Pydantic AI Agents

Pydantic AI Reference Skill

Targets Pydantic AI v2 (v2.0.0 released 2026-06-23). See the v1 → v2 migration notes at the end for the breaking changes this skill's examples already incorporate.

Pydantic AI Developer Guide

0. Environment Setup

Store API keys in a .env file and add it to .gitignore:

OPENAI_API_KEY=your_key
OPENROUTER_API_KEY=your_key
LOGFIRE_API_KEY=your_key

Load with python-dotenv: load_dotenv(). Never hardcode keys in source code.

1. Core Architecture

Pydantic AI agents have five key components:

Dependencies (deps):

• Reference: references/01_dependencies.py
• Use dataclasses to hold API keys, database connections, and user context
• Never use global variables for state

Instructions (instructions):

• Reference: references/02_prompts.py
• v2 recommended: use instructions= / @agent.instructions (instructions are NOT carried in message history, so they always reflect the current agent)
• Make them dynamic using the @agent.instructions decorator
• Inject data from ctx.deps into the prompt string
• Legacy system_prompt= / @agent.system_prompt still work; their text IS carried in message history - only use when replaying history across agents

Tools (@agent.tool / @agent.tool_plain):

• Reference: references/03_tools.py
• v2: @agent.tool REQUIRES ctx: RunContext as first parameter; use @agent.tool_plain for context-free tools (it raises otherwise)
• Use ctx.deps to access injected dependencies

Validators (output_type):

• Reference: references/04_validators.py
• Use Pydantic models to enforce structured output
• Use @field_validator for logic checks

Capabilities (capabilities) - NEW in v2:

• Reference: references/13_capabilities_native_tools.py
• THE core v2 primitive: one composable unit bundling tools, hooks, instructions, and model settings
• Provider-adaptive capabilities: Thinking, WebSearch, WebFetch, ImageGeneration, MCP
• Replaces v1's builtin_tools=, prepare_tools=, history_processors=, instrument=, event_stream_handler= Agent arguments

2. Promoting Instructions (System Prompt Engineering)

Follow these rules when writing system prompts:

1. Role Definition: Start with "You are a specialized agent for..."
2. Context Awareness: Explicitly mention the data available in the dependencies.
   • Bad: "I help users."
   • Good: "I help user {ctx.deps.user_name} (ID: {ctx.deps.user_id}) manage their account."
3. Tool Coercion: If tools are defined, instruct the model when to use them.
   • Example: "Use the lookup_order tool immediately if the user provides an order ID."
4. Failure Modes: Define what to do if a tool fails or data is missing.
   • Example: "If the database returns no results, politely ask the user for clarification."

3. OpenRouter Integration

Reference: references/06_openrouter.py

OpenRouter is an API gateway that provides access to multiple LLM models through a unified API. Since Pydantic AI v2 it has first-class support - do NOT use the old OpenAI-compatible base_url workaround anymore.

Key Points:

• Set OPENROUTER_API_KEY in your .env file
• Simplest: model string prefix 'openrouter:<vendor>/<model>'
• Explicit: OpenRouterModel + OpenRouterProvider
• OpenRouter model ids are always vendor-prefixed (e.g. openai/gpt-5.2, anthropic/claude-sonnet-4.6) - see https://openrouter.ai/models

Example Setup:

from pydantic_ai import Agent
from pydantic_ai.models.openrouter import OpenRouterModel
from pydantic_ai.providers.openrouter import OpenRouterProvider

# Simplest (reads OPENROUTER_API_KEY from environment):
agent = Agent('openrouter:anthropic/claude-sonnet-4.6')

# Explicit:
model = OpenRouterModel(
    'openai/gpt-5.2',
    provider=OpenRouterProvider(api_key=os.getenv('OPENROUTER_API_KEY', '')),
)

OpenRouterModelSettings adds provider routing, reasoning effort (openrouter_reasoning={'effort': 'high'}), and usage accounting.

4. Debugging with Logfire

Reference: references/07_logfire.py

Logfire is a platform tightly integrated with Pydantic AI for debugging and observability.

Key Features:

• Spans: Track execution time and context of operations
• Logging Levels: notice, info, debug, warn, error, fatal
• Exception Tracking: Capture stack traces and error context
• Tracing: Visualize agent execution flow

Setup:

1. Get API key from https://logfire.pydantic.dev/
2. Set LOGFIRE_API_KEY in your .env file
3. Configure: logfire.configure(token=LOGFIRE_API_KEY)
4. Instrument: logfire.instrument_pydantic_ai() - traces every agent run, model request, and tool call automatically

v2 note: the default instrumentation format is version 5; agent run spans report token usage under gen_ai.aggregated_usage.* (model request spans keep gen_ai.usage.*). Update dashboards that read usage from run spans.

Usage Pattern:

with logfire.span('Calling Agent') as span:
    result = agent.run_sync("user query")
    span.set_attribute('result', result.output)
    logfire.info('{result=}', result=result.output)

5. Advanced Patterns

Streaming Responses:

• Reference: references/08_streaming.py
• Use agent.run_stream() for real-time output
• Stream text with async for chunk in response.stream_text()
• Stream partial structured output with response.stream_output() (v2 rename of v1's .stream(); .stream_structured() is now .stream_response())
• Get final result with await response.get_output()
• v2: agent.run_stream_events() is a context manager only (async with agent.run_stream_events(...) as events:)

Result Validators & Retry:

• Reference: references/09_result_validators.py
• Use @agent.output_validator for custom validation
• Raise ModelRetry("feedback") to trigger retry with guidance
• Set retries=3 on Agent for auto-retry on validation failure

Model Settings & Usage:

• Reference: references/10_model_settings.py
• Pass model_settings={'temperature': 0.7, 'max_tokens': 500} to agent.run()
• Use low temperature (0.0-0.3) for factual tasks
• Use high temperature (0.7-1.0) for creative tasks
• v2: result.usage is a property (NOT result.usage()), with input_tokens/output_tokens (renamed from request/response tokens)

Multi-Agent Systems:

• Reference: references/11_multi_agent.py
• Orchestrate multiple specialized agents for complex tasks
• Use asyncio.gather() for parallel agent execution
• Implement routing for intent-based agent selection

Capabilities & Native Tools (NEW in v2):

• Reference: references/13_capabilities_native_tools.py
• Pass capabilities=[...] on the Agent; prefer provider-adaptive capabilities (Thinking(effort='high'), WebSearch(), WebFetch()) so code survives provider changes
• v2 renamed "builtin tools" to native tools: wrap them as NativeTool(WebSearchTool(...)) when provider-specific config matters
• WebSearch()/WebFetch() are native-only by default and RAISE on unsupported models; restore local fallback with WebSearch(local='duckduckgo') / WebFetch(local=True)
• Bundle instructions + tools with Capability(id=..., description=...); add defer_loading=True for on-demand loading (progressive disclosure)
• Subclass AbstractCapability for reusable bundles with model settings/hooks

Lifecycle Hooks (NEW in v2):

• Reference: references/14_hooks.py
• Create hooks = Hooks(), register with @hooks.on.before_model_request, @hooks.on.before_tool_execute(tools=['name']), etc., then pass capabilities=[hooks]
• Use for logging, metrics, auditing, and light interception
• Always return the (possibly modified) context/args from a hook

Toolsets & MCP (v2 API):

• Reference: references/15_toolsets_mcp.py
• Group reusable tools in a FunctionToolset; share across agents via Agent(toolsets=[...])
• MCP servers: pydantic_ai.mcp.MCPToolset (replaces v1's MCPServerStdio/ MCPServerSSE/MCPServerStreamableHTTP); manage connections with async with agent: (replaces agent.run_mcp_servers())
• Load config files with load_mcp_toolsets(...); provider-native MCP via the MCP(url=..., native=True) capability

6. Conversation History (Persistent Memory)

Reference: references/12_conversation_history.py

By default, each agent.run() call is stateless - the agent has no memory of previous interactions. To maintain conversation context across multiple turns, you must pass message_history.

Key Concepts:

1. Get messages from result: After each run(), call result.all_messages() to get the full conversation
2. Pass history to next call: Use message_history= parameter on subsequent run() calls
3. Messages are immutable: Each call returns a NEW list; the original is not modified

Basic Pattern:

from pydantic_ai import Agent, ModelMessage

agent = Agent(model=model, instructions="You are helpful.")

# First turn - no history
result1 = agent.run_sync("My name is Alice")
messages: list[ModelMessage] = result1.all_messages()

# Second turn - pass history so agent remembers
result2 = agent.run_sync(
    "What is my name?",
    message_history=messages  # Agent now knows "Alice"
)
messages = result2.all_messages()  # Updated history

# Third turn - continue the conversation
result3 = agent.run_sync(
    "Tell me a joke about my name",
    message_history=messages
)

Function Signature Pattern:

When building conversation loops, return both the output and messages:

from pydantic_ai import Agent, ModelMessage

def run_agent_with_history(
    user_input: str,
    message_history: list[ModelMessage] | None = None,
) -> tuple[str, list[ModelMessage]]:
    """Run agent and return output + updated history."""
    result = agent.run_sync(
        user_input,
        message_history=message_history or [],
    )
    return result.output, result.all_messages()

# Usage in a conversation loop
history = []
while True:
    user_input = input("You: ")
    response, history = run_agent_with_history(user_input, history)
    print(f"Agent: {response}")

Converting Custom Message Types:

If you store conversation history in your own format (e.g., database), convert to Pydantic AI format:

from datetime import timezone
from pydantic_ai import ModelMessage
from pydantic_ai.messages import (
    ModelRequest, ModelResponse,
    UserPromptPart, TextPart
)

def convert_to_model_messages(my_messages: list[MyMessage]) -> list[ModelMessage]:
    """Convert custom message format to Pydantic AI format."""
    result: list[ModelMessage] = []

    for msg in my_messages:
        # Ensure timezone-aware timestamp
        ts = msg.timestamp.replace(tzinfo=timezone.utc) if msg.timestamp.tzinfo is None else msg.timestamp

        if msg.role == "user":
            result.append(ModelRequest(
                parts=[UserPromptPart(content=msg.content, timestamp=ts)],
                kind="request",
            ))
        elif msg.role == "assistant":
            result.append(ModelResponse(
                parts=[TextPart(content=msg.content)],
                kind="response",
                timestamp=ts,
            ))

    return result

Important Notes:

• ModelMessage is a union type: It's ModelRequest | ModelResponse, not a class you instantiate directly
• User messages → ModelRequest with UserPromptPart
• Assistant messages → ModelResponse with TextPart
• Timestamps must be timezone-aware: Use timezone.utc
• Instructions are NOT in message_history: They're set on the Agent and injected automatically (legacy system_prompt text IS carried in history)

7. Testing Best Practices

Async/Sync Test Separation

Warning: When testing Pydantic AI agents, do NOT mix sync and async tests in the same file when using module-level agents.

Problem: Module-level agents create httpx clients at import time. When sync tests call run_sync(), they create/destroy temporary event loops which can corrupt the httpx connection pool. Subsequent async tests then fail with Connection error.

Solution: Separate async and sync tests into different files:

# tests/test_agent_sync.py
from src.agent import run_agent_sync

def test_sync_behavior():
    result = run_agent_sync("input")
    assert result.field == expected

# tests/test_agent_async.py (SEPARATE FILE)
# Does NOT import run_agent_sync!
import pytest
from pydantic_ai import Agent

@pytest.mark.asyncio
async def test_async_behavior():
    # Create fresh agent inside test
    agent = Agent(model=model, output_type=Response)
    result = await agent.run("input")
    assert result.output.field == expected

Alternative: Convert all tests to async to use the same event loop consistently.

8. Usage

Build a new agent by:

1. Reading the requirement
2. Selecting the relevant components from the references/ directory
3. Combining them into a single file following the pattern in references/05_main.py
4. Using OpenRouter (references/06_openrouter.py) for multi-model access
5. Adding Logfire (references/07_logfire.py) for debugging and monitoring
6. Adding conversation history (references/12_conversation_history.py) for multi-turn conversations
7. Adding capabilities/native tools (references/13_capabilities_native_tools.py) for web search, thinking, or reusable behavior bundles
8. Adding hooks (references/14_hooks.py) for logging, auditing, and interception
9. Adding toolsets or MCP servers (references/15_toolsets_mcp.py) for reusable/external tools
10. Adding advanced patterns (streaming, validators, multi-agent) as needed

9. Complete Example

See references/05_main.py for a complete working agent that demonstrates all patterns.

10. v1 → v2 Migration Notes

Key breaking changes in Pydantic AI v2 (all examples in this skill are v2):

Full upgrade guide: https://ai.pydantic.dev/changelog/