feat: serve Shiny apps as MCP Apps (SEP-1865)

.Rbuildignore

@@ -38,3 +38,5 @@
 ^LICENSE\.md$
 ^docs$
 ^\.context$
+^mcp$
+^\.playwright-mcp$

.gitignore

@@ -24,3 +24,4 @@ madge.svg
 .claude/settings.local.json
 /docs/
 .context
+.playwright-mcp/

DESCRIPTION

@@ -113,6 +113,8 @@ Suggests:
     markdown,
     mirai,
     otelsdk (>= 0.2.0),
+    pkgload,
+    processx,
     ragg,
     reactlog (>= 1.0.0),
     rmarkdown,
@@ -177,6 +179,11 @@ Collate:
     'insert-ui.R'
     'jqueryui.R'
     'knitr.R'
+    'mcp-app.R'
+    'mcp-server.R'
+    'mcp-session.R'
+    'mcp-stdio.R'
+    'mcp-tunnel.R'
     'middleware-shiny.R'
     'middleware.R'
     'timer.R'

NAMESPACE

@@ -160,6 +160,7 @@ export(is.reactive)
 export(is.reactivevalues)
 export(is.shiny.appobj)
 export(is.singleton)
+export(isMcpSession)
 export(isRunning)
 export(isTruthy)
 export(isolate)
@@ -171,6 +172,11 @@ export(makeReactiveBinding)
 export(markRenderFunction)
 export(markdown)
 export(maskReactiveContext)
+export(mcpHostContext)
+export(mcpRequestDisplayMode)
+export(mcpSendMessage)
+export(mcpToolInput)
+export(mcpUpdateModelContext)
 export(memoryCache)
 export(modalButton)
 export(modalDialog)

NEWS.md

@@ -1,5 +1,45 @@
 # shiny (development version)
 
+* Experimental support for serving a Shiny app as an MCP App (the Model
+  Context Protocol Apps extension, SEP-1865). Setting `options(shiny.mcp =
+  TRUE)` before the app starts mounts an MCP endpoint at `/mcp` on the app's
+  own port; MCP hosts that support the Apps extension (Claude, Claude
+  Desktop, VS Code Copilot, MCPJam, ...) can then call the app's tool and
+  render the live, fully reactive application in a sandboxed iframe, with
+  Shiny's websocket traffic tunneled over the host's postMessage channel.
+  HTTP side channels (file upload/download, server-side DataTables and
+  selectize, and dynamically inserted HTML dependencies) are tunneled too,
+  the host's theme and style variables are applied to the app, and assets
+  referenced by `url()` in stylesheets (e.g. icon webfonts) are embedded as
+  data URIs so they render inside the sandbox. Customize the model-visible
+  tool with `options(shiny.mcp.tool = list(name=, description=,
+  inputSchema=))`. New session API: `isMcpSession()`, `mcpToolInput()`,
+  `mcpHostContext()`, `mcpUpdateModelContext()`, and `mcpSendMessage()` let
+  apps react to the model's tool arguments and host theme, keep the model's
+  context up to date, and send messages to the conversation. Apps can also
+  expose additional model-callable R functions with
+  `options(shiny.mcp.tools = list(list(name=, description=, inputSchema=,
+  handler=)))`. Adding `options(shiny.mcp.stdio = TRUE)` also speaks MCP
+  over stdin/stdout, so local desktop hosts can launch the app process
+  directly, e.g. `claude mcp add my-app -- Rscript -e
+  "options(shiny.mcp=TRUE, shiny.mcp.stdio=TRUE);
+  shiny::runApp('app', launch.browser=FALSE)"` (not supported on Windows;
+  stdout is reserved for the protocol). On hosts that honor the declared
+  content security policy, the app connects over a real WebSocket for
+  native-latency reactivity and falls back to the tunnel automatically
+  (disable with `options(shiny.mcp.direct = FALSE)`). The websocket base
+  URL is path-aware, so apps deployed under a sub-path (e.g. Posit
+  Connect's `/content/<guid>`) connect directly too — derived from
+  `options(shiny.mcp.origin=)`, Connect's `RStudio-Connect-App-Base-Url`
+  (or `X-RSC-Request`) header, shinyapps.io's `X-Redx-Frontend-Name`
+  header, or a
+  deployment record next to the app (rsconnect `.dcf` or Posit Publisher
+  `.toml`). Apps can also request
+  fullscreen/picture-in-picture with `mcpRequestDisplayMode()`. Setting a
+  unique `options(shiny.mcp.appId=)` per app namespaces each app's
+  internal tools and resource URI so a gateway can merge several Shiny
+  apps into a single MCP server (one connector, many apps). (#4407)
+
 * `{watcher}` is now a required dependency and is always used for autoreload file watching, so it no longer needs to be installed separately. The legacy polling-based file watcher has been removed. (#4403)
 
 # shiny 1.14.0

R/html-deps.R

@@ -24,6 +24,19 @@ createWebDependency <- function(dependency, scrubFile = TRUE) {
   if (!inherits(dependency, "html_dependency"))
     stop("Unexpected non-html_dependency type")
 
+  # Packages like {htmlwidgets} call createWebDependency() directly for
+  # render-time dependencies, bypassing processDeps(). Inside an MCP Apps
+  # sandbox those URLs can't be fetched, so divert file-based dependencies
+  # to the inline representation. (href-only dependencies fall through:
+  # mcpInlineDependency() would send them right back here.)
+  if (
+    !is.null(dependency$src$file) &&
+      is.null(dependency$src$href) &&
+      isMcpSession(getDefaultReactiveDomain())
+  ) {
+    return(mcpInlineDependency(dependency))
+  }
+
   if (is.null(dependency$src$href)) {
     prefix <- paste(dependency$name, "-", dependency$version, sep = "")
     addResourcePath(prefix, dependency$src$file)
@@ -46,9 +59,16 @@ processDeps <- function(tags, session) {
   tags <- utils::getFromNamespace("tagify", "htmltools")(tags)
   ui <- takeSingletons(tags, session$singletons, desingleton = FALSE)$ui
   ui <- surroundSingletons(ui)
+  # Sessions running inside an MCP Apps sandbox can't fetch dependency
+  # files over HTTP, so inline them instead (see mcp-app.R).
+  processDep <- if (isMcpSession(session)) {
+    mcpInlineDependency
+  } else {
+    createWebDependency
+  }
   dependencies <- lapply(
     resolveDependencies(findDependencies(ui, tagify = FALSE)),
-    createWebDependency
+    processDep
   )
   names(dependencies) <- NULL