> ## Documentation Index
> Fetch the complete documentation index at: https://retrievalcontextprotocol.com/llms.txt
> Use this file to discover all available pages before exploring further.

# The agent stack

> Where RCP sits alongside MCP and ACP in a modern agent architecture — the third interoperability seam.

Modern agent architectures have three interoperability seams. RCP fills the one
that was missing.

```
  client  ⇄  ACP  ⇄  agent  ⇄  MCP  ⇄  tools
                       │
                       └──────  RCP  ⇄  retrieval engine(s)
```

<CardGroup cols={3}>
  <Card title="ACP" icon="window-maximize" href="https://agentclientprotocol.com">
    The **Agent Client Protocol**. Lets a code editor / IDE drive a coding agent.
    *(editor ⇄ agent)*
  </Card>

  <Card title="MCP" icon="plug" href="https://modelcontextprotocol.io">
    The **Model Context Protocol**. Lets an agent call tools and resources.
    *(agent ⇄ tools)*
  </Card>

  <Card title="RCP" icon="magnifying-glass" href="/get-started/introduction">
    The **Retrieval Context Protocol**. Lets any client pull grounding context
    from any RAG engine. *(agent ⇄ retrieval)*
  </Card>
</CardGroup>

## One shape, three seams

All three share JSON-RPC framing, an `initialize`/capabilities handshake, `_meta`
extensibility, and cursor-based pagination. Learn one and you are immediately
productive in the others.

|                   | ACP                 | MCP                 | RCP                            |
| ----------------- | ------------------- | ------------------- | ------------------------------ |
| **Connects**      | editor ⇄ agent      | agent ⇄ tools       | agent ⇄ retrieval              |
| **Framing**       | JSON-RPC 2.0        | JSON-RPC 2.0        | JSON-RPC 2.0                   |
| **Handshake**     | `initialize` + caps | `initialize` + caps | `initialize` + caps            |
| **Extensibility** | `_meta`             | `_meta`             | `_meta`                        |
| **Core noun**     | sessions, prompts   | tools, resources    | embed, retrieve, rerank, graph |

<Info>
  An agent runtime commonly speaks **all three at once**: ACP to its editor, MCP to
  its tools, and RCP to its knowledge bases. See
  [Relationship to MCP & ACP](/ecosystem/mcp-acp) for why retrieval earns its own
  protocol rather than riding on MCP tools.
</Info>