mcplite

mcplite exposes R functions as Model Context Protocol (MCP) tools through a lightweight standard-input/standard-output (stdio) server. Use it when a local MCP client needs to call R code for data lookups, calculations, report helpers, or automation.

Installation

Install the released version from CRAN:

install.packages("mcplite")

Install the development version from GitHub:

pak::pak("tosidata/mcplite")

Quick start

Save a complete server definition as server.R:

library(mcplite)

add_numbers <- tool(
  function(x, y) {
    x + y
  },
  name = "add_numbers",
  description = "Add two numbers and return the result.",
  arguments = list(
    x = type_number("First number."),
    y = type_number("Second number.")
  )
)

mcp_server(list(add_numbers))

Configure your MCP client to launch the script. The command must resolve to the Rscript executable. If Rscript is not discoverable, run Sys.which("Rscript") in R and use the returned path as the client’s command. Keep the server script path absolute too. A Claude Desktop-style configuration looks like this:

{
  "mcpServers": {
    "r-tools": {
      "command": "Rscript",
      "args": [
        "--vanilla",
        "/absolute/path/to/server.R"
      ]
    }
  }
}

Restart the client and check that it discovers add_numbers. As a first call, ask it to run add_numbers with x = 2 and y = 3; the result should be 5.

mcp_server() starts serving immediately and runs until standard input closes, so launch it through the MCP client rather than from an interactive R console.

Supported scope

mcplite is a tools-only, stdio-only MCP server. It supports modern MCP 2026-07-28 and legacy MCP 2024-11-05, 2025-06-18, and 2025-11-25. The same server definition works with modern and legacy clients.

Tools can return ordinary R values, rich MCP content, structured results, and tool-level errors. Compatible ellmer::tool() definitions can also be supplied directly.

The package focuses on local, client-launched tools. It does not provide an HTTP transport, prompt or resource endpoints, or multi-round-trip MCP feature families. Tools can still include supported embedded resources and resource links in their results.

Learn more

Choose the next task:

• Get started: save a server script, connect a client, and make a first call.
• Write tools: describe arguments, handle defaults and arrays, and report failures.
• Return results: choose ordinary values, structured content, or rich content blocks.

For installed help, start with ?mcplite. See ?tool and help("tool-types", package = "mcplite") for definitions and schemas, help("mcplite-results", package = "mcplite") for exact return-value mappings and payload options, and ?mcp_server for server use, compatibility, and tracing.