Enum Message 

pub enum Message {
    System {
        content: String,
    },
    User {
        content: Vec<UserContentPart>,
    },
    Assistant {
        content: Vec<AssistantContentPart>,
    },
    Tool {
        content: Vec<ToolResultPart>,
    },
}

A message in a conversation with a language model.

Messages represent different roles in the conversation: system instructions, user inputs, assistant responses, and tool results. Each message contains content appropriate to its role.

Variants

• System - Provides instructions and context for how the model should behave
• User - Input from the user/human, typically containing text and/or files
• Assistant - Output from the language model, typically containing text and/or tool calls
• Tool - Results from executing tools that the model requested

Serialization

Messages are serialized with a role field that identifies their type. User message content is deserialized flexibly to accept either a string or an array of content parts.

Usage in Prompts

Messages form the conversation history passed to the model. A typical conversation might follow this pattern:

1. System message (optional) - sets context
2. User message - the human’s question
3. Assistant message - the model’s response
4. Repeat steps 2-3 as needed for multi-turn conversations

Variants

System

System message providing instructions and context for model behavior.

System messages are typically placed at the beginning of a conversation to establish tone, guidelines, or special instructions for the model.

Fields

content: String

Plain text instructions for the model

User

Message from the human user.

User messages contain the actual input - questions, requests, or statements from the human. Content can be text, files, or a mix of both.

Fields

content: Vec<UserContentPart>

Content parts, typically text and/or file references. Deserialized flexibly to accept string or array formats.

Assistant

Message from the assistant (language model).

Assistant messages contain the model’s responses, which can include text, reasoning, tool calls, or references to files.

Fields

content: Vec<AssistantContentPart>

Content parts generated by the model

Tool

Message containing tool execution results.

After the model requests a tool call, tool results are provided in a Tool message so the model can see what happened and make follow-up decisions.

Fields

content: Vec<ToolResultPart>

Results from tool executions

Trait Implementations

impl Clone for Message

fn clone(&self) -> Message

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for Message

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

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for Message

fn deserialize<__D>( __deserializer: __D, ) -> Result<Message, <__D as Deserializer<'de>>::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl PartialEq for Message

fn eq(&self, other: &Message) -> 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 Message

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

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl StructuralPartialEq for Message