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

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

docs_src/apps/tutorial001.py

@@ -0,0 +1,38 @@
+from mcp import Client
+from mcp.server.apps import APP_MIME_TYPE, EXTENSION_ID, Apps, client_supports_apps
+from mcp.server.mcpserver import MCPServer
+from mcp.server.mcpserver.context import Context
+
+CLOCK_HTML = """\
+<!doctype html>
+<title>Clock</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>
+"""
+
+apps = Apps()
+
+
+@apps.tool(resource_uri="ui://clock/app.html", description="The current time.")
+def get_time(ctx: Context) -> str:
+    now = "2026-06-26T12:00:00Z"
+    if not client_supports_apps(ctx):
+        return f"The time is {now}."
+    return now
+
+
+apps.add_html_resource("ui://clock/app.html", CLOCK_HTML, title="Clock")
+
+mcp = MCPServer("clock", extensions=[apps])
+
+
+async def main() -> None:
+    async with Client(mcp, extensions={EXTENSION_ID: {"mimeTypes": [APP_MIME_TYPE]}}) as client:
+        result = await client.call_tool("get_time", {})
+        print(result.content)
+        # [TextContent(text='2026-06-26T12:00:00Z')]