A2A (Agent-to-Agent Protocol): What is it?

An open standard protocol enabling agents to collaborate and delegate tasks over networks

About This Document

This document explains A2A's core concepts, key differences from MCP, benefits and challenges, and future outlook. For detailed comparison with sub-agents, also refer to what-is-subagent.md.

What is A2A?

A2A (Agent-to-Agent Protocol) is an open standard protocol that enables different AI agents to communicate and collaborate in peer-to-peer relationships over networks.

Background and Leadership

Here is a brief history of A2A's origins and standardization.

• Google led the effort, announcing A2A in April 2025
• Subsequently, Linux Foundation will take over stewardship for open standardization
• Like MCP, it is positioned as foundational infrastructure for the agent economy

The Essence of A2A

A2A's defining characteristic is enabling agent ↔ agent peer communication:

• MCP: AI agent ↔ tools/APIs (master-slave relationship)
• A2A: AI agent ↔ AI agent (peer relationship)

In One Sentence

"A protocol allowing different AI agents to request work from each other"

Architectural Model

Three-layer structure for agent development:

• Build with ADK: Construct the agent itself
• Equip with MCP: Connect tools and APIs
• Communicate with A2A: Communicate with other agents

Why A2A?

Current Challenges

As AI agent technology evolves rapidly, companies and organizations develop and operate their own agents. However, there is no standard communication mechanism between these agents.

The Silo Problem:

• Internal Sales Analysis Agent → wants to query Salesforce AI Agent
• But there's no standard communication protocol
• MCP enables "agent ↔ tools" but not "agent ↔ agent"
• Result: agents operate in isolation, making cross-organization collaboration difficult

Before and After A2A

Before A2A:

• Agent-to-agent communication handled by custom implementations (API integration, etc.)
• Without standards, each integration requires custom specification negotiation
• Not scalable

After A2A:

• Standardized protocol enables agent-to-agent communication
• Authentication, authorization, and trust models are unified
• Agents from different organizations can collaborate seamlessly

Communication Flow

The diagram below illustrates the basic flow of agents from different organizations communicating via the A2A protocol.

Fundamental Differences from MCP

While both A2A and MCP enable "connections," their connection targets are fundamentally different. The comparison table below clarifies the distinctions.

Feature Comparison

The following table contrasts MCP and A2A.

Aspect	MCP	A2A
Led by	Anthropic	Google → Linux Foundation
Purpose	Agent ↔ Tools	Agent ↔ Agent
Connection Target	MCP servers (you manage)	Other agents (including external parties)
Communication Model	Master-Slave (agent directs)	Peer-to-peer (both can request)
Context Sharing	Shareable with parent agent	Fully isolated (expects opaque parties)
Owner	Self	Self or others
Trust Model	Implicit trust	Authentication/authorization required

Decision Flowchart: Which Should You Choose?

Use the following flowchart to determine the right choice among MCP, A2A, and custom sub-agents.

Core Concepts of A2A

Three key concepts are essential in A2A.

1. Agent Card

An agent's "self-introduction card" that provides information helping other agents and discovery systems understand what capabilities an agent offers.

Format: JSON

Included Information:

• Agent name and description
• Supported Capabilities
• Authentication method
• Supported A2A versions
• Endpoint information

Location: Discoverable via /.well-known/agent.json

2. Task

A "unit of work" agents request from each other. Handles both brief interactions and extended processes.

Lifecycle:

• submitted: Immediately after task receipt
• working: Processing
• input-required: Additional information needed
• completed: Success
• failed: Failure

Characteristics:

• Asynchronous execution
• Long-running task support
• Polling or callback mechanisms

3. Artifact

Data generated as a task execution result. Not limited to text—includes files and structured data.

Supported Formats:

• Text (plain text, markdown, etc.)
• Files (PDF, images, etc.)
• Structured data (JSON, XML, etc.)
• Multimodal (images, audio anticipated)

Benefits of A2A

Adopting A2A provides the following advantages.

✅ Standardized Cross-Organization Collaboration

Agents from different organizations can communicate via a standard protocol. This marks a shift from "custom per-company integration" to "standards-based collaboration."

✅ Agent Specialization and Cooperation

Each agent can specialize in its domain, while complex tasks benefit from multi-agent collaboration. For example:

• Sales Agent ← retrieves forecast data from Market Analysis Agent
• Sales Agent ← retrieves customer info from CRM Agent

✅ Multimodal Support

Information exchange isn't limited to text—images, audio, and files enable richer communication.

✅ Asynchronous Task Support

Long-running operations like report generation can execute in the background, with results retrieved later.

✅ Vendor Independence

As an open standard under Linux Foundation stewardship, there's no vendor lock-in.

Drawbacks and Challenges

A2A also comes with challenges that remain to be addressed.

❌ Implementation Complexity

Authentication, authorization, and encryption are mandatory—requirements stricter than MCP.

❌ Network Dependency

Requires handling network latency, connection dropouts, and other failures. Timeout configuration and retry logic are essential.

❌ Debugging Difficulty

Tracing inter-agent communication is complex, making troubleshooting challenging. Logging strategy is critical.

❌ Maturity is Still Low

Compared to MCP (released November 2024), A2A is even newer. Implementation patterns and operational best practices remain limited.

❌ Nascent Ecosystem

Only a limited number of agents and tools support A2A yet. Broader adoption will take time.

❌ Trust Challenges

You cannot fully guarantee the quality or security of agents owned by others. Risks of misuse by malicious agents must be considered.

Choosing Between Sub-agents and A2A

For collaboration between agents you manage, use "custom sub-agents." For external agent collaboration including other organizations, use "A2A."

Comparison

The following table contrasts custom sub-agents with A2A agents.

Dimension	Custom Sub-agent	A2A Agent
Location	Within same process	Over the network
Owner	Self	Self or others
Trust Relationship	Full trust	Authentication/authorization required
Context	Partially shared with parent	Fully isolated
Lifecycle	Session-scoped	Persistent service

Understanding via Metaphor

• Sub-agents = "In-house specialist department"

  • Located in the same building (process)
  • Under supervisor (parent agent) oversight
  • Fully trustworthy

• A2A agents = "Outsourced partner / vendor"

  • Located at separate facilities (separate processes)
  • Communicate over networks
  • Relationship based on contracts (authentication/authorization)

Complementary, Not Competing

Sub-agents and A2A agents don't compete—they complement each other. Most systems use both:

• Internal task decomposition → Custom sub-agents
• External resource utilization → A2A agents

Current Maturity and Future Outlook

Timeline

Key milestones from the emergence of MCP and A2A to the present.

• November 2024: Anthropic releases MCP
• April 2025: Google announces A2A
• 2025: Migration to Linux Foundation planned
• 2025–2026: Ecosystem development phase

Current Status

The 03-architecture.md document in this repository describes A2A's architectural positioning.

Future Projections

While A2A implementations remain limited, these scenarios are anticipated:

1. Early Stage (2025): Major players begin A2A support
2. Growth Stage (2025–2026): Mid-market companies start adopting
3. Maturity Stage (2026+): MCP + A2A combined use becomes standard

Recommended Architecture

Build with ADK, equip with MCP, communicate with A2A

This model is expected to become the standard for future agent development.

What to Read Next

Explore these documents to deepen your A2A understanding:

Purpose	Document
Sub-agent details	what-is-subagent.md
MCP details	what-is-mcp.md
Overall architecture	03-architecture.md
About Skills	what-is-skills.md

Last Updated: April 2025 Status: Initial Release (Post A2A Announcement)