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import {
AppBridge,
PostMessageTransport,
buildAllowAttribute,
type McpUiResourceCsp,
type McpUiResourcePermissions,
} from "@modelcontextprotocol/ext-apps/app-bridge";
import type { McpUiStyles } from "@modelcontextprotocol/ext-apps";
import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
import { grackleHostStyleVariables } from "../../utils/grackleHostStyleVariables.js";
/** Notification method the outer sandbox proxy posts once it is ready. */
const SANDBOX_PROXY_READY: string = "ui/notifications/sandbox-proxy-ready";
/** Default host identity reported to widgets during the MCP Apps handshake. */
const DEFAULT_HOST_INFO: Readonly<{ name: string; version: string }> = {
name: "Grackle",
version: "0.0.0",
};
/** URL schemes the default link handler is permitted to open. */
const SAFE_LINK_PROTOCOLS: ReadonlySet<string> = new Set(["http:", "https:"]);
/** True if `url` parses as an `http(s)` URL safe to open in a new browsing context. */
function isSafeHttpUrl(url: string): boolean {
try {
return SAFE_LINK_PROTOCOLS.has(new URL(url).protocol);
} catch {
return false;
}
}
/** A tool call the widget asked the host to run on its behalf. */
export interface McpAppWidgetCallToolParams {
name: string;
arguments?: Record<string, unknown>;
}
/**
* Props for {@link McpAppWidget}. Presentational only — all data and side
* effects arrive as props (no `useGrackle()`).
*/
export interface McpAppWidgetProps {
/** Widget HTML (`text/html;profile=mcp-app` contents) rendered in the sandbox. */
widgetHtml: string;
/** URL of the sandbox proxy, served from a DIFFERENT origin than the host. */
sandboxProxyUrl: string;
/** Tool input delivered to the widget via `ui/notifications/tool-input`. */
toolInput?: Record<string, unknown>;
/** Tool result delivered to the widget via `ui/notifications/tool-result`. */
toolResult?: CallToolResult;
/** Content-Security-Policy domains for the widget (from the resource `_meta.ui`). */
csp?: McpUiResourceCsp;
/** Permissions to grant the widget iframe (from the resource `_meta.ui`). */
permissions?: McpUiResourcePermissions;
/** Style variables for `hostContext.styles`; defaults to Grackle's live theme. */
hostStyleVariables?: McpUiStyles;
/** Color theme reported to the widget; defaults to the document's `data-theme`. */
theme?: "light" | "dark";
/** Host identity reported to the widget; defaults to {@link DEFAULT_HOST_INFO}. */
hostInfo?: { name: string; version: string };
/** Handle a tool call the widget requests (no MCP client is wired in T1). */
onCallTool?: (params: McpAppWidgetCallToolParams) => Promise<CallToolResult>;
/** Handle a link the widget asks the host to open. */
onOpenLink?: (url: string) => void;
/** Handle a message the widget asks the host to surface. */
onSendMessage?: (message: unknown) => void;
/** Handle a model-context update pushed by the widget. */
onUpdateModelContext?: (context: unknown) => void;
/** Notified when the widget requests a new size. */
onSizeChange?: (size: { width?: number; height?: number }) => void;
}
/** Resolve the active theme, preferring the prop, then the document, then light. */
function resolveTheme(theme: McpAppWidgetProps["theme"]): "light" | "dark" {
if (theme) {
return theme;
}
if (typeof document !== "undefined") {
const attr: string | null = document.documentElement.getAttribute("data-theme");
if (attr === "dark") {
return "dark";
}
}
return "light";
}
/**
* Point the iframe at the sandbox proxy and resolve once the proxy reports
* ready. Returns `false` if the iframe was already loaded (guards against an
* accidental double-invoke) or if `signal` aborts before the proxy responds.
*
* The ready listener is bound to `signal` so it is removed on effect cleanup,
* and only messages from the proxy window AND its expected origin are accepted.
*/
function loadSandboxProxy(
iframe: HTMLIFrameElement,
sandboxProxyUrl: string,
proxyOrigin: string,
csp: McpUiResourceCsp | undefined,
permissions: McpUiResourcePermissions | undefined,
signal: AbortSignal,
): Promise<boolean> {
Iif (iframe.src) {
return Promise.resolve(false);
}
iframe.setAttribute("sandbox", "allow-scripts allow-same-origin allow-forms");
const allow: string = buildAllowAttribute(permissions);
Iif (allow) {
iframe.setAttribute("allow", allow);
}
const ready: Promise<boolean> = new Promise<boolean>((resolve) => {
const listener = (event: MessageEvent): void => {
const data: { method?: string } | undefined = event.data as { method?: string } | undefined;
if (
event.source === iframe.contentWindow &&
event.origin === proxyOrigin &&
data?.method === SANDBOX_PROXY_READY
) {
resolve(true);
}
};
// `{ signal }` removes the listener automatically on effect cleanup, so it
// never outlives the component even if the proxy never responds.
window.addEventListener("message", listener, { signal });
signal.addEventListener("abort", () => resolve(false), { once: true });
});
const url: URL = new URL(sandboxProxyUrl, window.location.href);
Iif (csp) {
url.searchParams.set("csp", JSON.stringify(csp));
}
iframe.src = url.href;
return ready;
}
/** Swallow a settled promise's rejection (the host outlives transient widget errors). */
function ignoreRejection(promise: Promise<unknown> | void): void {
Promise.resolve(promise).catch(() => undefined);
}
/**
* Renders an MCP Apps widget (untrusted HTML) inside a double-iframe sandbox and
* drives it over the `ext-apps` `AppBridge` postMessage protocol.
*
* The Grackle chat pane is not an MCP client, so this constructs the bridge with
* `null` (no client) and serves tool input/result from props. Wiring the bridge
* to a live MCP server is a later ticket (#1238).
*/
export function McpAppWidget(props: McpAppWidgetProps): JSX.Element {
const {
widgetHtml,
sandboxProxyUrl,
csp,
permissions,
hostStyleVariables,
theme,
hostInfo,
toolInput,
toolResult,
onCallTool,
onOpenLink,
onSendMessage,
onUpdateModelContext,
onSizeChange,
} = props;
// Fail fast on the isolation-breaking misconfiguration: the sandbox proxy MUST
// be a different origin than the host, otherwise `window.top` is reachable and
// the double-iframe guarantee is void.
if (
typeof window !== "undefined" &&
new URL(sandboxProxyUrl, window.location.href).origin === window.location.origin
) {
throw new Error(
"McpAppWidget: sandboxProxyUrl must be served from a DIFFERENT origin than the host.",
);
}
const iframeRef = useRef<HTMLIFrameElement>(null);
const bridgeRef = useRef<AppBridge | undefined>(undefined);
// Flips true once the ui/initialize handshake completes; the tool-input/result
// effects below key on it so late-arriving props still reach the widget.
const [initialized, setInitialized] = useState<boolean>(false);
// Keep the latest callbacks in a ref so the setup effect can be keyed only on
// the widget identity (re-running it would reload the iframe).
const handlers = useRef({
onCallTool,
onOpenLink,
onSendMessage,
onUpdateModelContext,
onSizeChange,
});
handlers.current = { onCallTool, onOpenLink, onSendMessage, onUpdateModelContext, onSizeChange };
// csp/permissions are part of the widget identity (they affect the proxy URL
// and sendSandboxResourceReady), so the iframe must reload when they change.
const cspKey: string = JSON.stringify(csp ?? null);
const permissionsKey: string = JSON.stringify(permissions ?? null);
useEffect(() => {
const iframe: HTMLIFrameElement | null = iframeRef.current;
Iif (!iframe) {
return undefined;
}
// Origin of the proxy, used to validate inbound proxy messages. The
// different-origin invariant is enforced at render time (see above).
const proxyOrigin: string = new URL(sandboxProxyUrl, window.location.href).origin;
const abortController: AbortController = new AbortController();
const { signal } = abortController;
// Read abort state through a function so TS flow-analysis can't narrow it to
// a constant across the awaits below (cleanup flips it asynchronously).
const isAborted = (): boolean => signal.aborted;
let resizeObserver: ResizeObserver | undefined;
const run = async (): Promise<void> => {
const firstTime: boolean = await loadSandboxProxy(
iframe,
sandboxProxyUrl,
proxyOrigin,
csp,
permissions,
signal,
);
Eif (!firstTime || isAborted()) {
return;
}
const bridge = new AppBridge(
null,
hostInfo ?? DEFAULT_HOST_INFO,
{ openLinks: {}, updateModelContext: { text: {} } },
{
hostContext: {
theme: resolveTheme(theme),
platform: "web",
styles: { variables: hostStyleVariables ?? grackleHostStyleVariables() },
containerDimensions: { maxHeight: 6000 },
displayMode: "inline",
availableDisplayModes: ["inline"],
},
},
);
bridgeRef.current = bridge;
bridge.oncalltool = async (params): Promise<CallToolResult> => {
const handler = handlers.current.onCallTool;
if (handler) {
return handler({ name: params.name, arguments: params.arguments });
}
return {
isError: true,
content: [{ type: "text", text: "No MCP client is connected to this host." }],
};
};
bridge.onopenlink = async ({ url }): Promise<Record<string, never>> => {
const handler = handlers.current.onOpenLink;
if (handler) {
handler(url);
} else if (isSafeHttpUrl(url)) {
// Only http(s) — never javascript:/data: from an untrusted widget.
window.open(url, "_blank", "noopener,noreferrer");
}
return {};
};
bridge.onmessage = async (params): Promise<Record<string, never>> => {
handlers.current.onSendMessage?.(params);
return {};
};
bridge.onupdatemodelcontext = async (params): Promise<Record<string, never>> => {
handlers.current.onUpdateModelContext?.(params);
return {};
};
bridge.onsizechange = ({ width, height }): void => {
if (width !== undefined) {
iframe.style.minWidth = `min(${width}px, 100%)`;
}
if (height !== undefined) {
iframe.style.height = `${height}px`;
}
handlers.current.onSizeChange?.({ width, height });
};
bridge.onrequestdisplaymode = async (): Promise<{ mode: "inline" }> => ({ mode: "inline" });
const handshakeComplete: Promise<void> = new Promise<void>((resolve) => {
bridge.oninitialized = (): void => resolve();
});
await bridge.connect(
new PostMessageTransport(iframe.contentWindow as Window, iframe.contentWindow as Window),
);
if (isAborted()) {
return;
}
await bridge.sendSandboxResourceReady({ html: widgetHtml, csp, permissions });
await handshakeComplete;
if (isAborted()) {
return;
}
// Tool input/result are delivered (and kept in sync) by the effects below
// now that the handshake is complete.
setInitialized(true);
resizeObserver = new ResizeObserver(([entry]) => {
const width: number = Math.round(entry.contentRect.width);
if (width > 0) {
ignoreRejection(
bridge.sendHostContextChange({ containerDimensions: { width, maxHeight: 6000 } }),
);
}
});
resizeObserver.observe(iframe);
};
run().catch(() => undefined);
return (): void => {
abortController.abort();
setInitialized(false);
resizeObserver?.disconnect();
const bridge: AppBridge | undefined = bridgeRef.current;
bridgeRef.current = undefined;
Iif (bridge) {
ignoreRejection(bridge.teardownResource({}));
}
iframe.removeAttribute("src");
};
// Setup is keyed on the widget identity (html + proxy URL + csp/permissions).
// Tool input/result and theme updates flow via the dedicated effects below.
}, [widgetHtml, sandboxProxyUrl, cspKey, permissionsKey]);
// Deliver tool input after the handshake, and whenever it changes afterwards.
useEffect(() => {
Eif (!initialized) {
return;
}
ignoreRejection(bridgeRef.current?.sendToolInput({ arguments: toolInput ?? {} }));
}, [initialized, toolInput]);
// Deliver the tool result once it is available, and whenever it changes. The
// host commonly renders the widget before the result exists.
useEffect(() => {
Eif (!initialized || !toolResult) {
return;
}
ignoreRejection(bridgeRef.current?.sendToolResult(toolResult));
}, [initialized, toolResult]);
// Propagate theme + style-variable changes to a live widget. When
// hostStyleVariables is omitted, the Grackle token values themselves change
// with the theme, so the resolved variables are re-sent too.
useEffect(() => {
ignoreRejection(
bridgeRef.current?.sendHostContextChange({
theme: resolveTheme(theme),
styles: { variables: hostStyleVariables ?? grackleHostStyleVariables() },
}),
);
}, [theme, hostStyleVariables]);
return (
<iframe
ref={iframeRef}
data-testid="mcp-app-widget"
title="MCP App widget"
style={{ width: "100%", border: "none", display: "block" }}
/>
);
}
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