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RCP is one open, versioned JSON-RPC protocol so any RAG engine — any language, any vendor — can expose embed, rerank, retrieve, graph, and index, and any client can consume it uniformly. It is the retrieval companion to MCP and ACP, small enough for a thin client to fully consume and expressive enough for a frontier engine to fully express.

Get started

What RCP is, the problem it solves, and a working engine in five minutes.

Read the protocol

The wire format, capabilities, methods, and errors — one concept per page.

Pick an SDK

Native C++, Python, Node.js, and Rust SDKs that interoperate byte-for-byte.

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Why RCP

Every RAG stack reinvents the same wire: embed, rerank, retrieve, filter, cite. MCP standardised tools; ACP standardised agents; nothing standardised the retrieval layer that grounds them. RCP is that layer.

Frontier-native

Hybrid dense + sparse search, ColBERT/ColPali late interaction (MaxSim), rerankers, MMR diversification, GraphRAG, agentic multi-hop, citations, and metadata filtering are all first-class — not bolted on.

Transport-free

JSON-RPC 2.0 over newline-delimited stdio or HTTP. If it can read a line and parse JSON, it can speak RCP.

Capability-negotiated

A server advertises exactly what it can do; clients gate calls on capabilities and fail fast, client-side, before any I/O.

Composable

A registry (rcp.json) lets a client target one backend (Selector) or fuse many (Federation) with Reciprocal Rank Fusion.

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The missing third protocol

An agent runtime speaks ACP to its client, calls tools over MCP, and fetches grounding context over RCP:
All three share JSON-RPC framing, an initialize/capabilities handshake, _meta extensibility, and cursor pagination — learn one, you know the shape of all three. RCP isn’t a competitor to MCP — it’s the retrieval sibling. (Why not just an MCP tool?)

The retrieval pipeline

Transform → recall → rerank → diversify → cite, each stage independently negotiable behind a single retrieve call.

Why not just an MCP tool?

Six axes where retrieval outgrows an opaque search tool — and how the two protocols compose.

Conformance levels

L0 Base, L1 Retrieval, L2 SOTA — with a transport-agnostic test suite that validates any server in any language.