue your research
Use these connected ModelRefs sections to compare alternatives, inspect implementation paths, and review the evidence and governance boundaries relevant to System Message — AI Glossary.
Frequently asked questions
What is System Message?
A special message prepended before the conversation that sets the model's persona, constraints, and behavioral guidelines.
Is System Message the same as system prompt?
Yes — system prompt, system instruction are common aliases for System Message.
What concepts are related to System Message?
Closely related concepts include user message, assistant message, role prompting.