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ai-python-durable-execution

add durable execution to AI agents

Published by Vercel Labs Updated Jul 17
Covers Python Agents AI SDK

Description

Use when adding durable execution to AI SDK for Python, building durable agent loops, or serializing messages across workflow steps.

SKILL.md

ai-python-durable-execution

Use durable execution when an agent run must survive restarts, worker moves, or long waits.

The SDK does not provide durability by itself. Build a custom Agent.loop, and put side effects inside durable steps:

  • model calls
  • tool I/O
  • approval or resume boundaries

Keep the workflow replayable. Durable steps should take JSON inputs and return JSON outputs.

Serialize messages like this:

data = message.model_dump(mode="json")
message = ai.messages.Message.model_validate(data)

Model Step

A durable model step should drain ai.stream(...) inside the step and return one complete assistant Message.

@workflow.step
async def llm_step(
    model_data: dict[str, object],
    messages_data: list[dict[str, object]],
    tools_data: list[dict[str, object]],
) -> dict[str, object]:
    model = ai.Model.model_validate(model_data)
    messages = [
        ai.messages.Message.model_validate(message)
        for message in messages_data
    ]
    tools = [ai.Tool.model_validate(tool) for tool in tools_data]

    async with ai.stream(model, messages, tools=tools) as stream:
        async for _event in stream:
            pass

        if stream.message is None:
            raise RuntimeError("LLM stream ended without a message")

        return stream.message.model_dump(mode="json")

Durable Tools

Prefer wrapping the tool body in the durable step:

@ai.tool
@workflow.step
async def ask_mothership(question: str) -> str:
    response = await mothership_client.ask(question)
    return response.summary

If the workflow system needs separate activity dispatch, schedule a zero-arg callable that returns ai.tool_result(...). Do not call tool.fn directly.

Agent Loop

Use the model step result as a complete message. Do not wrap it in ai.Stream, ai.events.replay_message_events, or ai.util.merge, those utilities are used for fluent dispatch in non-durable applications, which is impossible in a workflow setting since streams are considered side-effects.

class DurableAgent(ai.Agent):
    async def loop(self, context: ai.Context):
        while context.keep_running():
            result = await llm_step(
                context.model.model_dump(mode="json"),
                [m.model_dump(mode="json") for m in context.messages],
                [t.model_dump(mode="json") for t in context.tools],
            )

            assistant_message = ai.messages.Message.model_validate(result)
            context.add(assistant_message)

            async with ai.ToolRunner() as runner:
                for tool_call in assistant_message.tool_calls:
                    runner.schedule(context.resolve(tool_call))

                async for event in runner.events():
                    yield event

                context.add(runner.get_tool_message())

This pattern does not stream model tokens to the caller. That is usually the right tradeoff for durable workflows, because many durable systems do not support async generators. You can build a queue-based side channel for streaming; however, that kind of stream can't be used to dispatch tools and affect control flow directly.

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