Struct MessageDelta 

pub struct MessageDelta {
    pub role: Option<String>,
    pub content: Option<String>,
    pub reasoning_content: Option<String>,
    pub tool_calls: Option<Vec<ToolCallDelta>>,
}

Incremental message content in a streaming response.

Each MessageDelta contains partial content that should be appended to build the complete message. Fields are Option because different chunks may contain different pieces (role, content, tool_calls).

Example

A complete streaming response might send these deltas:

1. {"role": "assistant"} - Start of assistant message
2. {"content": "Hello"} - First token(s)
3. {"content": " world"} - More tokens
4. {"content": "!"} - Final token

Fields

role: Option<String>

The role of the message (usually only present in first chunk).

content: Option<String>

Incremental text content to append.

reasoning_content: Option<String>

Incremental reasoning content (if supported).

tool_calls: Option<Vec<ToolCallDelta>>

Incremental tool call updates.

Trait Implementations

impl Debug for MessageDelta

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

Formats the value using the given formatter.

impl Default for MessageDelta

fn default() -> MessageDelta

Returns the “default value” for a type.

impl<'de> Deserialize<'de> for MessageDelta

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

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Serialize for MessageDelta

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

where __S: Serializer,

Serialize this value into the given Serde serializer.