Add a pluggable server extension API with MCP Apps (#3003)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

examples/stories/apps/server.py

@@ -0,0 +1,43 @@
+"""MCP Apps: a tool bound to a `ui://` resource the host renders as an interactive surface.
+
+`Apps` is an opt-in `Extension` passed to `MCPServer(extensions=[...])`. The
+`@apps.tool(resource_uri=...)` decorator stamps `_meta.ui.resourceUri` onto the
+tool; `add_html_resource` registers the matching `ui://` HTML resource. The tool
+degrades gracefully: `client_supports_apps(ctx)` reports whether the client
+negotiated Apps, so it returns text-only output otherwise.
+"""
+
+from mcp.server.apps import Apps, client_supports_apps
+from mcp.server.mcpserver import MCPServer
+from mcp.server.mcpserver.context import Context
+from stories._hosting import run_server_from_args
+
+RESOURCE_URI = "ui://get-time/app.html"
+CLOCK_HTML = """<!doctype html>
+<title>Current time</title>
+<h1 id="now">…</h1>
+<script>
+  window.addEventListener("message", (event) => {
+    const text = event.data?.result?.content?.[0]?.text;
+    if (text) document.getElementById("now").textContent = text;
+  });
+</script>
+"""
+
+
+def build_server() -> MCPServer:
+    apps = Apps()
+
+    @apps.tool(resource_uri=RESOURCE_URI, title="Get Time", description="Return the current time.")
+    def get_time(ctx: Context) -> str:
+        now = "2026-06-26T00:00:00Z"
+        if not client_supports_apps(ctx):
+            return f"The time is {now}."
+        return now
+
+    apps.add_html_resource(RESOURCE_URI, CLOCK_HTML, title="Clock")
+    return MCPServer("apps-example", extensions=[apps])
+
+
+if __name__ == "__main__":
+    run_server_from_args(build_server)