Enum MessageRole 

pub enum MessageRole {
    System,
    User,
    Assistant,
    Tool,
}

Identifies the sender/role of a message in the conversation.

This enum follows the standard chat completion role system used by most LLM APIs. The role determines how the message is interpreted and processed.

Serialization

Serializes to lowercase strings via serde ("system", "user", etc.) to match OpenAI API format.

Role Semantics

• System: Establishes context, instructions, and behavior
• User: Input from the human or calling application
• Assistant: Response from the AI model
• Tool: Results from tool/function execution

Variants

System

System message that establishes agent behavior and context.

Typically the first message in a conversation. Used for instructions, personality definition, and constraints that apply throughout the conversation.

User

User message representing human or application input.

The prompt or query that the agent should respond to. In multi-turn conversations, user messages alternate with assistant messages.

Assistant

Assistant message containing the AI model’s response.

Can include text, tool use requests, or both. When the model wants to call a tool, it includes ToolUseBlock content.

Tool

Tool result message containing function execution results.

Sent back to the model after executing a requested tool. Contains the tool’s output that the model can use in its next response.

Trait Implementations

impl Clone for MessageRole

fn clone(&self) -> MessageRole

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for MessageRole

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for MessageRole

fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl PartialEq for MessageRole

fn eq(&self, other: &MessageRole) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Serialize for MessageRole

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl Eq for MessageRole

impl StructuralPartialEq for MessageRole