Enum Message 

pub enum Message {
    User {
        content: String,
        attachments: Vec<Attachment>,
    },
    Assistant {
        content: String,
        tool_calls: Vec<ToolCall>,
        reasoning: Vec<ReasoningState>,
    },
    System {
        content: String,
    },
    Tool {
        content: String,
        tool_call_id: String,
    },
}

A message in a conversation.

Different message types have different fields:

• User/System: content with optional attachments
• Assistant: content with optional tool calls
• Tool: content with required tool_call_id

Variants

User

User message with content and optional attachments.

Fields

content: String

Text content of the message.

attachments: Vec<Attachment>

Attachment URLs (images, documents, etc.)

Assistant

Assistant message with content and optional tool calls.

Fields

content: String

Text content of the message.

tool_calls: Vec<ToolCall>

Tool calls made by the assistant.

reasoning: Vec<ReasoningState>

Opaque reasoning state produced with this turn.

Replayed to the provider on the next request. Order is significant — Anthropic rejects a turn whose thinking blocks were reordered or partially dropped — so append rather than rebuild.

System

System message with instructions/context.

Fields

content: String

Text content of the message.

Tool

Tool result message.

Fields

content: String

Result content from the tool.

tool_call_id: String

ID of the tool call this is responding to.

Implementations

impl Message

pub const fn role(&self) -> Role

Returns the message sender role.

pub fn content(&self) -> &str

Returns the text content of the message.

pub fn attachments(&self) -> &[Attachment]

Returns the typed attachments (only for User messages).

pub fn tool_calls(&self) -> &[ToolCall]

Returns tool calls made by the assistant (only for Assistant messages).

pub fn tool_call_id(&self) -> Option<&str>

Returns the tool call ID (only for Tool messages).

pub fn user(content: impl Into<String>) -> Self

Creates a new user message.

pub fn assistant(content: impl Into<String>) -> Self

Creates a new assistant message.

pub fn assistant_with_tool_calls( content: impl Into<String>, tool_calls: Vec<ToolCall>, ) -> Self

Creates an assistant message with tool calls.

pub fn assistant_with_reasoning( content: impl Into<String>, tool_calls: Vec<ToolCall>, reasoning: Vec<ReasoningState>, ) -> Self

Creates an assistant message carrying the reasoning state of its turn.

Use this when rebuilding a conversation for another request: without the reasoning, providers that verify their own thinking see a turn that has lost the reasoning behind its tool calls.

pub fn reasoning(&self) -> &[ReasoningState]

Reasoning state recorded on this message, if any.

Only assistant turns carry reasoning; every other role returns empty.

pub fn system(content: impl Into<String>) -> Self

Creates a new system message.

pub fn tool(tool_call_id: impl Into<String>, content: impl Into<String>) -> Self

Creates a new tool result message.

pub fn with_attachment(self, attachment: Attachment) -> Self

Adds a typed attachment to the message (only works for User messages).

pub fn with_attachments( self, values: impl IntoIterator<Item = Attachment>, ) -> Self

Adds multiple typed attachments to the message.

pub fn with_tool_calls(self, calls: Vec<ToolCall>) -> Self

Adds tool calls to the message (only works for Assistant messages).

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

Formats the value using the given formatter.

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

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

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Eq for Message

impl PartialEq for Message

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

Equality operator ==.

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

Inequality operator !=.

impl Serialize for Message

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

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl StructuralPartialEq for Message