PydanticAI Tool System Tool Registration

Two decorators based on whether you need context:

from pydantic_ai import Agent, RunContext

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

@agent.tool - First param MUST be RunContext

@agent.tool async def get_user_data(ctx: RunContext[MyDeps], user_id: int) -> str: """Get user data from database.

Args:
    ctx: The run context with dependencies.
    user_id: The user's ID.
"""
return await ctx.deps.db.get_user(user_id)

@agent.tool_plain - NO context parameter allowed

@agent.tool_plain def calculate_total(prices: list[float]) -> float: """Calculate total price.

Args:
    prices: List of prices to sum.
"""
return sum(prices)

Critical Rules @agent.tool: First parameter MUST be RunContext[DepsType] @agent.tool_plain: MUST NOT have RunContext parameter Docstrings: Required for LLM to understand tool purpose Google-style docstrings: Used for parameter descriptions Docstring Formats

Google style (default):

@agent.tool_plain async def search(query: str, limit: int = 10) -> list[str]: """Search for items.

Args:
    query: The search query.
    limit: Maximum results to return.
"""

Sphinx style:

@agent.tool_plain(docstring_format='sphinx') async def search(query: str) -> list[str]: """Search for items.

:param query: The search query.
"""

Tool Return Types

Tools can return various types:

String (direct)

@agent.tool_plain def get_info() -> str: return "Some information"

Pydantic model (serialized to JSON)

@agent.tool_plain def get_user() -> User: return User(name="John", age=30)

Dict (serialized to JSON)

@agent.tool_plain def get_data() -> dict[str, Any]: return {"key": "value"}

ToolReturn for custom content types

from pydantic_ai import ToolReturn, ImageUrl

@agent.tool_plain def get_image() -> ToolReturn: return ToolReturn(content=[ImageUrl(url="https://...")])

Accessing Context

RunContext provides:

@agent.tool async def my_tool(ctx: RunContext[MyDeps]) -> str: # Dependencies db = ctx.deps.db api = ctx.deps.api_client

# Model info
model_name = ctx.model.model_name

# Usage tracking
tokens_used = ctx.usage.total_tokens

# Retry info
attempt = ctx.retry  # Current retry attempt (0-based)
max_retries = ctx.max_retries

# Message history
messages = ctx.messages

return "result"

Tool Prepare Functions

Dynamically modify tools per-request:

from pydantic_ai.tools import ToolDefinition

async def prepare_tools( ctx: RunContext[MyDeps], tool_defs: list[ToolDefinition] ) -> list[ToolDefinition]: """Filter or modify tools based on context.""" if ctx.deps.user_role != 'admin': # Hide admin tools from non-admins return [t for t in tool_defs if not t.name.startswith('admin_')] return tool_defs

agent = Agent('openai:gpt-4o', prepare_tools=prepare_tools)

Toolsets

Group and compose tools:

from pydantic_ai import FunctionToolset, CombinedToolset

Create a toolset

db_tools = FunctionToolset()

@db_tools.tool def query_users(name: str) -> list[dict]: """Query users by name.""" ...

@db_tools.tool def update_user(id: int, data: dict) -> bool: """Update user data.""" ...

Use in agent

agent = Agent('openai:gpt-4o', toolsets=[db_tools])

Combine toolsets

all_tools = CombinedToolset([db_tools, api_tools])

Common Mistakes Wrong: Context in tool_plain @agent.tool_plain async def bad_tool(ctx: RunContext[MyDeps]) -> str: # ERROR! ...

Wrong: Missing context in tool @agent.tool def bad_tool(user_id: int) -> str: # ERROR! ...

Wrong: Context not first parameter @agent.tool def bad_tool(user_id: int, ctx: RunContext[MyDeps]) -> str: # ERROR! ...

Async vs Sync

Both work, but async is preferred for I/O:

Async (preferred for I/O operations)

@agent.tool async def fetch_data(ctx: RunContext[Deps]) -> str: return await ctx.deps.client.get('/data')

Sync (fine for CPU-bound operations)

@agent.tool_plain def compute(x: int, y: int) -> int: return x * y