Protocols & Standards

Summary

Three protocols dominate: MCP (agent-to-tools/data, vertical), A2A (agent-to-agent, horizontal), ACP (agent commerce). Most production systems layer them: agents use MCP to access their tools, A2A to delegate to peers, and optionally ACP for purchases. Supporting standards (OpenTelemetry, llms.txt) add observability and API discovery.

Orchestrator (uses A2A)
  ├── MCP → tools
  ├── MCP → data
  ├── A2A → Specialist 1
  │    └── MCP → Specialist's tools
  └── A2A → Specialist 2
       └── MCP → Specialist's tools

MCP 2025-11-25: tools, resources, prompts, async tasks
A2A v1.0 RC: task delegation across organizational boundaries
ACP beta: agent-initiated commerce with payment delegation
Publish: /.well-known/agent.json, /.well-known/acp.json, /AGENTS.md, /llms.txt
Transport: stdio (local), Streamable HTTP (remote)

Three mature protocols and several complementary standards define how agents discover capabilities, execute work, and integrate across organizational boundaries.

Quick Start

Does your agent need tools? → Use MCP (spec 2025-11-25). This is the baseline.

Multiple agents delegating work? → Add A2A (v1.0 RC) for peer-to-peer task delegation.

Agent initiating purchases? → Add ACP (beta) for commerce with payment boundaries.

Complex multi-step workflows? → Use Arazzo to choreograph API sequences.

Need observability? → Instrument with OpenTelemetry GenAI semconv — stable and widely adopted.

The Protocols

Protocol	Purpose	Learn More
MCP 2025-11-25	Agent-to-tool / agent-to-data (vertical)	MCP docs
A2A v1.0 RC	Agent-to-agent delegation (horizontal)	A2A docs
ACP beta	Agent-initiated commerce	ACP docs
Arazzo 1.0.1	Multi-step API workflow sequences	Comparison page
OpenTelemetry GenAI	Observability & tracing	Emerging standards
AGENTS.md	Agent context documentation	Emerging standards
llms.txt	LLM-friendly API docs	Emerging standards

Architecture Pattern

Most production systems use MCP + A2A + supporting standards:

Orchestrator Agent (uses A2A)
  ├── MCP → Internal tools
  ├── MCP → Data sources
  ├── A2A → Specialist Agent 1
  │    └── MCP → Specialist's tools
  └── A2A → Specialist Agent 2
       └── MCP → Specialist's tools

Discovery Checklist

Publish these endpoints for agents to find you:

/.well-known/agent.json — A2A AgentCard (for multi-agent scenarios)
/.well-known/acp.json — ACP capabilities (if you accept agent-initiated payments)
/AGENTS.md — Agent context & MCP servers (always useful)
/llms.txt or /llms-full.txt — LLM-friendly docs
Structured data as JSON-LD (schema.org markup) for auto-discovery

Full Comparison & Decision Guide

See Protocol Comparison for maturity, transport details, auth models, and when to use each.

See Emerging Standards for Arazzo workflows, observability, semantic metadata, and draft specs to monitor.