MCP vs A2A vs ACP: Which Protocol Should Your AI Agent Support?
MCP, A2A, and ACP are the three protocols shaping agent interoperability in 2026. We break down what each does, when to use them, and which ones your agent actually needs.
Laurent Yew
Founder
The Three Protocols Explained
If you're building or deploying AI agents in 2026, you need to understand three protocols: MCP (Model Context Protocol), A2A (Agent2Agent), and ACP (Agent Communication Protocol). They're not competing — they solve different problems. MCP connects agents to tools. A2A connects agents to each other. ACP provides a REST-native communication layer. Most production agents will need at least one, and many will need two or all three.
MCP: Connecting Agents to Tools
The Model Context Protocol (MCP) was created by Anthropic as an open standard for connecting AI models to external data sources and tools. It uses a client-server architecture: the agent (client) connects to an MCP server that exposes resources, tools, and prompts. Resources are data sources (files, databases, APIs). Tools are functions the agent can call. Prompts are pre-configured instruction templates.
MCP is the most widely adopted protocol — 10 of the 15 registries we index support it. If your agent needs to read from a database, call an external API, or access files, MCP is the standard way to do it. Read our full MCP explainer for technical details.
A2A: Connecting Agents to Each Other
The Agent2Agent Protocol (A2A) was developed by Google to solve a different problem: how do agents discover and collaborate with each other? A2A uses Agent Cards — JSON-LD documents that describe an agent's capabilities, endpoint, and authentication requirements. When Agent A needs help with a task, it can search for agents with the right capabilities, read their Agent Cards, and delegate subtasks.
A2A is essential for multi-agent systems. Six of the 15 registries support it, including LangChain Hub, Google AI Hub, and Pinecone Marketplace. If you're building a system where agents collaborate (e.g., a research agent that delegates to a coding agent), A2A is what makes that possible. Read our full A2A explainer.
ACP: REST-Native Agent Communication
The Agent Communication Protocol (ACP) was developed by Cisco and BeeAI as a REST-native alternative. Unlike MCP's client-server model and A2A's peer-to-peer model, ACP uses standard REST endpoints with OpenAPI specifications. Every agent exposes a predictable REST API, making integration straightforward for developers already familiar with REST patterns.
ACP is the newest protocol and has the smallest footprint — primarily Google AI Hub. However, its REST-native approach makes it the easiest to implement for teams with existing API infrastructure. Read our full ACP explainer.
Side-by-Side Comparison
| Feature | MCP | A2A | ACP |
|---|---|---|---|
| Creator | Anthropic | Cisco / BeeAI | |
| Primary Use | Agent-to-tool | Agent-to-agent | Agent-to-agent (REST) |
| Architecture | Client-server | Peer-to-peer | REST API |
| Discovery Mechanism | Manual config | Agent Cards (JSON-LD) | OpenAPI specs |
| Transport | JSON-RPC over stdio/SSE | HTTP + JSON-LD | HTTP REST |
| Registry Support | 10 of 15 registries | 6 of 15 registries | 1 of 15 registries |
| Best For | Tool integration | Multi-agent collaboration | REST-native teams |
Which Protocol Should Your Agent Support?
The answer depends on what your agent does and who uses it. Here's a decision framework.
Support MCP If...
- Your agent needs to access external data (databases, files, APIs).
- You want your agent listed on OpenAI, Anthropic, AWS Bedrock, or Microsoft Copilot Store.
- Your users expect tool-use capabilities (code execution, web browsing, file operations).
- You want the broadest registry compatibility — MCP is supported by 10 of 15 registries.
Support A2A If...
- Your agent should be discoverable by other agents for task delegation.
- You're building a multi-agent system where agents collaborate.
- You want to be listed on Google AI Hub or Pinecone Marketplace.
- Your agent provides a service that other agents would benefit from calling.
Support ACP If...
- Your team has strong REST API experience and wants a familiar pattern.
- You need OpenAPI-compatible documentation for compliance.
- You're integrating with Cisco or BeeAI ecosystems.
- You want the simplest possible implementation path.
// TIP
Most agents should support at least MCP. If you're building a multi-agent system, add A2A. ACP is optional but valuable if your team prefers REST. Supporting multiple protocols increases your agent's discoverability across registries — browse the registry to see which agents support which protocol combinations.
How Protocols Affect Registry Discoverability
Protocol support directly affects where your agent can be listed and how users find it. Agents supporting only MCP can be listed on 10 registries. Agents supporting only A2A can be listed on 6. Agents supporting both MCP and A2A can be listed on all 15 registries that support either protocol — maximizing discoverability.
When users search AgentResourceDB's registry, they can filter by protocol. An agent that supports MCP will appear in MCP-filtered searches; an agent that supports both MCP and A2A appears in both. More protocol support means more search visibility, which means more potential users.
Do MCP, A2A, and ACP Work Together?
Yes, and they're designed to be complementary. A common architecture: an agent uses MCP to connect to tools (databases, APIs), uses A2A to discover and delegate to other agents, and optionally exposes an ACP REST endpoint for traditional API consumers. AgentResourceDB's protocol compliance score verifies that agents correctly implement whatever protocols they claim — so when you browse agents by protocol, you can trust the filtering is accurate.
Explore agents by protocol on our registry, or learn more about each protocol in depth: MCP, A2A, ACP.
// Ready to explore?
Browse the full AgentResourceDB registry with 104,000+ AI agents across 15 registries.
Browse the Registry// Author
Laurent Yew
Founder
Laurent Yew is the founder of AgentResourceDB, where he leads the platform's vision of building a unified, trust-first discovery layer for the AI agent ecosystem. With over a decade of experience scaling AI and SaaS products, Laurent has dedicated his career to making complex developer infrastructure accessible, transparent, and reliable. He writes about agent registries, protocol interoperability, and the future of agent-to-agent collaboration, drawing from hands-on work building evaluation frameworks that help developers cut through the noise of 100,000+ agents. Through AgentResourceDB, he is committed to establishing the trust standards the industry needs as AI agents move from experimentation to production.
// Frequently Asked Questions
What is the difference between MCP and A2A?
MCP (Model Context Protocol) connects AI agents to external tools and data sources using a client-server architecture. A2A (Agent2Agent) enables agents to discover and communicate with each other using Agent Cards. MCP is agent-to-tool; A2A is agent-to-agent. They are complementary, not competing.
Which AI agent protocol should I support first?
Start with MCP. It has the broadest registry support (10 of 15 registries) and covers the most common use case: connecting agents to external tools and data. Add A2A if your agent needs to collaborate with other agents. Add ACP if your team prefers REST-native APIs.
Can an agent support MCP, A2A, and ACP simultaneously?
Yes. The three protocols are designed to be complementary. An agent can use MCP for tool integration, A2A for agent discovery and delegation, and ACP for REST-native API access. Supporting multiple protocols increases discoverability across registries.
Is MCP replacing A2A or ACP?
No. MCP, A2A, and ACP solve different problems. MCP handles agent-to-tool communication, A2A handles agent-to-agent discovery, and ACP provides REST-native communication. They are evolving in parallel and are designed to work together in multi-protocol agent architectures.
How do I verify an agent's protocol support?
AgentResourceDB's trust score includes a protocol compliance factor that tests whether agents actually implement the protocols they claim. Browse the registry and filter by protocol — every listed agent has been verified through automated protocol tests.