Struct Delta 

pub struct Delta {
    pub role: Option<String>,
    pub content: Option<String>,
    pub reasoning_content: Option<String>,
    pub tool_calls: Option<Vec<ToolCallMessage>>,
}

Represents incremental content updates in streaming responses.

The delta contains partial content that should be appended to the accumulated response. Different fields may be present depending on the chunk type and model capabilities.

Fields

• role - Message role, typically “assistant” on first chunk
• content - Partial text content to append
• reasoning_content - Reasoning traces for thinking models

Fields

role: Option<String>

Role of the message sender.

Typically “assistant” on the first chunk of a response, may be omitted on subsequent chunks.

content: Option<String>

Partial text content that should be appended to the response.

This field contains the incremental text content for the current chunk.

reasoning_content: Option<String>

Reasoning content for models with thinking capabilities.

This field contains step-by-step reasoning traces when the model is operating in thinking mode with reasoning enabled.

tool_calls: Option<Vec<ToolCallMessage>>

Streaming tool call payload for tool invocation.

When tool_stream is enabled and the model emits tool calling information, providers often stream this as an array of objects with partial fields. Use a flexible Value here to accept strings/arrays/objects without failing deserialization on type mismatch across increments.

Trait Implementations

impl Clone for Delta

fn clone(&self) -> Delta

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for Delta

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

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for Delta

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

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Serialize for Delta

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

where __S: Serializer,

Serialize this value into the given Serde serializer.