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Agent Series (11): A2A Protocol — How Agents Collaborate with Each Other

#agents #llm #opensource #multiagent

MCP Solved Agent ↔ Tool. Who Solves Agent ↔ Agent?

The previous article covered MCP: an Agent connects to tool services via a standard protocol. Tools are passive — they wait to be called, execute, return a result.

But some scenarios require delegating to another Agent with autonomous decision-making, not just a tool:

  • You have a Research Agent, expert at collecting and analyzing technical material
  • You have a Writing Agent, expert at turning analysis into readable documents
  • You have a Code Review Agent, expert at finding security issues

When these three Agents need to collaborate, how do they communicate? Who knows who exists? How do they hand off work?

This is the problem A2A (Agent-to-Agent) Protocol solves.

MCP:  Agent  ←→  Tools/Data (vertical integration, Agent invokes tools)
A2A:  Agent  ←→  Agent      (horizontal collaboration, Agent delegates to Agent)
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A2A's Three Core Concepts

AgentCard: An Agent's Business Card

Each Agent publishes an AgentCard describing its capabilities:

from a2a.types import AgentCard, AgentSkill

research_card = AgentCard()
research_card.name = "research-agent"
research_card.description = "Gathers factual background on technical topics"

skill = AgentSkill()
skill.id = "research"
skill.name = "Research"
skill.description = "Collect key facts on a topic"
skill.tags.extend(["research", "facts"])
research_card.skills.append(skill)
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Key AgentCard fields: name, description, skills (each skill has tags for discovery).

Think of it as the Agent equivalent of OpenAPI Spec — a machine-readable capability declaration that humans can also read.

Task: The Unit of Work

Agents don't pass function calls to each other — they pass Task objects:

from a2a.types import Task, TaskState, TaskStatus, Message, Part, Role

task = Task()
task.id = str(uuid.uuid4())
task.status.state = TaskState.TASK_STATE_SUBMITTED

# Input: a User-role Message
msg = Message()
msg.role = Role.ROLE_USER
part = Part(); part.text = "Should I use Python or Go?"
msg.parts.append(part)
task.history.append(msg)
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Task has a lifecycle: SUBMITTED → WORKING → COMPLETED / FAILED. On completion, the Agent appends the result as a ROLE_AGENT Message.

Registry: The Agent Yellow Pages

AgentRegistry stores all registered AgentCards and supports discovery by tag:

class AgentRegistry:
    def register(self, card: AgentCard, handler: Callable[[Task], Task]) -> None:
        self._agents[card.name] = AgentEntry(card=card, handler=handler)

    def discover(self, tag: str) -> list[AgentCard]:
        """Return all agents whose skills include the given tag."""
        ...

    def delegate(self, agent_name: str, input_text: str) -> Task:
        """Create a Task and execute it via the registered handler."""
        ...
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Demo 1: Direct Calls — Simple but Tightly Coupled

Without a protocol, the orchestrator calls three Python functions directly:

def direct_orchestrator(question: str) -> str:
    research = research_agent_fn(question)    # hard dependency
    analysis = analysis_agent_fn(research)   # hard dependency
    answer   = writing_agent_fn(analysis)    # hard dependency
    return answer
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Real execution output:

→ calling research_agent (direct)
→ calling analysis_agent (direct)
→ calling writing_agent (direct)

Answer: Choose Python if you need rapid development with broad libraries.
        Select Go if performance and concurrency are critical...
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Works perfectly. The problem is structural: the orchestrator has three hardcoded function references. Replace any one Agent, and you must edit the orchestrator. If the orchestrator lives in a different service, that means a cross-service code change.

Demo 2: A2A AgentCard Registry

Register three Agents, each with distinct skill tags:

[registry] registered: research-agent
[registry] registered: analysis-agent
[registry] registered: writing-agent
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Discovery test:

researchers = registry.discover("research")
# → Found: research-agent — Gathers factual background on technical topics

writers = registry.discover("writing")
# → Found: writing-agent — Composes clear technical prose from analysis output
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The orchestrator never writes an agent name — it discovers by tag:

def a2a_orchestrator(question: str) -> str:
    researchers = registry.discover("research")
    t1 = registry.delegate(researchers[0].name, question)

    analysts = registry.discover("analysis")
    t2 = registry.delegate(analysts[0].name, task_output(t1))

    writers = registry.discover("writing")
    t3 = registry.delegate(writers[0].name, task_output(t2))
    return task_output(t3)
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Real execution output:

→ delegating to research-agent (discovered via tag)
→ delegating to analysis-agent (discovered via tag)
→ delegating to writing-agent (discovered via tag)

Answer: Choose Python for rapid development; Go for high-throughput performance...
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The critical difference: the orchestrator code contains no agent names. Register a writing-agent-v2 with the same writing tag, and the orchestrator discovers and uses it immediately — zero code changes.

Demo 3: LLM-Driven Agent Routing

A2A's most powerful use case: the LLM reads the AgentCard catalog and decides which Agents to call and in what order.

Show the LLM the agent catalog:

Available agents:
  research-agent: Gathers factual background [skills: Research(research, facts)]
  analysis-agent: Analyzes research notes    [skills: Analysis(analysis, tradeoffs)]
  writing-agent:  Composes technical prose   [skills: Writing(writing, prose)]
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LLM outputs an execution plan:

["research-agent", "analysis-agent", "writing-agent"]
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Execute the plan:

Executing 3 agents:
  → delegating to research-agent
  → delegating to analysis-agent
  → delegating to writing-agent

Final answer: Choose Python for rapid development; Go for high-throughput...
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This is A2A's end state: no pre-configured orchestrator pipeline. The LLM reads the task requirements and the AgentCard descriptions, then plans the collaboration chain at runtime.

MCP vs A2A vs ANP: Protocol Selection Matrix 

Dimension         MCP                          A2A
──────────────────────────────────────────────────────────────────────
Problem solved    Agent ↔ Tool/Data            Agent ↔ Agent
Discovery         list_tools() (tool catalog)  discover() (Agent registry)
Unit of work      Tool call (sync)             Task (async-ready)
Coupling          Agent invokes tool directly  Orchestrator delegates task
Other end         Passive tool service         Autonomous Agent with logic
Cross-service     Tool is an independent proc  Agent is an independent service
Who decides       Agent (tool use)             Orchestrator (delegation)
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Complete four-way selection guide:

Scenario                                  Recommended
──────────────────────────────────────────────────────────────────────
Same codebase, call target known          Direct function call
Agent needs external tools                MCP (tools as service)
Agent delegates to specialist Agents      A2A (agents as service)
Cross-org large-scale Agent network       ANP (decentralized discovery)
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Design Checklist

AgentCard Design

  • [ ] description: one sentence about what the Agent is good at — the LLM reads it for routing decisions
  • [ ] skill.tags: use semantically clear tags (research, analysis, writing), not version numbers or internal IDs
  • [ ] AgentCard should be machine-readable and human-readable (think OpenAPI style)

Task Design

  • [ ] Task.id: use UUID for tracking and idempotent retry
  • [ ] Use history (Message chain) for context passing — don't concatenate everything into part.text
  • [ ] Distinguish ROLE_USER (input) from ROLE_AGENT (output) messages

Registry and Discovery

  • [ ] One tag can map to multiple Agents (load balancing, A/B testing)
  • [ ] Support tag-based filter AND exact-name lookup
  • [ ] Production: use a persistent registry (database or service registry), not an in-memory dict

LLM-Driven Routing

  • [ ] Keep the agent catalog concise: name + description + skill tags. Don't dump the full AgentCard JSON to the LLM
  • [ ] Validate the LLM's output: handle unknown agent names with a fallback path
  • [ ] Log the LLM-generated execution plan for post-hoc analysis and debugging

Summary

Five core takeaways:

  1. A2A and MCP are complementary, not competing: MCP is Agent-invokes-tool; A2A is Agent-delegates-to-Agent. A single system can use both protocols
  2. AgentCard is the heart of A2A: it transforms an Agent from a hardcoded dependency into a discoverable, composable service unit
  3. Task is a richer unit of work than a function call: it has a lifecycle, is async-ready, and carries structured history
  4. The direct-call vs A2A dividing line: same team, same codebase → direct call; cross-service, need decoupling → A2A
  5. LLM-driven routing is A2A's highest form: Agents plan their own collaboration chain based on task descriptions, with no pre-configured orchestrator pipeline

Up next: Agent Evaluation Framework — how to systematically test Agents, which metrics matter, and how to use DeepEval.

References

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