Enum Message 

pub enum Message {
    System(String),
    User(Vec<ContentBlock>),
    Assistant {
        content: String,
        reasoning: Option<String>,
        tool_calls: Vec<ToolCall>,
    },
    ToolResult {
        id: String,
        content: String,
    },
}

A single message in a conversation.

This type carries context uniformly between Provider / Memory / Agent: what Memory stores and returns, and what Provider sends and receives, are all Message sequences; Provider implementations map them to the vendor’s wire format.

Note: the model may request several tools in one turn, and these requests must stay in a single Message::Assistant message — splitting them across messages breaks vendor wire validation (some vendors require tool results to immediately follow the assistant message carrying them, e.g. DeepSeek).

Example

Organize a conversation history with the convenience constructors (in real scenarios the agent loop and Memory do this automatically):

use molo::Message;

let history = vec![
    Message::system("You are a helpful assistant"),
    Message::user("How is the weather in Beijing today?"),
    Message::assistant("Let me check the weather."),
    Message::tool_result("call_1", "Sunny, 23°C"),
];

assert_eq!(history.len(), 4);
assert_eq!(history[1], Message::user("How is the weather in Beijing today?"));

Variants

System(String)

System instruction describing the agent’s role and behavior constraints.

User(Vec<ContentBlock>)

User input, made of content blocks (text / images / vendor-shaped pass-through blocks).

Assistant

Assistant reply: text + reasoning + requested tool calls (multiple requests in one turn stay together).

Fields

content: String

Reply text.

reasoning: Option<String>

The model’s reasoning; provided by thinking models (e.g. DeepSeek / Qwen3), None for other models.

This field is a vendor extension and must be passed back verbatim when sending the conversation history, otherwise the API rejects the request (e.g. DeepSeek reports “The reasoning_content in the thinking mode must be passed back to the API.”).

tool_calls: Vec<ToolCall>

Tools requested by the model with this reply; execution results are passed back right after as Message::ToolResult messages.

ToolResult

Tool execution result, passed back to the model.

Fields

id: String

The id corresponding to ToolCall::id.

content: String

Text of the execution result (on failure, the error text; the model decides what to do next).

Implementations

impl Message

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

A system instruction message.

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

A plain-text user message (a single text block, equivalent to user_blocks(vec![ContentBlock::Text(content)])).

pub fn user_blocks(blocks: Vec<ContentBlock>) -> Self

User input made of content blocks (text / images / vendor-shaped pass-through blocks).

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

An assistant reply message (no reasoning, no tool requests).

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

An assistant reply with reasoning (for thinking models; pass reasoning back verbatim when sending history, otherwise the API rejects the request).

pub fn tool_result(id: impl Into<String>, content: impl Into<String>) -> Self

A tool execution result passed back to the model; id corresponds to the ToolCall::id in the Message::Assistant message that carried the request.

Example

use molo::Message;

let result = Message::tool_result("call_1", "Sunny, 23°C");
assert_eq!(result, Message::ToolResult {
    id: "call_1".into(),
    content: "Sunny, 23°C".into(),
});

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 Hash for Message

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

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