building-pydantic-ai-agents

Building AI Agents with Pydantic AI

Pydantic AI is a Python agent framework for building production-grade Generative AI applications. This skill provides patterns, architecture guidance, and tested code examples for building applications with Pydantic AI.

When to Use This Skill

Invoke this skill when:

• User asks to build an AI agent, create an LLM-powered app, or mentions Pydantic AI
• User wants to add tools, capabilities (thinking, web search), or structured output to an agent
• User asks to define agents from YAML/JSON specs or use template strings
• User wants to stream agent events, delegate between agents, or test agent behavior
• Code imports pydantic_ai or references Pydantic AI classes (Agent, RunContext, Tool)
• User asks about hooks, lifecycle interception, or agent observability with Logfire
• The agent design includes optional instructions, specialist workflows, long-tail tools, or any context the model does not need on most turns

Do not use this skill for:

• The Pydantic validation library alone (pydantic/BaseModel without agents)
• Other AI frameworks (LangChain, LlamaIndex, CrewAI, AutoGen)
• General Python development unrelated to AI agents

Quick-Start Patterns

Create a Basic Agent

from pydantic_ai import Agent

agent = Agent(
    'anthropic:claude-sonnet-4-6',
    name='hello_world_agent',
    instructions='Be concise, reply with one sentence.',
)

result = agent.run_sync('Where does "hello world" come from?')
print(result.output)
"""
The first known use of "hello, world" was in a 1974 textbook about the C programming language.
"""

Add Tools to an Agent

import random

from pydantic_ai import Agent, RunContext

agent = Agent(
    'google:gemini-3-flash-preview',
    name='dice_game_agent',
    deps_type=str,
    instructions=(
        "You're a dice game, you should roll the die and see if the number "
        "you get back matches the user's guess. If so, tell them they're a winner. "
        "Use the player's name in the response."
    ),
)


@agent.tool_plain
def roll_dice() -> str:
    """Roll a six-sided die and return the result."""
    return str(random.randint(1, 6))


@agent.tool
def get_player_name(ctx: RunContext[str]) -> str:
    """Get the player's name."""
    return ctx.deps


dice_result = agent.run_sync('My guess is 4', deps='Anne')
print(dice_result.output)
#> Congratulations Anne, you guessed correctly! You're a winner!