Enum MessageChunk 

#[non_exhaustive]
pub enum MessageChunk {
    Delta(String),
    ToolCall {
        id: String,
        name: String,
        arguments: String,
    },
    ToolResult {
        id: String,
        name: String,
        content: String,
    },
    Done(RunSummary),
    Cancelled,
}

Message chunks for a streaming run — the streaming output of one run, sliced into pieces.

Delta / ToolCall / ToolResult are the streaming projection of the message record (text the model is generating, recorded Assistant tool requests, and returned ToolResult messages); Done / Cancelled are terminal markers. These are not “events” — real events are the application-level event abstraction AgentEvent(trait, where each Agent implementation defines its own event variants (the framework doesn’t anticipate them), flowing through an event pipeline.

Reasoning

Reasoning produces no chunks: reasoning deltas from thinking models do not appear in this enum — matching on MessageChunk::Reasoning won’t compile, and that’s intentional. To surface reasoning, attach an EventChannel and subscribe to ReActEvent::Reasoning, or consume StreamEvent::Reasoning at the Provider layer.

The enum carries #[non_exhaustive] (reserved for extension): matches must include a wildcard arm.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

Delta(String)

An increment of the reply text; increments within the same round are concatenated in order.

ToolCall

The model requested a tool call (the tool_calls of a recorded Assistant message).

Fields

id: String

The id of this call, matching ToolCall::id in the recorded memory; used to pair multiple calls of the same tool within a round.

name: String

The tool name.

arguments: String

The arguments generated by the model (JSON text).

ToolResult

A tool execution completed (records a ToolResult message; on failure the content is the error text).

Fields

id: String

The id of this execution, matching Message::ToolResult in the recorded memory; paired with the ToolCall id in the same round.

name: String

The tool name.

content: String

The execution result text (the error text on failure).

Done(RunSummary)

The run ended normally; carries the execution summary for this run (RunSummary); the stream produces no further chunks afterwards.

Cancelled

The run was cooperatively cancelled (via the CancellationToken passed to run/run_stream); terminal chunk, the stream produces no further chunks afterwards (no Done).

Trait Implementations

impl Clone for MessageChunk

fn clone(&self) -> MessageChunk

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for MessageChunk

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

Formats the value using the given formatter.

impl Eq for MessageChunk

impl PartialEq for MessageChunk

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

Equality operator ==.

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

Inequality operator !=.

impl StructuralPartialEq for MessageChunk