A demo implementation of the MCP client protocol in R

mcp_client_demo.R

# an example server: acquaint's MCP server for R sessions
r_process <- callr::r_bg(
  function() acquaint::mcp_server(),
  stdout = "|",
  stdin = "|"
)

Sys.sleep(1)

log_cat_client <- function(x, append = TRUE) {
  log_file <- "~/mcp_client_test.txt"
  cat(x, "\n\n", sep = "", append = append, file = log_file)
}

send_and_receive <- function(process, message) {
  # send the message
  json_msg <- jsonlite::toJSON(message, auto_unbox = TRUE)
  log_cat_client(c("FROM CLIENT: ", json_msg))
  process$write_input(paste0(json_msg, "\n"))

  # poll for response
  output <- NULL
  attempts <- 0
  max_attempts <- 20

  while (length(output) == 0 && attempts < max_attempts) {
    Sys.sleep(0.2)
    output <- process$read_output_lines()
    attempts <- attempts + 1
  }

  if (!is.null(output) && length(output) > 0) {
    log_cat_client(c("FROM SERVER: ", output[1]))
    return(fromJSON(output[1]))
  }

  log_cat_client(c("ALERT: No response received after ", attempts, " attempts"))
  return(NULL)
}

# step 1: Initialize the MCP connection
initialize_request <- list(
  jsonrpc = "2.0",
  id = 1,
  method = "initialize",
  params = list(
    protocolVersion = "2024-11-05",
    capabilities = list(
      tools = list(
        listChanged = FALSE
      )
    ),
    clientInfo = list(
      name = "MCP Test Client",
      version = "0.1.0"
    )
  )
)

init_response <- send_and_receive(r_process, initialize_request)

# step 2: Send initialized notification
# For some reason, if I send this, the buffer gets tied up and the following
# tools/list isn't appropriately responded to.
# Don't expect a response here, so no `receive()`
# initialized_notification <- list(
#   jsonrpc = "2.0",
#   method = "notifications/initialized"
# )
#
# r_process$write_input(jsonlite::toJSON(initialized_notification, auto_unbox = TRUE))

# step 3: request the list of tools
tools_request <- list(
  jsonrpc = "2.0",
  id = 2,
  method = "tools/list"
)

tools_response <- send_and_receive(r_process, tools_request)

# step 4: call a tool
platform_request <- list(
  jsonrpc = "2.0",
  id = 3,
  method = "tools/call",
  params = list(
    name = "btw_tool_session_platform_info"
  )
)

platform_response <- send_and_receive(r_process, platform_request)

# clean up
r_process$kill()