building-pydantic-ai-agents

name	building-pydantic-ai-agents
description	Build AI agents with Pydantic AI — tools, capabilities (including on-demand loading), structured output, streaming, testing, and multi-agent patterns. Use when the user mentions Pydantic AI, imports pydantic_ai, or asks to build an AI agent, add tools/capabilities, defer capability loading, stream output, define agents from YAML, or test agent behavior.
license	MIT
compatibility	Requires Python 3.10+
metadata	{"version":"1.1.0","author":"pydantic"}

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 runtime debugging
• 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',
    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',
    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!

Structured Output with Pydantic Models

from pydantic import BaseModel

from pydantic_ai import Agent


class CityLocation(BaseModel):
    city: str
    country: str


agent = Agent('google:gemini-3-flash-preview', output_type=CityLocation)
result = agent.run_sync('Where were the olympics held in 2012?')
print(result.output)
#> city='London' country='United Kingdom'
print(result.usage)
#> RunUsage(input_tokens=57, output_tokens=8, requests=1)

Dependency Injection

from datetime import date

from pydantic_ai import Agent, RunContext

agent = Agent(
    'openai:gpt-5.2',
    deps_type=str,
    instructions="Use the customer's name while replying to them.",
)


@agent.instructions
def add_the_users_name(ctx: RunContext[str]) -> str:
    return f"The user's name is {ctx.deps}."


@agent.instructions
def add_the_date() -> str:
    return f'The date is {date.today()}.'


result = agent.run_sync('What is the date?', deps='Frank')
print(result.output)
#> Hello Frank, the date today is 2032-01-02.

Testing with TestModel

from pydantic_ai import Agent
from pydantic_ai.models.test import TestModel

my_agent = Agent('openai:gpt-5.2', instructions='...')


async def test_my_agent():
    """Unit test for my_agent, to be run by pytest."""
    m = TestModel()
    with my_agent.override(model=m):
        result = await my_agent.run('Testing my agent...')
        assert result.output == 'success (no tool calls)'
    assert m.last_model_request_parameters.function_tools == []

Use Capabilities

Capabilities are reusable, composable units of agent behavior — bundling tools, hooks, instructions, and model settings.

from pydantic_ai import Agent
from pydantic_ai.capabilities import Thinking, WebSearch

agent = Agent(
    'anthropic:claude-opus-4-6',
    instructions='You are a research assistant. Be thorough and cite sources.',
    capabilities=[
        Thinking(effort='high'),
        WebSearch(),
    ],
)

Add Lifecycle Hooks

Use Hooks to intercept model requests, tool calls, and runs with decorators — no subclassing needed.

from pydantic_ai import Agent, RunContext
from pydantic_ai.capabilities.hooks import Hooks
from pydantic_ai.models import ModelRequestContext

hooks = Hooks()


@hooks.on.before_model_request
async def log_request(ctx: RunContext[None], request_context: ModelRequestContext) -> ModelRequestContext:
    print(f'Sending {len(request_context.messages)} messages')
    return request_context


agent = Agent('openai:gpt-5.2', capabilities=[hooks])

Define Agent from YAML Spec

Use Agent.from_file to load agents from YAML or JSON — no Python agent construction code needed.

from pydantic_ai import Agent

# agent.yaml:
# model: anthropic:claude-opus-4-6
# instructions: You are a helpful research assistant.
# capabilities:
#   - WebSearch
#   - Thinking:
#       effort: high

agent = Agent.from_file('agent.yaml')

Task Routing Table

Load only the most relevant reference first. Read additional references only if the task spans multiple areas.

I want to...	Reference
Create/configure agents, choose output types, use deps, define specs, or pick run methods	Agents Core
Bundle reusable behavior or intercept lifecycle events	Capabilities and Hooks
Decide what should load eagerly vs on demand, apply progressive disclosure, defer capability loading, or explain load_capability	Capabilities on Demand
Add function tools, toolsets, MCP servers, or explicit search tools	Tools Core
Use provider-native web search, web fetch, or code execution	Native Tools
Use advanced tool features such as approval, retries, ToolReturn, validators, timeouts, or tool search	Tools Advanced
Work with multimodal input, message history, or context trimming	Input and History
Test or debug agent behavior	Testing and Debugging
Coordinate multiple agents or build graph workflows	Orchestration and Integrations
Call the model directly, expose A2A, use durable execution, embeddings, evals, or third-party integrations	Orchestration and Integrations
Compare abstractions, output modes, decorators, or model-string patterns	Architecture and Decision Guide
Follow an older link into COMMON-TASKS.md	Task Reference Map

Architecture and Decisions

Load Architecture and Decision Guide only when the user is choosing between abstractions or wants comparison tables and decision trees:

Topic	What it covers
Decision Trees	Tool registration, output modes, multi-agent patterns, capabilities, testing approaches, extensibility
Comparison Tables	Output modes, model provider prefixes, tool decorators, built-in capabilities, agent methods
Architecture Overview	Execution flow, generic types, construction patterns, lifecycle hooks, model string format

Quick reference — model string format: "provider:model-name" (e.g., "openai:gpt-5.2", "anthropic:claude-sonnet-4-6", "google:gemini-3-pro-preview")

Quick reference — key agent methods: run(), run_sync(), run_stream(), run_stream_sync(), run_stream_events(), iter()

Key Practices

• Python 3.10+ compatibility required
• Progressive disclosure by default: For every capability, explicitly consider whether defer_loading=True would benefit the agent before choosing eager loading. Do not eagerly load specialist instructions, rarely used tool schemas, or domain context unless the model needs them on most turns. Prefer capabilities on demand for named instruction+tool bundles, and tool search for large flat tool catalogs.
• Debugging: Use deterministic model overrides and capture_run_messages() to inspect failed runs without coupling the project to a specific telemetry backend.
• Testing: Use TestModel for deterministic tests, FunctionModel for custom logic

Common Gotchas

These are mistakes agents commonly make with Pydantic AI. Getting these wrong produces silent failures or confusing errors.

• @agent.tool requires RunContext as first param; @agent.tool_plain must not have it. Mixing these up causes runtime errors. Use tool_plain when you don't need deps, usage, or messages.
• Model strings need the provider prefix: 'openai:gpt-5.2' not 'gpt-5.2'. Without the prefix, Pydantic AI can't resolve the provider.
• TestModel requires agent.override(): Don't set agent.model directly. Always use the context manager: with agent.override(model=TestModel()):.
• str in output_type allows plain text to end the run: If your union includes str (or no output_type is set), the model can return plain text instead of structured output. Omit str from the union to force tool-based output.
• Hook decorator names on .on don't repeat on_: Use hooks.on.run_error and hooks.on.model_request_error — not hooks.on.on_run_error.
• history_processors is deprecated; use capabilities=[ProcessHistory(p), ...], or hook before_model_request directly via capabilities=[Hooks(before_model_request=fn)]. ProcessHistory is a thin wrapper around that hook — the hook itself is the underlying primitive. The kwarg still works in 1.x but emits a PydanticAIDeprecationWarning and will be removed in v2.

Task-Family References

Load exactly one of these unless the task clearly spans multiple families:

Task family	Reference
Core agent setup, output, deps, specs, models, run methods	Agents Core
Capabilities, hooks, and reusable behavior	Capabilities and Hooks
Progressive disclosure, deferred capabilities, capabilities on demand, and load_capability semantics	Capabilities on Demand
Function tools, toolsets, MCP, explicit search tools	Tools Core
Provider-native tools	Native Tools
Approval, retries, validators, timeouts, rich tool returns, tool search, and tool-level deferred loading	Tools Advanced
Multimodal input, message history, history processors	Input and History
Testing, request inspection, and runtime debugging	Testing and Debugging
Multi-agent patterns, graphs, direct API, A2A, durable execution, embeddings, evals, third-party integrations	Orchestration and Integrations

Use Task Reference Map only for compatibility with older links or when you need a pointer from an old section name to the new file.