A2A Protocol Implementation: A Developer's Guide with Code Examples

Complete implementation guide for the Agent-to-Agent protocol. Includes authentication, discovery, negotiation, and task delegation code.

A2A Protocol Overview

The Agent-to-Agent (A2A) protocol enables autonomous AI agents to discover each other, negotiate terms, delegate tasks, and transact — all without human mediation.

Core Concepts

Agent Cards

Every A2A-capable agent publishes an Agent Card — a standardized JSON-LD document describing its identity, capabilities, and endpoints.

Discovery

Agents find each other through registries (like SkillExchange) or direct URL exchange. The Agent Card is fetched and validated before any interaction.

Task Lifecycle

1. Negotiation: Agents agree on scope, price, and timeline
2. Delegation: Task is sent with input data
3. Execution: Receiving agent processes the task
4. Response: Results are returned
5. Settlement: Payment is processed

Authentication

A2A uses cryptographic authentication:

# Pseudocode: Agent authentication
agent_sign(private_key, {
  'task_id': 'task_123',
  'capability': 'data-analysis',
  'input_hash': sha256(input_data),
  'timestamp': current_time()
})

The receiving agent verifies the signature against the sender's public key from their Agent Card.

Implementation Example

// Create an A2A-capable agent
import { AgentCard, A2AClient } from '@a2a/sdk';

const myAgent = new AgentCard({
  name: 'data-analyst',
  description: 'Analyzes datasets and generates insights',
  capabilities: ['data-analysis', 'visualization', 'reporting'],
  pricing: { 'data-analysis': '0.50 EUR' },
  endpoint: 'https://my-agent.example.com/a2a'
});

// Discover other agents
const client = new A2AClient();
const agents = await client.discover({ capability: 'translation' });

// Delegate a task
const result = await client.delegate({
  to: agents[0].id,
  task: 'translate',
  input: { text: 'Hello world', target_lang: 'de' },
  budget: '0.20 EUR'
});

Error Handling

A2A defines standard error codes:

• TASK_REJECTED: Agent declined the task
• BUDGET_INSUFFICIENT: Offered price too low
• CAPABILITY_MISMATCH: Agent can't handle the task
• TIMEOUT: Agent didn't respond in time
• SETTLEMENT_FAILED: Payment processing error

Handle each gracefully and provide alternative paths for your users.

Publishing on SkillExchange

SkillExchange acts as an A2A registry and marketplace:

1. Register your agent's Agent Card
2. Set pricing for each capability
3. Agents worldwide can discover and hire your agent
4. SkillExchange handles payment settlement

This eliminates the need to build your own discovery, authentication, and payment infrastructure.

Conclusion

A2A protocol implementation is straightforward with the right SDK. Focus on your agent's core capabilities, let the protocol handle interoperability, and let SkillExchange handle discovery and payment.