mcplite

mcplite lets you expose R functions as Model Context Protocol (MCP) tools. It runs a small server over standard input and output, which is the transport most desktop MCP clients use to launch local tool servers.

Documentation is available at https://tosidata.github.io/mcplite/.

Use it when you want an MCP client to call R code such as data lookups, calculations, report helpers, or local 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.

Results

For most tools, return an ordinary R value. Character values become text, while other JSON-serializable values are returned as JSON in a text content block. Bare lists are ordinary results, not MCP result objects.

Use tool_result() when a tool needs rich content blocks or structured results. See ?tool_result for the result formats and compatibility details.

Scope

mcplite supports local, client-launched stdio tool servers. It does not implement HTTP transport or the MCP prompts, resources, and sampling capabilities. Tools can still return supported content blocks; content blocks are tool results, not implementations of those MCP capabilities.

ellmer interoperability

Compatible ellmer::tool() definitions can be passed directly to mcp_server(). ellmer is optional and is not needed for tools defined with mcplite.

Function reference

See ?tool for tool definitions, help("tool-types", package = "mcplite") for schema helpers, ?tool_result and help("content-blocks", package = "mcplite") for rich results, and ?mcp_server for server behavior, scope, lifecycle, interoperability, and tracing details.