building-pydantic-ai-agents

Category: Tools & Productivity | Uploader: pydantic | Downloads: 0 | Version: v1.0（Latest）
Build AI agents with Pydantic AI — tools, capabilities, 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, stream output, define agents from YAML, or test agent behavior.
Changelog: Source: GitHub https://github.com/pydantic/skills
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Current level: tree/main/plugins/ai/skills/building-pydantic-ai-agents/
📁 references/
- 📄 ARCHITECTURE.md 8.8 KB
- 📄 COMMON-TASKS.md 9.2 KB
📄 SKILL.md 13.9 KB
SKILL.md

---
name: building-pydantic-ai-agents
description: >
  Build AI agents with Pydantic AI — tools, capabilities, 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, stream output, define agents from YAML, or test agent behavior.
license: MIT
compatibility: Requires Python 3.10+
metadata:
  version: "1.0.0"
  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 observability with Logfire

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

```python
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

```python
import random

from pydantic_ai import Agent, RunContext

agent = Agent(
    'google-gla: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

```python
from pydantic import BaseModel

from pydantic_ai import Agent


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


agent = Agent('google-gla: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

```python
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

```python
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.

```python
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.

```python
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.

```python
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

| I want to... | Documentation |
|---|---|
| Create or configure agents | [Agents](https://ai.pydantic.dev/agents/) |
| Bundle reusable behavior (tools, hooks, instructions) | [Capabilities](https://ai.pydantic.dev/capabilities/) |
| Intercept model requests, tool calls, or runs | [Hooks](https://ai.pydantic.dev/hooks/) |
| Define agents in YAML/JSON without Python code | [Agent Specs](https://ai.pydantic.dev/agent-spec/) |
| Use template strings in agent instructions | [Template Strings](https://ai.pydantic.dev/agent-spec/#template-strings) |
| Let my agent call external APIs or functions | [Tools](https://ai.pydantic.dev/tools/) |
| Organize or restrict which tools an agent can use | [Toolsets](https://ai.pydantic.dev/toolsets/) |
| Give my agent web search with automatic provider fallback | [WebSearch Capability](https://ai.pydantic.dev/capabilities/#provider-adaptive-tools) |
| Give my agent URL fetching with automatic provider fallback | [WebFetch Capability](https://ai.pydantic.dev/capabilities/#provider-adaptive-tools) |
| Give my agent web search or code execution (builtin tools) | [Built-in Tools](https://ai.pydantic.dev/builtin-tools/) |
| Search with DuckDuckGo/Tavily/Exa | [Common Tools](https://ai.pydantic.dev/common-tools/) |
| Ensure my agent returns data in a specific format | [Structured Output](https://ai.pydantic.dev/output/#structured-output) |
| Pass database connections, API clients, or config to tools | [Dependencies](https://ai.pydantic.dev/dependencies/) |
| Access usage stats, message history, or retry count in tools | [RunContext](https://ai.pydantic.dev/tools/) |
| Choose or configure models | [Models](https://ai.pydantic.dev/models/) |
| Automatically switch to backup model when primary fails | [Fallback Model](https://ai.pydantic.dev/models/#fallback-model) |
| Show real-time progress as my agent works | [Streaming Events and Final Output](https://ai.pydantic.dev/agents/#streaming-events-and-final-output) |
| Work with messages and multimedia | [Message History](https://ai.pydantic.dev/message-history/) |
| Reduce token costs by trimming or filtering conversation history | [Processing Message History](https://ai.pydantic.dev/message-history/#processing-message-history) |
| Keep long conversations manageable without losing context | [Summarize Old Messages](https://ai.pydantic.dev/message-history/#summarize-old-messages) |
| Use MCP servers | [MCP](https://ai.pydantic.dev/mcp/) |
| Build multi-step graphs | [Graph](https://ai.pydantic.dev/graph/) |
| Debug a failed agent run or see what went wrong | [Model Errors](https://ai.pydantic.dev/agents/#model-errors) |
| Make my agent resilient to temporary failures | [Retries](https://ai.pydantic.dev/retries/) |
| Understand why my agent made specific decisions | [Using Logfire](https://ai.pydantic.dev/logfire/#using-logfire) |
| Write deterministic tests for my agent | [Unit testing with TestModel](https://ai.pydantic.dev/testing/#unit-testing-with-testmodel) |
| Enable thinking/reasoning across any provider | [Thinking](https://ai.pydantic.dev/thinking/) · [Thinking Capability](https://ai.pydantic.dev/capabilities/#thinking) |
| Systematically verify my agent works correctly | [Evals](https://ai.pydantic.dev/evals/) |
| Use embeddings for RAG | [Embeddings](https://ai.pydantic.dev/embeddings/) |
| Use durable execution | [Durable Execution](https://ai.pydantic.dev/durable_execution/overview/) |
| Have one agent delegate tasks to another | [Agent Delegation](https://ai.pydantic.dev/multi-agent-applications/#agent-delegation) |
| Route requests to different agents based on intent | [Programmatic Agent Hand-off](https://ai.pydantic.dev/multi-agent-applications/#programmatic-agent-hand-off) |
| Require tool approval (human-in-the-loop) | [Deferred Tools](https://ai.pydantic.dev/deferred-tools/) |
| Use images, audio, video, or documents | [Input](https://ai.pydantic.dev/input/) |
| Use advanced tool features | [Advanced Tools](https://ai.pydantic.dev/tools-advanced/) |
| Validate or require approval before tool execution | [Advanced Tools](https://ai.pydantic.dev/tools-advanced/) |
| Call the model without using an agent | [Direct API](https://ai.pydantic.dev/direct/) |
| Expose agents as HTTP servers (A2A) | [A2A](https://ai.pydantic.dev/a2a/) |
| Handle network errors and rate limiting automatically | [Retries](https://ai.pydantic.dev/retries/) |
| Use LangChain or ACI.dev tools | [Third-Party Tools](https://ai.pydantic.dev/third-party-tools/) |
| Publish reusable agent extensions as packages | [Extensibility](https://ai.pydantic.dev/extensibility/) |
| Build custom toolsets, models, or agents | [Extensibility](https://ai.pydantic.dev/extensibility/) |
| Debug common issues | [Troubleshooting](https://ai.pydantic.dev/troubleshooting/) |
| Migrate from deprecated APIs | [Changelog](https://ai.pydantic.dev/changelog/) |
| See advanced real-world examples | [Examples](https://ai.pydantic.dev/examples/) |
| Look up an import path | [API Reference](https://ai.pydantic.dev/api/) |

## Architecture and Decisions

Load [Architecture and Decision Guide](./references/ARCHITECTURE.md) for detailed decision trees, comparison tables, and architecture overview:

| 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-gla: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
- **Observability**: Pydantic AI has first-class integration with [Logfire](https://logfire.pydantic.dev/) for tracing agent runs, tool calls, and model requests. Add it with `logfire.instrument_pydantic_ai()`. For deeper HTTP-level visibility, `logfire.instrument_httpx(capture_all=True)` captures the exact payloads sent to model providers.
- **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 plural**: The Agent parameter is `history_processors=[...]`, not `history_processor=`.

## Common Tasks

Load [Common Tasks Reference](./references/COMMON-TASKS.md) for detailed implementation guidance with code examples:

| Task | Section |
|---|---|
| Add capabilities (Thinking, WebSearch, etc.) | Add Capabilities to an Agent |
| Intercept model requests and tool calls | Intercept Agent Lifecycle with Hooks |
| Define agents from YAML/JSON config files | Define Agents Declaratively with Specs |
| Enable thinking/reasoning across providers | Enable Thinking Across Providers |
| Trim or filter conversation history | Manage Context Size |
| Stream events and show real-time progress | Show Real-Time Progress |
| Auto-switch providers on failure | Handle Provider Failures |
| Write deterministic tests | Test Agent Behavior |
| Delegate tasks between agents | Coordinate Multiple Agents |
| Instrument with Logfire for debugging | Debug and Validate Agent Behavior |
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