# A Reference Architecture — MCP & Agent-to-Agent Protocols

Source: https://www.geekswithgeeks.com/en/mcp-a2a/t-arch

> Place MCP servers, agents and gateways in one picture.

## Agents as both hosts and peers

A practical enterprise layout: a **front-door agent** talks to the user and acts as an **MCP host** for its own tools (search, ticketing, internal APIs via MCP servers) and as an **A2A client** that delegates specialised, long-running work (billing, legal review, logistics) to **remote agents** owned by other teams. Each remote agent has its own MCP servers behind it. Between boundaries sit **gateways** that enforce authentication, rate limits, logging and policy, and a **registry** lists approved MCP servers and Agent Cards. Keep **identity** consistent (who is the end user?) by passing delegated, scoped credentials rather than shared super-keys, and keep **context minimal** across each hop.

## Tools down, peers across

A real system uses MCP for tool access and A2A for collaboration, plus common engineering discipline.

![Four concerns: architecture, testing, governance, evolution.](assets/figures/mcp-a2a/section-7-map.svg) — Figure 7.1 — Architecture, testing, governance and evolution.

## The picture

Vertical links are MCP; horizontal links are A2A.

```text
user -> [ Front-door agent ] ---- A2A (via gateway) ----> [ Billing agent ] -- MCP --> invoices DB
                |                                           [ Legal agent   ] -- MCP --> contracts
                | MCP (host)
                +--> search server   +--> ticketing server   +--> internal API server

gateway: authN/authZ, rate limits, logging, redaction     registry: approved servers + Agent Cards
identity: end-user identity -> delegated, scoped tokens per hop (never a shared super-key)
```

**Quiz:** In this architecture, what do the A2A links connect?

- [x] Agents to other agents
- [ ] Agents to databases directly
- [ ] Browsers to GPUs
- [ ] Models to tokenizers

*Answer:* Agents to other agents. A2A is the agent-to-agent layer; MCP reaches tools and data.