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

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

docs_src/apps/report.html

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+<!doctype html>
+<title>Report</title>
+<p>Quarterly numbers render here.</p>

docs_src/apps/tutorial001.py

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+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')]

docs_src/apps/tutorial002.py

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+from mcp.server.apps import Apps, ResourceCsp, ResourcePermissions
+from mcp.server.mcpserver import MCPServer
+
+DASHBOARD_HTML = "<!doctype html><title>Dashboard</title><canvas id='chart'></canvas>"
+
+apps = Apps()
+
+
+@apps.tool(resource_uri="ui://dashboard/app.html", visibility=["app"])
+def refresh_dashboard() -> str:
+    """Refresh the dashboard data."""
+    return "refreshed"
+
+
+apps.add_html_resource(
+    "ui://dashboard/app.html",
+    DASHBOARD_HTML,
+    title="Dashboard",
+    csp=ResourceCsp(connect_domains=["https://api.example.com"]),
+    permissions=ResourcePermissions(clipboard_write={}),
+    domain="dashboard.example.com",
+    prefers_border=True,
+)
+
+mcp = MCPServer("dashboard", extensions=[apps])

docs_src/apps/tutorial003.py

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+from pathlib import Path
+
+from mcp.server.apps import Apps
+from mcp.server.mcpserver import MCPServer
+from mcp.server.mcpserver.resources import FileResource
+
+REPORT_HTML = Path(__file__).parent / "report.html"
+
+apps = Apps()
+
+
+@apps.tool(resource_uri="ui://report/app.html")
+def refresh_report() -> str:
+    """Refresh the report data."""
+    return "report refreshed"
+
+
+apps.add_resource(FileResource(uri="ui://report/app.html", name="report", path=REPORT_HTML))
+
+mcp = MCPServer("report", extensions=[apps])