Agent-to-Agent (A2A) Protocol Architecture Diagrams (2026)

The Agent-to-Agent (A2A) protocol, introduced by Google in 2025 and now adopted by 150+ organizations, is the emerging standard for structured communication between AI agents. Where MCP (Model Context Protocol) standardizes how agents call tools, A2A standardizes how agents delegate work to other agents — defining the message format, transport layer, capability discovery, and task lifecycle for agent-to-agent interactions. Together, MCP and A2A form the two-protocol backbone of the modern agentic stack in 2026.

As multi-agent systems move to production, clear architecture diagrams that show A2A communication patterns have become essential for design reviews, security audits, and cross-team alignment. This guide explains the A2A protocol components, contrasts them with MCP, and provides prompt templates for diagramming your A2A-based system.

A2A vs. MCP: two halves of the agentic stack

Core A2A protocol concepts

Agent Card

Every A2A-compatible agent publishes an Agent Card — a JSON document served at a well-known URL (typically/.well-known/agent.json) that describes: the agent's name and description, its input/output modalities (text, images, files, structured data), the tasks it can perform, its authentication requirements, and the endpoint URL for task submission. Agent Cards enable discovery: an orchestrator can fetch a remote agent's card to understand what it can do before delegating a task. Show Agent Cards in your diagram as a discovery side-channel separate from the main task submission flow.

Task unit

A2A tasks have a defined lifecycle: the caller submits a Task (with a unique ID, input message, and context), the remote agent transitions the task through states — submitted → working → completed or failed — and the caller can poll for status or stream progress via SSE. Long-running tasks can be paused (input-required) when the remote agent needs more information. Draw task state transitions explicitly in your diagram.

Transport: HTTPS + SSE

A2A tasks are submitted via HTTPS POST. Status updates and streamed partial results flow back to the caller over Server-Sent Events (SSE). Unlike MCP's stdio transport (suitable for local tool servers), A2A is designed for network-separated, independently deployed agents — the remote agent may be deployed by a different team, in a different cloud region, or by a third-party vendor.

The MCP + A2A combined stack

In production 2026 architectures, MCP and A2A serve complementary roles:

• Orchestrator agent uses MCP to call local tools (file system, database, code execution) and uses A2A to delegate complex subtasks to remote specialist agents
• Specialist agents expose their capabilities via Agent Cards over A2A, and internally use MCP to call their own tools
• An Agent Registry (optional) maintains a catalog of available agents and their Agent Cards, similar to a service registry in microservices

Prompt templates for A2A architecture diagrams

Orchestrator with A2A specialist delegation

Cross-organization A2A federation

What to show in your A2A diagram

• Agent boundaries: Each agent as a distinct box — label its model, deployment platform, and role
• Agent Card discovery: A dashed arrow from the orchestrator to each remote agent's /.well-known/agent.json endpoint
• A2A task submission: HTTPS POST arrows from orchestrator to remote agents, labeled with task type
• SSE streams: Return arrows showing streamed task state updates and partial results
• MCP tool calls: Distinguished from A2A connections (different arrow style or color)
• Trust boundaries: Organizational or trust-tier boundaries drawn with dashed boxes
• Authentication mechanism: JWT, OAuth2, or mTLS annotated on cross-boundary connections

Related guides: MCP architecture diagram, AI agent architecture diagrams, AI agent orchestration use cases, and securing agentic AI systems.