Preview an MCP App in a web browser — preview

Starts a local HTTP server and opens the MCP App in a browser. A lightweight host page emulates the MCP Apps postMessage protocol so that tools are fully functional — inputs trigger tool calls, and outputs update in real time, just like they would inside Claude Desktop.

Usage

preview_app(app, port = NULL, launch = TRUE)

Arguments

app: An McpApp object, or a path to a directory containing an MCP App app.R (which will be source()d to obtain the app object).
port: Port for the local server. NULL (the default) picks a random available port.
launch: Whether to open the browser automatically (default TRUE).

Value

Invisibly, a list with url (the preview URL) and stop() (a function to shut down the server).

Examples

if (FALSE) { # \dontrun{
app <- mcp_app(
  ui = htmltools::tags$div(
    mcp_text_input("name", "Your name"),
    mcp_text("greeting")
  ),
  tools = list(
    list(
      name = "greet",
      fun = function(name = "world") {
        list(greeting = paste0("Hello, ", name, "!"))
      }
    )
  )
)

# Opens in browser with working inputs/outputs
srv <- preview_app(app)

# Stop when done
srv$stop()
} # }