6. Interacting with the Server

6. Interacting with the Server

With the Helloworld A2A server running, let’s send some requests to it. The SDK includes a client (A2AClient) that simplifies these interactions.

The Helloworld Test Client

The test_client.py script demonstrates how to:

1. Fetch the Agent Card from the server.
2. Create an A2AClient instance.
3. Send both non-streaming (message/send) and streaming (message/stream) requests.

Open a new terminal window, activate your virtual environment, and navigate to the a2a-samples directory.

Activate virtual environment (Be sure to do this in the same directory where you created the virtual environment):

=== “Mac/Linux”

```sh
source .venv/bin/activate
```

=== “Windows”

```powershell
.venv\Scripts\activate
```

Run the test client:

# from the a2a-samples directory
python samples/python/agents/helloworld/test_client.py

Understanding the Client Code

Let’s look at key parts of test_client.py:

1. Fetching the Agent Card & Initializing the Client:

   --8<-- "https://raw.githubusercontent.com/a2aproject/a2a-samples/refs/heads/main/samples/python/agents/helloworld/test_client.py:A2ACardResolver"

   The A2ACardResolver class is a convenience. It first fetches the AgentCard from the server’s /.well-known/agent-card.json endpoint (based on the provided base URL) and then initializes the client with it.

2. Sending a Non-Streaming Message (send_message):

   --8<-- "https://raw.githubusercontent.com/a2aproject/a2a-samples/refs/heads/main/samples/python/agents/helloworld/test_client.py:send_message"

   • The send_message_payload constructs the data for MessageSendParams.
   • This is wrapped in a SendMessageRequest.
   • It includes a message object with the role set to “user” and the content in parts.
   • The Helloworld agent’s execute method will enqueue a single “Hello World” message. The DefaultRequestHandler will retrieve this and send it as the response.
   • The response will be a SendMessageResponse object, which contains either a SendMessageSuccessResponse (with the agent’s Message as the result) or a JSONRPCErrorResponse.

3. Handling Task IDs (Illustrative Note for Helloworld):

   The Helloworld client (test_client.py) doesn’t attempt get_task or cancel_task directly because the simple Helloworld agent’s execute method, when called via message/send, results in the DefaultRequestHandler returning a direct Message response rather than a Task object. More complex agents that explicitly manage tasks (like the LangGraph example) would return a Task object from message/send, and its id could then be used for get_task or cancel_task.

4. Sending a Streaming Message (send_message_streaming):

   --8<-- "https://raw.githubusercontent.com/a2aproject/a2a-samples/refs/heads/main/samples/python/agents/helloworld/test_client.py:send_message_streaming"

   • This method calls the agent’s message/stream endpoint. The DefaultRequestHandler will invoke the HelloWorldAgentExecutor.execute method.
   • The execute method enqueues one “Hello World” message, and then the event queue is closed.
   • The client will receive this single message as one SendStreamingMessageResponse event, and then the stream will terminate.
   • The stream_response is an AsyncGenerator.

Expected Output

When you run test_client.py, you’ll see JSON outputs for:

• The non-streaming response (a single “Hello World” message).
• The streaming response (a single “Hello World” message as one chunk, after which the stream ends).

The id fields in the output will vary with each run.

// Non-streaming response
{"jsonrpc":"2.0","id":"xxxxxxxx","result":{"message":{"role":"ROLE_AGENT","parts":[{"text":"Hello World"}],"messageId":"yyyyyyyy"}}}
// Streaming response (one chunk)
{"jsonrpc":"2.0","id":"zzzzzzzz","result":{"message":{"role":"ROLE_AGENT","parts":[{"text":"Hello World"}],"messageId":"wwwwwwww"}}}

(Actual IDs like xxxxxxxx, yyyyyyyy, zzzzzzzz, wwwwwwww will be different UUIDs/request IDs)

This confirms your server is correctly handling basic A2A interactions with the updated SDK structure!

Now you can shut down the server by typing Ctrl+C in the terminal window where __main__.py is running.