Skip to content

3. Agent Skills & Agent Card

Before an A2A agent can do anything, it needs to define what it can do (its skills) and how other agents or clients can find out about these capabilities (its Agent Card).

We'll use the helloworld example located in a2a-samples/samples/python/agents/helloworld/.

Agent Skills

An Agent Skill describes a specific capability or function the agent can perform. It's a building block that tells clients what kinds of tasks the agent is good for.

Attributes of an AgentSkill (defined in a2a.types):

  • id: A unique identifier for the skill.
  • name: A human-readable name.
  • description: A more detailed explanation of what the skill does.
  • tags: Keywords for categorization and discovery.
  • examples: Sample prompts or use cases.
  • input_modes / output_modes: Supported Media Types for input and output (e.g., "text/plain", "application/json").
  • security_requirements: Security schemes necessary for this skill.

In __main__.py, you can see how a skill for the Helloworld agent is defined:

skill = AgentSkill(
    id='hello_world',
    name='Returns hello world',
    description='just returns hello world',
    tags=['hello world'],
    examples=['hi', 'hello world'],
)

This skill is very simple: it's named "Returns hello world" and primarily deals with text.

Agent Card

The Agent Card is a JSON document that an A2A Server makes available, typically at a .well-known/agent-card.json endpoint. It's like a digital business card for the agent.

Key attributes of an AgentCard (defined in a2a.types):

  • name, description, version: Basic identity information.
  • icon_url: A URL to an icon representing the agent.
  • supported_interfaces: Ordered list of endpoints and protocols where the A2A service can be reached.
  • capabilities: Supported A2A features like streaming or extended_agent_card.
  • default_input_modes / default_output_modes: Default Media Types for the agent.
  • skills: A list of AgentSkill objects that the agent offers.

The helloworld example defines its Agent Card like this:

# This will be the public-facing agent card
public_agent_card = AgentCard(
    name='Hello World Agent',
    description='Just a hello world agent',
    icon_url='http://localhost:9999/',
    version='1.0.0',
    default_input_modes=['text'],
    default_output_modes=['text'],
    capabilities=AgentCapabilities(
        streaming=True, extended_agent_card=True
    ),
    supported_interfaces=[
        AgentInterface(
            protocol_binding='JSONRPC',
            url='http://localhost:9999',
        )
    ],
    skills=[skill],  # Only the basic skill for the public card
)

This card tells us the agent is named "Hello World Agent", runs at http://localhost:9999 using JSONRPC, supports text interactions, and has the hello_world skill. Its capabilities also indicate support for streaming and an extended authenticated configuration.

Understanding the Agent Card is crucial because it's how a client discovers an agent and learns how to interact with it.