Introduction to Google’s Agent2Agent (A2A) Protocol/

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Building A2A Compliant Client and Server

Building A2A Compliant Client and Server

Explore how A2A compliant systems work behind-the-scenes.

We'll cover the following...

Now that we understand A2A’s core concepts, it’s time to build something real. From scratch, we’ll create a complete A2A-compliant system: a simple echo agent that receives messages from A2A clients over HTTP and responds with echoed content, paired with a client that discovers and communicates with it.

We’ll build incrementally, starting with the bare minimum and adding A2A compliance step-by-step. By the end, we’ll have a working agent that any A2A client can discover and use, and a client that can talk to any A2A-compliant agent. Along the way, we’ll explain each library, pattern, and design choice so you understand what to code and why.

What are we building?

Our goal is straightforward: create the simplest possible A2A interaction. We’ll build an echo agent that receives text messages and responds with “You said: [your message].” This might seem trivial, but it demonstrates all the essential A2A patterns:

  • Agent discovery through a standard endpoint.

  • JSON-RPC 2.0 communication protocol.

  • Task life cycle management (submitted to completed).

  • Structured message exchange with proper parts.

  • Error handling for malformed requests.

Think of this as the “hello world” of agent-to-agent communication, simple enough to understand completely, but comprehensive enough to serve as a foundation for more complex agents.

Before we start coding, ensure that we have the right tools. We’ll use Python with a few key libraries that handle the heavy lifting:

# Create a clean workspace
mkdir a2a-echo-example
cd a2a-echo-example
# Set up virtual environment
python3 -m venv venv
source venv/bin/activate  # On Windows: venv\Scripts\activate
# Install core dependencies
pip install fastapi uvicorn pydantic requests
Package installation

Here’s what each library does.

  • FastAPI: A modern Python web framework that makes building APIs easy. It handles HTTP routing, request validation, and automatic API documentation. We chose it because it’s beginner-friendly, has excellent type checking, and integrates seamlessly with A2A’s JSON-based communication.

  • Uvicorn: An ASGI server that runs our FastAPI application. Think of it as the engine that listens for HTTP requests and routes them to our agent code. It’s lightweight, fast, and perfect for development.

  • Pydantic: A data validation library which ensures that our ...