Stream responses

Stream an agent’s output as it happens instead of waiting for the final response: text tokens, tool calls, and lifecycle events, as they occur. This is what drives responsive user interfaces and real-time monitoring.

Strands emits the same set of events regardless of how you consume them; you choose the consumption model that fits your application. Async iterators suit asynchronous server frameworks, and callback handlers suit synchronous Python.

Stream an agent’s response

Async IteratorsConsume the event stream with async iterators, for async server frameworks like FastAPI and Express.

Callback HandlersIntercept events with a callback function in synchronous Python.

Stream event typesReference for every event the stream emits: lifecycle, model, tool, and multi-agent.

A running stream

The smallest real thing streaming does is print an agent’s answer token by token as the model produces it. Iterate over the stream and forward the text chunks:

Python

import asyncio
from strands import Agent

agent = Agent(callback_handler=None)


async def main():
    async for event in agent.stream_async("Tell me about agents in one sentence."):
        if "data" in event:
            print(event["data"], end="", flush=True)


asyncio.run(main())

TypeScript

const agent = new Agent({ tools: [notebook] })

for await (const event of agent.stream('Calculate 2+2')) {
  if (
    event.type === 'modelStreamUpdateEvent' &&
    event.event.type === 'modelContentBlockDeltaEvent' &&
    event.event.delta.type === 'textDelta'
  ) {
    // Print out the model text delta event data
    process.stdout.write(event.event.delta.text)
  }
}
console.log('\nDone!')

Each data chunk is one piece of the model’s output. The event reference lists everything else the stream carries: tool calls, lifecycle signals, and multi-agent coordination.

Where to go next

New to streaming? Start with Async Iterators to consume the stream in an async server, or Callback Handlers if your Python application is synchronous. Both receive the same events; the event reference is the complete list of what you can react to.

Tags

streaming3