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Pydantic AI is a Python agent framework built on Pydantic. Braintrust provides native integration to trace Pydantic AI agents, capturing inputs, outputs, tool calls, and performance metrics.
This guide covers manual instrumentation. For quicker setup, use auto-instrumentation.

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Setup

Install the Braintrust SDK and Pydantic AI: 
pip install braintrust pydantic-ai

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Trace Pydantic AI agents

Use setup_pydantic_ai() to automatically instrument all Pydantic AI agents and direct API calls: 
from braintrust.wrappers.pydantic_ai import setup_pydantic_ai
from pydantic_ai import Agent

# Initialize Braintrust tracing for Pydantic AI
setup_pydantic_ai(project_name="my-pydantic-project")

# Use agents as normal - they're automatically traced
agent = Agent("openai:gpt-4o", system_prompt="You are a helpful assistant.")
result = await agent.run("What is the capital of France?")
This automatically traces:
  • Agent runs: agent.run(), agent.run_sync(), agent.run_stream(), and agent.run_stream_sync()
  • Direct API calls: model_request(), model_request_sync(), and streaming variants
  • Model interactions: Individual LLM calls made by agents
  • Tool calls: Any tools defined on your agents

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Example with tools and streaming

from braintrust.wrappers.pydantic_ai import setup_pydantic_ai
from pydantic_ai import Agent, ModelSettings

setup_pydantic_ai(project_name="my-pydantic-project")

agent = Agent(
    "openai:gpt-4o",
    model_settings=ModelSettings(max_tokens=500),
)

@agent.tool_plain
def get_weather(city: str) -> str:
    """Get the current weather for a city."""
    return f"22°C and sunny in {city}"

# Streaming is fully traced, including time-to-first-token
async with agent.run_stream("What's the weather in Paris?") as result:
    async for text in result.stream_text(delta=True):
        print(text, end="", flush=True)

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Using with existing spans

If you already have a Braintrust span context (e.g., from @traced or start_span), Pydantic AI traces will nest under it: 
from braintrust import traced
from braintrust.wrappers.pydantic_ai import setup_pydantic_ai
from pydantic_ai import Agent

setup_pydantic_ai(project_name="my-pydantic-project")

agent = Agent("openai:gpt-4o")

@traced
async def my_workflow(question: str):
    # Agent traces appear as children of this span
    result = await agent.run(question)
    return result.output

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Alternative: OpenTelemetry integration

You can also use Braintrust’s OpenTelemetry support with Pydantic AI’s built-in instrumentation: 
from braintrust.otel import BraintrustSpanProcessor
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from pydantic_ai import Agent

provider = TracerProvider()
trace.set_tracer_provider(provider)
provider.add_span_processor(BraintrustSpanProcessor())

Agent.instrument_all()
This requires pip install "braintrust[otel]". See the OpenTelemetry guide for more details.

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