A2A vs MCP: What's the Difference? (And Do You Need Both?)

If you've been following AI agent infrastructure in 2026, you've seen two protocols come up constantly: MCP (Model Context Protocol, from Anthropic) and A2A (Agent-to-Agent, from Google). Both claim to be the standard for AI agent interoperability. Both are backed by major companies. Both are being adopted fast.

They are not competitors. They solve completely different problems — and most production agent systems will eventually use both.

The one-line version

MCP is how an agent talks to tools. A2A is how agents talk to each other.

That distinction sounds small but it's load-bearing. MCP is about giving a single agent access to external capabilities — databases, APIs, search indexes, documentation. A2A is about coordinating work across multiple agents that may be built by different teams, on different infrastructure, using different models.

MCP: agent to tool

MCP (Model Context Protocol) defines a standard JSON-RPC interface between an AI agent and a server that exposes tools or data. You build an MCP server, expose tools like search_docs or ask_site, and any MCP client — Claude, Cursor, a custom agent — can connect and call them.

// MCP: one agent, one server, one tool call
Agent → MCP Server → tool executes → result back to agent

POST /mcp
{ "method": "tools/call", "params": { "name": "ask_site", "arguments": { "domain": "stripe.com", "query": "What are the API rate limits?" } } }

MCP is synchronous and stateless (as of the 2026 RC spec). The agent makes a request, the server responds. No awareness of other agents, no task delegation, no coordination. Just a clean tool call.

A2A: agent to agent

A2A (Agent-to-Agent protocol, announced by Google in April 2025, now at 150+ supporting organisations) defines how two agents — which may be built by different vendors, on different infrastructure — discover each other, delegate tasks, and exchange results.

// A2A: one agent delegates to another agent
Orchestrator Agent → A2A → Specialist Agent → result back

// Discovery via Agent Card
GET /.well-known/agent.json
→ { "name": "ResearchAgent", "capabilities": ["web_research", "summarisation"], "endpoint": "..." }

// Task delegation
POST /a2a/tasks
{ "task": "Summarise the latest MCP spec changes", "context": "..." }

A2A uses HTTP, Server-Sent Events, and JSON-RPC 2.0 — the same transport layer as MCP — but the semantics are about task coordination between peers, not tool execution by a client.

A concrete example of using both

Imagine an AI system that monitors your product's documentation, detects when it goes stale, and files a GitHub issue:

• MCP is used when the doc-monitor agent calls ask_site("docs.yourproduct.com", "What changed in the last refresh?") — that's an agent talking to a tool.
• A2A is used when the doc-monitor agent delegates the GitHub issue creation to a separate GitHub specialist agent, which is built by a different team and runs independently — that's an agent talking to another agent.

You can't do the second with MCP, and the first doesn't need A2A. They're different layers.

Side-by-side comparison

Do you need both?

If you're building a tool or service that agents consume: you need MCP. A2A is irrelevant — you're the tool layer, not the coordination layer. Build an MCP server and expose your functionality.

If you're building a single-agent application: you need MCP (to give your agent tools). You don't need A2A unless your agent needs to hand work off to another agent.

If you're building a multi-agent system: you need both. MCP for each agent's tool access. A2A for how your orchestrator delegates subtasks to specialist agents.

The ecosystem picture in mid-2026

MCP has broader adoption right now — Claude Desktop, Cursor, and most developer tooling support it natively. The 2026-07-28 release candidate is the biggest protocol revision since launch, adding stateless servers, MCP Apps, and the Tasks extension.

A2A has hit 150+ supporting organisations but is mostly in production at larger enterprise deployments. The tooling ecosystem is less mature — there's no "Claude Desktop for A2A" yet. But the spec is solid (v1.0 shipped in early 2026) and adoption is accelerating.

The two protocols are also converging technically. Both use JSON-RPC 2.0 and HTTP. The MCP Tasks extension (in the 2026 RC) covers some of the async task coordination use cases that previously required A2A. Watch for the boundary between them to blur over the next 12 months.

Quick decision guide

Start with MCP if:

• You're exposing a tool, API, or knowledge base to AI agents
• You want Claude Desktop or Cursor to be able to call your service
• You're building a single-agent app and need tool access

Add A2A when:

• You need multiple agents to coordinate on a shared task
• Your agents are built by different teams or vendors
• You need to delegate work asynchronously between independent agents