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

ACP

The Agent Client Protocol. Lets a code editor / IDE drive a coding agent. (editor ⇄ agent)

MCP

The Model Context Protocol. Lets an agent call tools and resources. (agent ⇄ tools)

RCP

The Retrieval Context Protocol. Lets any client pull grounding context from any RAG engine. (agent ⇄ retrieval)

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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.
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 for why retrieval earns its own protocol rather than riding on MCP tools.