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Talk to the model from a Shiny app served with shinymcp

Source: R/runtime.R
mcp_model_context.Rd

In a Shiny app served as an MCP App with as_mcp_app(), the server function runs in shinymcp's live runtime. These helpers let it reach the model and the chat. In a normal Shiny session they do nothing and return FALSE, so the same app still runs with shiny::runApp().

  • mcp_model_context() sets what the model knows about this view of the app. Hosts give it to the model on its next turn. Each call replaces the last. Without it, shinymcp sends the input values and a short summary of each output. Keep it small and factual: identifiers and the numbers the user is looking at, not whole data sets.

  • mcp_send_message() posts a message into the chat as the user, which usually prompts a reply from the model. Hosts may ask the user first.

  • is_mcp_session() is TRUE when the server function is running in shinymcp's runtime.

Usage

mcp_model_context(
  text = NULL,
  data = NULL,
  session = shiny::getDefaultReactiveDomain()
)

mcp_send_message(text, session = shiny::getDefaultReactiveDomain())

is_mcp_session(session = shiny::getDefaultReactiveDomain())

Arguments

text

Text for the model (or the chat message).

data

A named list of structured data for the model.

session

The Shiny session. The default works inside a server function and in modules.

Value

mcp_model_context() and mcp_send_message() invisibly return TRUE when the message will be delivered, FALSE otherwise. is_mcp_session() returns a logical.

Shiny's own MCP support

Shiny is gaining MCP support of its own (https://github.com/rstudio/shiny/pull/4407). Once it is released, it will be the way to put a live Shiny app in a chat, and shinymcp will stop serving live apps. as_mcp_app(), mcp_endpoint(), and mcp_tool_module() will take only apps built from tools, and bindMcp() and the helpers for server functions (mcp_model_context(), mcp_host_context(), and the rest) will be removed. For something the model should be able to use on its own, rewrite that part of the app as tools with mcp_app(); see vignette("rewriting-as-tools").

See also

Other server function helpers: mcp_host_context()

Examples

server <- function(input, output, session) {
  filtered <- shiny::reactive(mtcars[mtcars$cyl == input$cyl, ])

  shiny::observe({
    mcp_model_context(
      text = paste("Showing", nrow(filtered()), "cars with", input$cyl, "cylinders."),
      data = list(cyl = input$cyl, n = nrow(filtered()))
    )
  })
}