PydanticAI is a library for building AI applications with Pydantic models. It features built-in, optional support for OpenTelemetry, allowing detailed tracing of agent runs and model interactions. LangWatch, being an OpenTelemetry-compatible observability platform, can seamlessly ingest these traces. This guide explains how to configure PydanticAI and LangWatch to capture this observability data. For more background on PydanticAI’s observability features, refer to their debugging and monitoring documentation.

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Prerequisites

  1. Install LangWatch SDK: 
    pip install langwatch
  2. Install PydanticAI: 
    pip install pydantic-ai

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Instrumenting PydanticAI with LangWatch

The primary way to integrate PydanticAI with LangWatch is by leveraging PydanticAI’s native OpenTelemetry emission in an environment configured by langwatch.setup().

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Using PydanticAI’s Built-in OpenTelemetry with LangWatch Global Setup

When langwatch.setup() is called, it initializes a global OpenTelemetry environment, including a trace exporter configured for LangWatch. If PydanticAI’s instrumentation is enabled (via Agent(instrument=True) or Agent.instrument_all()), it will emit OpenTelemetry traces that are automatically captured by LangWatch. 
import langwatch
from pydantic_ai import Agent
from pydantic_ai.agent import InstrumentationSettings # Optional, for event_mode
import os
import asyncio

# 1. Initialize LangWatch
# This sets up the global OpenTelemetry environment for LangWatch.
langwatch.setup()

# 2. Enable PydanticAI Instrumentation
# Option A: Instrument all agents globally
Agent.instrument_all()

# Option B: Instrument a specific agent instance
# For this example, we'll instrument a specific agent.
# If targeting a generic OTel collector like LangWatch, event_mode='logs' is recommended
# as it aligns with OTel semantic conventions for capturing message events.
# The default mode might use a custom attribute format for events.
instrumentation_settings_for_langwatch = InstrumentationSettings(event_mode='logs')
agent = Agent(model_name='openai:gpt-4o-mini', instrument=instrumentation_settings_for_langwatch)

@langwatch.trace(name="PydanticAI - City Capital Query")
async def get_capital_city(country: str):
    langwatch.get_current_trace().update(metadata={"country_queried": country})

    try:
        # PydanticAI agent calls will now generate OpenTelemetry spans
        # that LangWatch captures under the "PydanticAI - City Capital Query" trace.
        result = await agent.run(f'What is the capital of {country}?')
        return result.output
    except Exception as e:
        if current_trace:
            current_trace.record_exception(e)
            current_trace.set_status("error", str(e))
        raise

async def main():
    try:
        capital = await get_capital_city("France")
        print(f"The capital of France is: {capital}")
    except Exception as e:
        print(f"Error getting capital of France: {e}")

    try:
        capital_error = await get_capital_city("NonExistentCountry") # Example to show error tracing
        print(f"Query for NonExistentCountry returned: {capital_error}")
    except Exception as e:
        print(f"Correctly caught error for NonExistentCountry: {e}")

if __name__ == "__main__":
    asyncio.run(main())
Key points for this approach:
  • langwatch.setup(): Essential for initializing the OpenTelemetry environment that LangWatch uses.
  • Agent(instrument=True) or Agent.instrument_all(): Activates PydanticAI’s OpenTelemetry signal emission.
  • InstrumentationSettings(event_mode='logs'): As per PydanticAI documentation, using event_mode='logs' aligns message capture with OpenTelemetry Semantic Conventions for Generative AI, which might be better for generic OTel collectors. The default mode (json_array) uses a custom attribute format for events.
  • @langwatch.trace(): Creates a parent trace in LangWatch, under which PydanticAI’s operation spans will be nested.
By following these steps, you can effectively monitor your PydanticAI applications using LangWatch, gaining insights into agent behavior, model performance, and overall application flow.