10-custom-function-tools

10. Custom Function Tools

Building custom Python function tools for ADK agents.

Blog Post: https://arjunprabhulal.com/adk-custom-tools-function/

Table of Contents

1. Overview
2. Prerequisites
3. Setup Steps
4. Creating a Function Tool
5. Running the Agent
6. Next Steps

Overview

Custom function tools are Python functions that agents can call. ADK automatically wraps them - no need for FunctionTool().

Prerequisites

• Python 3.10+
• Gemini API key from AI Studio

Setup Steps

1. Navigate to this module:

cd 10-custom-function-tools

2. Create and activate a virtual environment:

python -m venv .venv
source .venv/bin/activate  # On Windows: .venv\Scripts\activate

3. Install dependencies:

pip install -r ../requirements.txt

4. Set up environment variables in custom_tool_agent/.env:

GOOGLE_API_KEY=your-api-key-here

Creating a Function Tool

1. Define a Python function with type hints
2. Add a docstring describing the function
3. Return a dictionary with the result
4. Pass the function directly to the agent's tools list

def get_order_status(order_id: str) -> dict:
    """Retrieves the status of an order by order ID."""
    return {"order_id": order_id, "status": "Shipped"}

agent = Agent(
    model="gemini-2.5-flash",
    tools=[get_order_status],  # ADK auto-wraps it
)

Running the Agent

Using ADK Web

adk web

Open http://127.0.0.1:8000 and select custom_tool_agent.

Test Queries:

• "What is the status of order 12345?"
• "Check order ABC123"

Using ADK CLI

adk run custom_tool_agent

Next Steps

Continue to 11. OpenAPI Tools