Vercel AI Data Stream Protocol

Pydantic AI natively supports the Vercel AI Data Stream Protocol to receive agent run input from, and stream events to, a Vercel AI Elements frontend.

Usage

The VercelAIAdapter class is responsible for transforming agent run input received from the frontend into arguments for Agent.run_stream_events(), running the agent, and then transforming Pydantic AI events into Vercel AI events. The event stream transformation is handled by the VercelAIEventStream class, but you typically won't use this directly.

If you're using a Starlette-based web framework like FastAPI, you can use the VercelAIAdapter.dispatch_request() class method from an endpoint function to directly handle a request and return a streaming response of Vercel AI events. This is demonstrated in the next section.

If you're using a web framework not based on Starlette (e.g. Django or Flask) or need fine-grained control over the input or output, you can create a VercelAIAdapter instance and directly use its methods. This is demonstrated in "Advanced Usage" section below.

Usage with Starlette/FastAPI

Besides the request, VercelAIAdapter.dispatch_request() takes the agent, the same optional arguments as Agent.run_stream_events(), and an optional on_complete callback function that receives the completed AgentRunResult and can optionally yield additional Vercel AI events.

dispatch_request.py

from fastapi import FastAPI
from starlette.requests import Request
from starlette.responses import Response

from pydantic_ai import Agent
from pydantic_ai.ui.vercel_ai import VercelAIAdapter

agent = Agent('openai:gpt-5')

app = FastAPI()

@app.post('/chat')
async def chat(request: Request) -> Response:
    return await VercelAIAdapter.dispatch_request(request, agent=agent)

Advanced Usage

If you're using a web framework not based on Starlette (e.g. Django or Flask) or need fine-grained control over the input or output, you can create a VercelAIAdapter instance and directly use its methods, which can be chained to accomplish the same thing as the VercelAIAdapter.dispatch_request() class method shown above:

1. The VercelAIAdapter.build_run_input() class method takes the request body as bytes and returns a Vercel AI RequestData run input object, which you can then pass to the VercelAIAdapter() constructor along with the agent.
   • You can also use the VercelAIAdapter.from_request() class method to build an adapter directly from a Starlette/FastAPI request.
2. The VercelAIAdapter.run_stream() method runs the agent and returns a stream of Vercel AI events. It supports the same optional arguments as Agent.run_stream_events() and an optional on_complete callback function that receives the completed AgentRunResult and can optionally yield additional Vercel AI events.
   • You can also use VercelAIAdapter.run_stream_native() to run the agent and return a stream of Pydantic AI events instead, which can then be transformed into Vercel AI events using VercelAIAdapter.transform_stream().
3. The VercelAIAdapter.encode_stream() method encodes the stream of Vercel AI events as SSE (HTTP Server-Sent Events) strings, which you can then return as a streaming response.
   • You can also use VercelAIAdapter.streaming_response() to generate a Starlette/FastAPI streaming response directly from the Vercel AI event stream returned by run_stream().

Note

This example uses FastAPI, but can be modified to work with any web framework.

run_stream.py

import json
from http import HTTPStatus

from fastapi import FastAPI
from fastapi.requests import Request
from fastapi.responses import Response, StreamingResponse
from pydantic import ValidationError

from pydantic_ai import Agent
from pydantic_ai.ui import SSE_CONTENT_TYPE
from pydantic_ai.ui.vercel_ai import VercelAIAdapter

agent = Agent('openai:gpt-5')

app = FastAPI()


@app.post('/chat')
async def chat(request: Request) -> Response:
    accept = request.headers.get('accept', SSE_CONTENT_TYPE)
    try:
        run_input = VercelAIAdapter.build_run_input(await request.body())
    except ValidationError as e:
        return Response(
            content=json.dumps(e.json()),
            media_type='application/json',
            status_code=HTTPStatus.UNPROCESSABLE_ENTITY,
        )

    adapter = VercelAIAdapter(agent=agent, run_input=run_input, accept=accept)
    event_stream = adapter.run_stream()

    sse_event_stream = adapter.encode_stream(event_stream)
    return StreamingResponse(sse_event_stream, media_type=accept)