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Version: 0.3

Quick Start

Ship a streaming, tool-using agent in five minutes. We'll build a weather agent that calls a Python function as a tool, streams Claude's response token-by-token, and exits cleanly.

Prerequisites​

  • Python 3.11+
  • cubepi installed (pip install cubepi)
  • An ANTHROPIC_API_KEY in your environment

The full script​

Save this as weather_agent.py:

weather_agent.py
import asyncio
import os
from pydantic import BaseModel

from cubepi import Agent, AgentTool, AgentToolResult, Model, TextContent
from cubepi.providers.anthropic import AnthropicProvider


class GetWeatherParams(BaseModel):
    city: str


async def get_weather(tool_call_id, params: GetWeatherParams, *, signal=None, on_update=None):
    # In a real app: call an HTTP weather API. Here we hard-code a reply.
    return AgentToolResult(
        content=[TextContent(text=f"72°F and sunny in {params.city}")]
    )


async def main():
    provider = AnthropicProvider(api_key=os.environ["ANTHROPIC_API_KEY"])

    agent = Agent(
        provider=provider,
        model=Model(id="claude-sonnet-4-5-20250929", provider="anthropic"),
        system_prompt="You are a concise weather assistant.",
        tools=[
            AgentTool(
                name="get_weather",
                description="Get current weather for a city",
                parameters=GetWeatherParams,
                execute=get_weather,
            ),
        ],
    )

    # Subscribe BEFORE prompt() — that's how you see streaming events.
    def on_event(event, signal=None):
        if event.type == "message_update" and event.stream_event.type == "text_delta":
            print(event.stream_event.delta, end="", flush=True)
        elif event.type == "agent_end":
            print()  # final newline

    agent.subscribe(on_event)
    await agent.prompt("What's the weather in Tokyo?")


asyncio.run(main())

Run it:

python weather_agent.py

You should see Claude stream a sentence like "The weather in Tokyo is currently 72°F and sunny…", with the tool result threaded through.

What just happened​

CubePi ran a loop that looks (conceptually) like this:

  1. agent.prompt("What's the weather in Tokyo?") enqueued a UserMessage and called the model.
  2. The model decided to invoke get_weather(city="Tokyo") — CubePi parsed the JSON args with your Pydantic model, called your async def, and fed the result back as a ToolResultMessage.
  3. The model produced a final assistant response, streamed back as text_delta events.
  4. The loop emitted agent_end and returned.

agent.subscribe(...) registered a callback for every event the runtime emits: agent_start, turn_start, message_start, text_delta, tool_execution_start, tool_execution_end, message_end, turn_end, agent_end. The script only cared about text_delta, but you can hang any UI off the rest.

Sources of confusion (read before you debug)​

  • Subscribe before you prompt. Listeners only receive events emitted after subscribe was called. Calling prompt first means the early events are gone.
  • Model(id=…, provider=…) is required. The agent uses model.provider to clamp thinking levels and route metadata. The id must match a model the provider supports.
  • The execute signature is fixed. Keep (tool_call_id, params, *, signal, on_update) even if you don't use the keyword-only arguments. CubePi always passes them.
  • agent.prompt() can only run one prompt at a time. While it's running, use agent.steer() to inject a follow-up, or agent.follow_up() to queue one for after the current run.

Next steps​