# Agent libraries – Pydantic AI

[Pydantic AI](https://pydantic.dev/docs/ai/overview/) is a Python agent framework built by the Pydantic team that uses Pydantic validation for [structured output](https://pydantic.dev/docs/ai/core-concepts/output/#structured-output) schema generation and validation.
By specifying an `output_type` on an Agent, you can constrain the LLM to return data that matches your Pydantic model schema.

## LLM Structured Output

```python
from pydantic_ai import Agent

from pydantic import BaseModel, Field, ValidationInfo, field_validator


class City(BaseModel):
    name: str
    country: str
    population: int = Field(description='Estimated population', gt=0)

    @field_validator('country')
    @classmethod
    def country_must_be_valid(cls, v: str, info: ValidationInfo) -> str:
        valid_countries: list[str] = info.context or []
        if v not in valid_countries:
            raise ValueError(f'Unknown country: {v!r}')
        return v


agent = Agent(
    'openai:gpt-5-mini',
    output_type=list[City],
    # Pydantic validation context (not sent to the model)
    validation_context=['Japan', 'United States', 'Germany'],
)

result = agent.run_sync('List the 3 largest cities in Japan')
print(result.output)
#> [City(name='Tokyo', country='Japan', population=13960000), ...]
```

When validation fails (say the model returns a country your validator rejects), Pydantic AI feeds the
validation errors back to the model and asks it to retry. To see this happening in a real application,
instrument it with [Logfire](/guides/integrations-logfire), which records
[Pydantic AI](https://pydantic.dev/docs/ai/integrations/logfire/) runs and the validations inside them: each agent run
shows the output the model produced, the errors your validators raised, and the retry that followed.

## Related pages

- [API Documentation](./api-documentation-index.md)
- [Concepts](./concepts-index.md)
- [Dev Tools](./dev-tools-index.md)
- [Error Messages](./error-messages-index.md)
- [Examples](./examples-index.md)
- [Integrations](./integrations-index.md)
- [Internals](./internals-index.md)
- [Production Tools](./production-tools-index.md)
- [Pydantic](./pydantic-index.md)
- [Pydantic Core](./pydantic-core-index.md)

# Agent Instructions

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