---
title: Define and customize tools
slug: guides/agents/custom-tools
---

You can create custom tools in Letta using the Python SDK, as well as via the [ADE tool builder](/guides/ade/tools).

For your agent to call a tool, Letta constructs an OpenAI tool schema (contained in `json_schema` field) from the function you define. Letta can either parse this automatically from a properly formatting docstring, or you can pass in the schema explicitly by providing a Pydantic object that defines the argument schema.

## Creating a custom tool

### Specifying tools via Pydantic models
To create a custom tool, you can extend the `BaseTool` class and specify the following:
* `name` - The name of the tool
* `args_schema` - A Pydantic model that defines the arguments for the tool
* `description` - A description of the tool
* `tags` - (Optional) A list of tags for the tool to query
You must also define a `run(..)` method for the tool code that takes in the fields from the `args_schema`.

Below is an example of how to create a tool by extending `BaseTool`:
```python title="python" maxLines=50
from letta_client import Letta
from letta_client.client import BaseTool
from pydantic import BaseModel
from typing import List, Type

class InventoryItem(BaseModel):
    sku: str  # Unique product identifier
    name: str  # Product name
    price: float  # Current price
    category: str  # Product category (e.g., "Electronics", "Clothing")

class InventoryEntry(BaseModel):
    timestamp: int  # Unix timestamp of the transaction
    item: InventoryItem  # The product being updated
    transaction_id: str  # Unique identifier for this inventory update

class InventoryEntryData(BaseModel):
    data: InventoryEntry
    quantity_change: int  # Change in quantity (positive for additions, negative for removals)


class ManageInventoryTool(BaseTool):
    name: str = "manage_inventory"
    args_schema: Type[BaseModel] = InventoryEntryData
    description: str = "Update inventory catalogue with a new data entry"
    tags: List[str] = ["inventory", "shop"]

    def run(self, data: InventoryEntry, quantity_change: int) -> bool:
        print(f"Updated inventory for {data.item.name} with a quantity change of {quantity_change}")
        return True

# create a client to connect to your local Letta server
client = Letta(
  base_url="http://localhost:8283"
)
# create the tool
tool_from_class = client.tools.add(
    tool=ManageInventoryTool(),
)
```

### Specifying tools via function docstrings
You can create a tool by passing in a function with a [Google Style Python docstring](https://google.github.io/styleguide/pyguide.html#383-functions-and-methods) specifying the arguments and description of the tool:
```python title="python" maxLines=50
# install letta_client with `pip install letta-client`
from letta_client import Letta

# create a client to connect to your local Letta server
client = Letta(
  base_url="http://localhost:8283"
)

# define a function with a docstring
def roll_dice() -> str:
    """
    Simulate the roll of a 20-sided die (d20).

    This function generates a random integer between 1 and 20, inclusive,
    which represents the outcome of a single roll of a d20.

    Returns:
        str: The result of the die roll.
    """
    import random

    dice_role_outcome = random.randint(1, 20)
    output_string = f"You rolled a {dice_role_outcome}"
    return output_string

# create the tool
tool = client.tools.create_from_function(
    func=roll_dice
)
```
The tool creation will return a `Tool` object. You can update the tool with `client.tools.upsert_from_function(...)`.


### Specifying arguments via Pydantic models
To specify the arguments for a complex tool, you can use the `args_schema` parameter.

```python title="python" maxLines=50
# install letta_client with `pip install letta-client`
from letta_client import Letta

class Step(BaseModel):
    name: str = Field(
        ...,
        description="Name of the step.",
    )
    description: str = Field(
        ...,
        description="An exhaustic description of what this step is trying to achieve and accomplish.",
    )


class StepsList(BaseModel):
    steps: list[Step] = Field(
        ...,
        description="List of steps to add to the task plan.",
    )
    explanation: str = Field(
        ...,
        description="Explanation for the list of steps.",
    )

def create_task_plan(steps, explanation):
    """ Creates a task plan for the current task. """
    return steps


tool = client.tools.upsert_from_function(
    func=create_task_plan,
    args_schema=StepsList
)
```
Note: this path for updating tools is currently only supported in Python.

### Creating a tool from a file
You can also define a tool from a file that contains source code. For example, you may have the following file:
```python title="custom_tool.py" maxLines=50
from typing import List, Optional
from pydantic import BaseModel, Field


class Order(BaseModel):
    order_number: int = Field(
        ...,
        description="The order number to check on.",
    )
    customer_name: str = Field(
        ...,
        description="The customer name to check on.",
    )

def check_order_status(
    orders: List[Order]
):
    """
    Check status of a provided list of orders

    Args:
        orders (List[Order]): List of orders to check

    Returns:
        str: The status of the order (e.g. cancelled, refunded, processed, processing, shipping).
    """
    # TODO: implement
    return "ok"

```
Then, you can define the tool in Letta via the `source_code` parameter:
```python title="python" maxLines=50
tool = client.tools.create(
    source_code = open("custom_tool.py", "r").read()
)
```
Note that in this case, `check_order_status` will become the name of your tool, since it is the last Python function in the file. Make sure it includes a [Google Style Python docstring](https://google.github.io/styleguide/pyguide.html#383-functions-and-methods) to define the tool’s arguments and description.

# (Advanced) Accessing Agent State
<Warning>
Tools that use `agent_state` currently do not work in the ADE live tool tester (they will error when you press "Run"), however if the tool is correct it will work once you attach it to an agent.
</Warning>
If you need to directly access the state of an agent inside a tool, you can use the reserved `agent_state` keyword argument, for example:
```python title="python"
def get_agent_id(agent_state: "AgentState") -> str:
    """
    A custom tool that returns the agent ID

    Returns:
        str: The agent ID
    """
    return agent_state.id
```
