Creating PydanticAI Agents#

Quick Start#

python

from pydantic_ai import Agent

# Minimal agent (text output)
agent = Agent('openai:gpt-4o')
result = agent.run_sync('Hello!')
print(result.output)  # str

Model Selection#

Model strings follow provider:model-name format:

python

# OpenAI
agent = Agent('openai:gpt-4o')
agent = Agent('openai:gpt-4o-mini')

# Anthropic
agent = Agent('anthropic:claude-sonnet-4-5')
agent = Agent('anthropic:claude-haiku-4-5')

# Google
agent = Agent('google-gla:gemini-2.0-flash')
agent = Agent('google-vertex:gemini-2.0-flash')

# Others: groq:, mistral:, cohere:, bedrock:, etc.

Structured Outputs#

Use Pydantic models for validated, typed responses:

python

from pydantic import BaseModel
from pydantic_ai import Agent

class CityInfo(BaseModel):
    city: str
    country: str
    population: int

agent = Agent('openai:gpt-4o', output_type=CityInfo)
result = agent.run_sync('Tell me about Paris')
print(result.output.city)  # "Paris"
print(result.output.population)  # int, validated

Agent Configuration#

python

from pydantic_ai import Agent
from pydantic_ai.settings import ModelSettings

agent = Agent(
    'openai:gpt-4o',
    output_type=MyOutput,           # Structured output type
    deps_type=MyDeps,               # Dependency injection type
    instructions='You are helpful.',  # Static instructions
    retries=2,                      # Retry attempts for validation
    name='my-agent',                # For logging/tracing
    model_settings=ModelSettings(   # Provider settings
        temperature=0.7,
        max_tokens=1000
    ),
    end_strategy='early',           # How to handle tool calls with results
)

Running Agents#

Three execution methods:

python

# Async (preferred)
result = await agent.run('prompt', deps=my_deps)

# Sync (convenience)
result = agent.run_sync('prompt', deps=my_deps)

# Streaming
async with agent.run_stream('prompt') as response:
    async for chunk in response.stream_output():
        print(chunk, end='')

Instructions vs System Prompts#

python

# Instructions: Concatenated, for agent behavior
agent = Agent(
    'openai:gpt-4o',
    instructions='You are a helpful assistant. Be concise.'
)

# Dynamic instructions via decorator
@agent.instructions
def add_context(ctx: RunContext[MyDeps]) -> str:
    return f"User ID: {ctx.deps.user_id}"

# System prompts: Static, for model context
agent = Agent(
    'openai:gpt-4o',
    system_prompt=['You are an expert.', 'Always cite sources.']
)

Common Patterns#

Parameterized Agent (Type-Safe)#

python

from dataclasses import dataclass
from pydantic_ai import Agent, RunContext

@dataclass
class Deps:
    api_key: str
    user_id: int

agent: Agent[Deps, str] = Agent(
    'openai:gpt-4o',
    deps_type=Deps,
)

# deps is now required and type-checked
result = agent.run_sync('Hello', deps=Deps(api_key='...', user_id=123))

No Dependencies (Satisfy Type Checker)#

python

# Option 1: Explicit type annotation
agent: Agent[None, str] = Agent('openai:gpt-4o')

# Option 2: Pass deps=None
result = agent.run_sync('Hello', deps=None)

Verification gates#

Run these in order before depending on an agent in production code:

1. Smoke run — Execute agent.run_sync('Reply with OK.') (or await agent.run(...) in async code). Pass: the call completes without raising and result.output is present.
2. Structured output — If you set output_type, prompt for a response that should satisfy the schema. Pass: result.output is an instance of your Pydantic model; repeated validation failures mean tightening instructions or retries, not adding features yet.
3. Dependencies — If you set deps_type, call run / run_sync with deps= of that type. Pass: the invocation type-checks and completes (or fails only for model/API reasons, not a missing or wrong deps value).

Decision Framework#

ScenarioConfigurationSimple text responsesAgent(model)Structured data extractionAgent(model, output_type=MyModel)Need external servicesAdd deps_type=MyDepsValidation retries neededIncrease retries=3Debugging/monitoringSet instrument=True