Callbacks - PraisonAI

Stream events let you react to agent activity - update UI, log activity, or integrate with other systems.

Quick Start

Create Stream Events

use praisonai::{StreamEvent, StreamEventType};

// Create a text delta event
let event = StreamEvent::delta_text("Hello, ");

// Create events for different stages
let start = StreamEvent::request_start();
let first = StreamEvent::first_token("The");
let end = StreamEvent::stream_end();

Handle Events

use praisonai::{StreamEvent, StreamEventType};

fn handle_event(event: &StreamEvent) {
    match event.event_type {
        StreamEventType::RequestStart => println!("Starting..."),
        StreamEventType::FirstToken => println!("First token: {:?}", event.content),
        StreamEventType::DeltaText => print!("{}", event.content.as_deref().unwrap_or("")),
        StreamEventType::ToolCallEnd => println!("Tool complete"),
        StreamEventType::StreamEnd => println!("\nDone!"),
        StreamEventType::Error => eprintln!("Error: {:?}", event.error),
        _ => {}
    }
}

Track Metrics

use praisonai::StreamMetrics;

let mut metrics = StreamMetrics::new();
metrics.start();

// ... process stream ...

metrics.end();
println!("Time to First Token: {:?}ms", metrics.time_to_first_token());
println!("Total Tokens: {}", metrics.total_tokens());

StreamEventType

Events emitted during streaming.

pub enum StreamEventType {
    RequestStart,    // Before API call
    HeadersReceived, // HTTP headers arrived
    FirstToken,      // First content (TTFT marker)
    DeltaText,       // Text content chunk
    DeltaToolCall,   // Tool call in progress
    ToolCallEnd,     // Tool call complete
    LastToken,       // Final content chunk
    StreamEnd,       // Stream completed
    Error,           // Error occurred
}

Event	When Triggered
`RequestStart`	Before API call
`HeadersReceived`	HTTP headers received
`FirstToken`	First token arrives
`DeltaText`	Each text chunk
`DeltaToolCall`	Tool call delta
`ToolCallEnd`	Tool execution done
`LastToken`	Final token
`StreamEnd`	Stream finishes
`Error`	Error occurs

StreamEvent

Structure for streaming events.

pub struct StreamEvent {
    pub event_type: StreamEventType,
    pub content: Option<String>,
    pub tool_call: Option<ToolCallData>,
    pub error: Option<String>,
    pub timestamp: DateTime<Utc>,
    // ... more fields
}

Factory Methods

Method	Description
`request_start()`	Create start event
`first_token(content)`	Create first token event
`delta_text(content)`	Create text delta event
`stream_end()`	Create end event
`error_event(msg)`	Create error event

Builder Methods

Method	Description
`.content(text)`	Set content
`.tool_call(data)`	Set tool call data
`.error(msg)`	Set error
`.agent_id(id)`	Set agent ID
`.session_id(id)`	Set session ID
`.metadata(k, v)`	Add metadata

Best Practices

Handle events efficiently

Process events quickly - avoid blocking the stream.

Use for observability

Great for logging, metrics, progress updates, and debugging.

Track TTFT

Use FirstToken event to measure Time To First Token.

Hooks

Intercept and modify behavior

Streaming

Enable response streaming

​Quick Start

​StreamEventType

​StreamEvent

​Factory Methods

​Builder Methods

​Best Practices

​Related