Create an MCP App from a Shiny module — mcp_tool

Wraps a standard Shiny module (UI function + server function) as an McpApp. The module UI is rendered with MCP-compatible attributes, and a tool definition is created that maps to the module's inputs and outputs. If a handler is provided, the tool is fully functional; otherwise, a stub handler is generated as a placeholder.

Usage

mcp_tool_module(
  module_ui,
  module_server,
  name,
  description,
  handler = NULL,
  arguments = NULL,
  version = "0.1.0",
  ...
)

Arguments

module_ui: A Shiny module UI function that accepts an id argument (e.g., function(id) { ns <- NS(id); tagList(...) }).
module_server: A Shiny module server function. Currently stored as metadata for future headless Shiny session support, which will allow the module server to execute reactively when tools are called.
name: Tool/app name. Used in ui:// resource URIs.
description: Human-readable description of what the tool does.
handler: Optional tool handler function. If provided, this function is called when the MCP tool is invoked. Its arguments should match the module's input IDs. If NULL, a stub handler is generated.
arguments: Optional list of ellmer::type_string(), ellmer::type_number(), etc. for the tool's input schema. If NULL, arguments are auto-detected from the rendered module UI.
version: App version string.
...: Additional arguments stored as module metadata (e.g., shared reactive values to pass to the module server when headless support lands).

Value

An McpApp object.

Details

This mirrors shinychat::chat_tool_module() for the MCP runtime — the same module can be used in both contexts.

Examples

if (FALSE) { # \dontrun{
library(shiny)

# Define a standard Shiny module
hist_ui <- function(id) {
  ns <- NS(id)
  tagList(
    sliderInput(ns("bins"), "Bins:", min = 5, max = 50, value = 25),
    plotOutput(ns("plot"), height = "250px")
  )
}

hist_server <- function(id, dataset) {
  moduleServer(id, function(input, output, session) {
    output$plot <- renderPlot({
      hist(dataset(), breaks = input$bins, col = "#007bc2")
    })
  })
}

# Create and serve as MCP App
app <- mcp_tool_module(
  hist_ui, hist_server,
  name = "histogram",
  description = "Show an interactive histogram",
  handler = function(bins = 25) {
    tmp <- tempfile(fileext = ".png")
    grDevices::png(tmp, width = 600, height = 250)
    hist(faithful$eruptions, breaks = bins, col = "#007bc2")
    grDevices::dev.off()
    list(plot = base64enc::base64encode(tmp))
  }
)
serve(app)
} # }