Enum MessageEvent 

pub enum MessageEvent {
    Start {
        partial: Message,
    },
    TextDelta {
        delta: String,
        partial: Message,
    },
    ThinkingDelta {
        delta: String,
        partial: Message,
    },
    ToolCallStart {
        index: usize,
        partial: Message,
    },
    ToolCallDelta {
        index: usize,
        delta: String,
        partial: Message,
    },
    ToolCallEnd {
        index: usize,
        tool_call: ToolCall,
        partial: Message,
    },
    Done {
        message: Message,
    },
    Error {
        message: Message,
        error: String,
    },
}

A typed streaming event describing the progress of one assistant Message.

Providers emit these events while generating a response so downstream agents can distinguish incremental deltas from the final message instead of relying on repeated partial-Message re-sends. Each incremental event carries both the delta (just the new fragment) and the accumulated partial message, so consumers can either append deltas or replace the whole message — a single accumulation loop suffices.

Serialized as an internally tagged JSON object: the type field holds the snake_case variant name (e.g. "text_delta", "tool_call_end", "done") and the variant fields are inlined alongside it.

Variants

• Start - Generation began; partial is the (usually empty) initial message
• TextDelta - New text content was appended to partial’s text content
• ThinkingDelta - New thinking text was appended to partial’s thinking block
• ToolCallStart - The assistant began emitting the tool call at index
• ToolCallDelta - New argument text for the tool call at index
• ToolCallEnd - The tool call at index is complete and parsed
• Done - Generation finished; message is the final complete message
• Error - Generation failed; message holds what was accumulated so far

Variants

Start

Generation of a new assistant message has started.

Fields

partial: Message

The initial (typically empty) accumulated message.

TextDelta

A fragment of text content was generated.

Fields

delta: String

The newly generated text fragment.

partial: Message

The accumulated message including this delta.

ThinkingDelta

A fragment of thinking/reasoning text was generated.

Fields

delta: String

The newly generated thinking fragment.

partial: Message

The accumulated message including this delta.

ToolCallStart

The assistant started emitting a tool call.

Fields

index: usize

Zero-based position of the tool call within the message.

partial: Message

The accumulated message so far.

ToolCallDelta

A fragment of tool-call arguments was generated.

Fields

index: usize

Zero-based position of the tool call within the message.

delta: String

The newly generated argument text fragment.

partial: Message

The accumulated message so far.

ToolCallEnd

A tool call is complete and its arguments have been parsed.

Fields

index: usize

Zero-based position of the tool call within the message.

tool_call: ToolCall

The completed tool call.

partial: Message

The accumulated message including this tool call.

Done

Generation finished successfully.

Fields

message: Message

The final complete message.

Error

Generation failed.

Fields

message: Message

The message accumulated before the failure.

error: String

Description of the failure.

Trait Implementations

impl Clone for MessageEvent

fn clone(&self) -> MessageEvent

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for MessageEvent

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

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for MessageEvent

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

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl PartialEq for MessageEvent

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

Equality operator ==.

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

Inequality operator !=.

impl Serialize for MessageEvent

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 MessageEvent

impl TryFrom<MessageEvent> for AgentValue

type Error = AgentError

The type returned in the event of a conversion error.

fn try_from(event: MessageEvent) -> Result<Self, AgentError>

Performs the conversion.