ModelRefs / Message Role — AI Glossary
Message Role — AI Glossary
The designation (system, user, assistant, tool) of a message in a chat completion request, controlling how the model interprets its content.
Overview
Standard roles: system (behavioral instructions, invisible to users), user (human input), assistant (model output, used for few-shot prefilling), tool (result returned from a function call). Anthropic uses an equivalent structure. Some providers add: function (legacy OpenAI), developer (differentiated instructions vs. operator instructions in newer APIs).
Reference details
| Topic | ecosystem |
|---|---|
| Last reviewed | 2026-06-24 |
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Frequently asked questions
What is Message Role?
The designation (system, user, assistant, tool) of a message in a chat completion request, controlling how the model interprets its content.
What concepts are related to Message Role?
Closely related concepts include system message, user message, assistant message.