fix(apps): address review of app-rendered elicitations

Eight findings from the Copilot review, all accepted.

Ownership across connections was the substantive one, in three parts. A
mid-session transport close reaches teardown only through
`clearAndAnnouncePendingPeerRequests`, whose emptiness check looked at the
native queues alone — so a lone app elicitation survived a dropped connection
with its modal still open; the check now counts the active set too. The web
bridge factory resolves its client at call time, so an entry queued by a
replaced `InspectorClient` could rebuild against the *next* one and answer
through a different server; the host now drops every entry synchronously on
the swap (`AppElicitationController.failAll`) rather than racing core's
awaited teardown. And the request id was only monotonic within one client, so
each replacement client's first request was `app-elicitation-1` — settling
that id could resolve the wrong server's request. It now carries a
per-instance prefix.

Result validation now parses the whole value with `ElicitResultSchema` before
the requested-schema check, instead of inspecting `action` by hand: a
`{ action: "decline", content: { x: {} } }` used to pass, though the standard
result permits only primitives and string arrays in `content`.

The dialog's accessible name now includes the request id and the prompt — two
concurrent elicitations can be for the same app URI, and the previous name
made those indistinguishable, which is what the comment beside it claimed to
prevent.

`observeAppCapabilities` takes a minimal structural `onmessage` contract
(generic over the message type, since a handler typed for a narrower message
is not assignable to one typed for `unknown`), removing the double cast at the
call site.

`smoke:web:elicit` now fails on an uncaught page error after a successful
drive, and captures the async half of that class off the console channel, as
its two sibling smokes do — it could otherwise print OK over a broken bundle.

Adds the tests each fix needs, including the two the review asked for
directly: that `advertiseElicitation` reaches the `AppBridge` constructor, and
that it is absent by default.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/elements/AppRenderer/appCapabilities.test.ts

@@ -70,6 +70,30 @@ describe("appCapabilities (#1854)", () => {
     expect(appAdvertisesElicitation(bridge)).toBe(false);
   });
 
+  it("ignores frames that are not shaped like ui/initialize at all", () => {
+    // The frame comes from sandboxed view code, so nothing about its shape is
+    // guaranteed — a non-object, or an initialize with no params, must not
+    // throw on the way through to the bridge's own handler.
+    const bridge = makeBridge({});
+    const { transport, inner } = makeTransport();
+    observeAppCapabilities(bridge, transport);
+    transport.onmessage?.("not-a-frame" as unknown as JSONRPCMessage);
+    transport.onmessage?.(null as unknown as JSONRPCMessage);
+    transport.onmessage?.({
+      jsonrpc: "2.0",
+      id: 1,
+      method: "ui/initialize",
+    } as unknown as JSONRPCMessage);
+    transport.onmessage?.({
+      jsonrpc: "2.0",
+      id: 2,
+      method: "ui/initialize",
+      params: null,
+    } as unknown as JSONRPCMessage);
+    expect(appAdvertisesElicitation(bridge)).toBe(false);
+    expect(inner).toHaveBeenCalledTimes(4);
+  });
+
   it("keeps bridges independent", () => {
     const a = makeBridge({});
     const b = makeBridge({});

clients/web/src/components/elements/AppRenderer/appCapabilities.ts

@@ -1,5 +1,4 @@
 import type { AppBridge } from "@modelcontextprotocol/ext-apps/app-bridge";
-import type { Transport } from "@modelcontextprotocol/client";
 
 /**
  * The app capabilities exactly as the view sent them, per bridge.
@@ -22,10 +21,41 @@ import type { Transport } from "@modelcontextprotocol/client";
  */
 const rawAppCapabilities = new WeakMap<AppBridge, Record<string, unknown>>();
 
-/** Shape of the `ui/initialize` frame this reads — nothing else is touched. */
-interface UiInitializeFrame {
-  method?: unknown;
-  params?: { appCapabilities?: unknown };
+/**
+ * The only part of a transport this needs: the inbound-message callback.
+ *
+ * Structural, and generic over the message type, on purpose. ext-apps'
+ * `PostMessageTransport` implements the SDK *v1* `Transport` — a different
+ * nominal type from the v2 client's, though runtime-identical — so naming
+ * either would force a cast at the call site. Generic rather than
+ * `(message: unknown)` because a handler typed for a narrower message is not
+ * assignable to one typed for `unknown` (contravariance), which would put the
+ * cast back; `rest` is `never[]` for the same reason, accepting any trailing
+ * parameter list without claiming to know it. The message is only ever read
+ * through {@link recordAdvertisedCapabilities}, which narrows from `unknown`.
+ */
+export interface MessageObservable<TMessage = unknown> {
+  onmessage?: (message: TMessage, ...rest: never[]) => void;
+}
+
+/**
+ * Record `params.appCapabilities` if `message` is a `ui/initialize` frame.
+ * Narrows from `unknown` rather than asserting a shape: the frame is whatever
+ * the sandboxed view posted.
+ */
+function recordAdvertisedCapabilities(
+  bridge: AppBridge,
+  message: unknown,
+): void {
+  if (typeof message !== "object" || message === null) return;
+  const frame = message as { method?: unknown; params?: unknown };
+  if (frame.method !== "ui/initialize") return;
+  if (typeof frame.params !== "object" || frame.params === null) return;
+  const advertised = (frame.params as { appCapabilities?: unknown })
+    .appCapabilities;
+  if (typeof advertised === "object" && advertised !== null) {
+    rawAppCapabilities.set(bridge, advertised as Record<string, unknown>);
+  }
 }
 
 /**
@@ -36,20 +66,14 @@ interface UiInitializeFrame {
  * wraps. That ordering is safe: the view cannot send `ui/initialize` until the
  * host has pushed its HTML into the sandbox, which happens later still.
  */
-export function observeAppCapabilities(
+export function observeAppCapabilities<TMessage>(
   bridge: AppBridge,
-  transport: Transport,
+  transport: MessageObservable<TMessage>,
 ): void {
   const inner = transport.onmessage?.bind(transport);
-  transport.onmessage = (message, extra) => {
-    const frame = message as UiInitializeFrame;
-    if (frame.method === "ui/initialize") {
-      const advertised = frame.params?.appCapabilities;
-      if (typeof advertised === "object" && advertised !== null) {
-        rawAppCapabilities.set(bridge, advertised as Record<string, unknown>);
-      }
-    }
-    inner?.(message, extra);
+  transport.onmessage = (message: TMessage, ...rest: never[]) => {
+    recordAdvertisedCapabilities(bridge, message);
+    inner?.(message, ...rest);
   };
 }
 

clients/web/src/components/elements/AppRenderer/createAppBridgeFactory.test.ts

@@ -158,6 +158,33 @@ describe("createAppBridgeFactory", () => {
     }
   });
 
+  it("does not advertise hostCapabilities.elicitation by default (#1854)", async () => {
+    // An App-tool frame is never handed an elicitation, so claiming the host
+    // capability there would tell those apps something untrue about the host.
+    const factory = createAppBridgeFactory({
+      getClient: () => fakeClient,
+      readResource: vi.fn(),
+    });
+    await factory(makeIframe(), { kind: "tool", tool });
+    expect(bridgeInstances[0].ctorArgs[2]).not.toHaveProperty("elicitation");
+  });
+
+  it("advertises hostCapabilities.elicitation when opted in (#1854)", async () => {
+    // The value reaches the AppBridge CONSTRUCTOR, which is what the bridge
+    // echoes in its `ui/initialize` response — an app reads it there to decide
+    // whether this host will forward an elicitation to it at all.
+    const factory = createAppBridgeFactory({
+      advertiseElicitation: true,
+      getClient: () => fakeClient,
+      readResource: vi.fn(),
+    });
+    await factory(makeIframe(), {
+      kind: "resource",
+      resourceUri: "ui://demo/pick.html",
+    });
+    expect(bridgeInstances[0].ctorArgs[2]).toMatchObject({ elicitation: {} });
+  });
+
   it("on sandboxready, reads the UI resource, wraps the html with the per-app CSP, and echoes the approved sandbox config", async () => {
     const readResource = vi.fn().mockResolvedValue(
       uiResource("<h1>weather</h1>", {

clients/web/src/components/elements/AppRenderer/createAppBridgeFactory.ts

@@ -8,7 +8,7 @@ import type {
   McpUiHostCapabilities,
   McpUiResourceMeta,
 } from "@modelcontextprotocol/ext-apps/app-bridge";
-import type { Client, Transport } from "@modelcontextprotocol/client";
+import type { Client } from "@modelcontextprotocol/client";
 import type {
   EmbeddedResource,
   Implementation,
@@ -373,7 +373,7 @@ export function createAppBridgeFactory(
     // Record the view's raw `ui/initialize` capabilities before the bridge's
     // own schema strips the keys it predates (#1854). Must follow `connect`,
     // which is what installs the handler this wraps.
-    observeAppCapabilities(bridge, transport as unknown as Transport);
+    observeAppCapabilities(bridge, transport);
     return bridge;
   };
 }