feat(apps): negotiated app-rendered form elicitations

Let a server hand a form `elicitation/create` to an MCP App and return the
app's ordinary `ElicitResult`, with the native elicitation form as the
fallback on every failure.

No second extension, no custom method, and no custom result shape: the only
new wire surface is a nested `elicitation` flag on the existing
`io.modelcontextprotocol/ui` extension on each side, plus
`_meta.ui.resourceUri` on the request. App rendering is selected only when
all four gates hold — client `elicitation.form`, client MCP Apps MIME type,
the nested `elicitation` setting on BOTH peers, and a valid absolute `ui://`
URI on the request.

Only the web client advertises the nested client-side setting, and only
because it has a sandbox renderer: supplying
`InspectorClientOptions.appElicitation` is what opts a client in, so CLI and
TUI keep advertising the MIME type without ever claiming they can resolve an
elicitation through an app.

Routing lives in the one funnel both entry points already use
(`enqueuePendingElicitation`), so the inbound handler and the MRTR driver
cannot diverge. Ownership is request-scoped — a per-request id keys the
renderer, iframe and bridge — so two concurrent elicitations can never
resolve through each other's bridges. An explicit `decline`/`cancel` is a
completed elicitation and goes back to the server; everything else (absent or
malformed metadata, resource/sandbox/bridge failure, an app with no
elicitation capability, a timeout, an invalid result or one that fails the
requested schema) falls back.

The Inspector speaks the ext-apps#733 / SEP-3118 wire protocol but cannot yet
consume its helpers — the released `@modelcontextprotocol/ext-apps` (1.7.5)
predates that PR. `core/mcp/appElicitation.ts` and
`AppRenderer/requestAppElicitation.ts` mirror it exactly and are marked for
deletion once a release containing it ships. One consequence needed its own
seam: ext-apps 1.7.5 parses the view's `ui/initialize` through a schema that
strips `elicitation`, so an app that correctly advertises it would look like
one that did not, silently turning every negotiated elicitation into a
fallback. `AppRenderer/appCapabilities.ts` records the raw frame instead, and
prefers the bridge's own value once it carries the key.

`AppRenderer` now takes an `AppRenderSource` union rather than a `Tool`, so
the same renderer and bridge factory serve an App tool and an elicitation
without either faking the other's shape.

Adds the public fixture (`app_choose_option` + a self-contained
`ui://demo/choose-option.html` app covering accept/decline/cancel), its two
showcase configs, and `smoke:web:elicit`, which drives the negotiated path
end to end in headless Chromium and then the same tool against a server that
never advertised the capability, asserting the native form takes it.

Closes #1854

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

core/mcp/inspectorClient.ts

@@ -132,6 +132,13 @@ import {
   type ModernDetailedTask,
 } from "./modernTaskSchemas.js";
 import { buildClientExtensions } from "./extensions.js";
+import {
+  getElicitationUiResourceUri,
+  isFormElicitation,
+  supportsAppElicitation,
+  validateAppElicitResult,
+  type AppElicitationRenderer,
+} from "./appElicitation.js";
 import {
   EmptyResultSchema,
   CallToolResultSchema,
@@ -562,6 +569,27 @@ export class InspectorClient extends InspectorClientEventTarget {
   // Per-extension advertise overrides (#1738); undefined key falls back to the
   // registry default in ADVERTISABLE_EXTENSIONS.
   private readonly advertisedExtensions?: Record<string, boolean>;
+  /**
+   * Host-supplied renderer for app-rendered form elicitations (#1854), or
+   * undefined on a client that cannot host MCP Apps. Its presence is what
+   * advertises the nested MCP Apps `elicitation` capability, so this is the one
+   * fact both the advertisement and the routing gate read.
+   */
+  private readonly appElicitationRenderer?: AppElicitationRenderer;
+
+  /**
+   * Monotonic counter behind the request-scoped id handed to the renderer. The
+   * association MUST be per request (a map keyed by this id owns the resource
+   * URI, renderer instance and promise on the host side) so two concurrent
+   * elicitations cannot resolve through each other's bridges.
+   */
+  private appElicitationSeq = 0;
+  /**
+   * Abort controllers for app-rendered elicitations still awaiting an answer.
+   * Aborted alongside the native pending queue on disconnect, so a rendered app
+   * cannot outlive the connection that asked for it.
+   */
+  private activeAppElicitations = new Set<AbortController>();
   private receiverTaskTtlMs: number | (() => number);
   private receiverTaskRecords: Map<string, ReceiverTaskRecord> = new Map();
   // OAuth support (config owned by oauthManager; client delegates and uses !!oauthManager for "is OAuth configured")
@@ -616,6 +644,7 @@ export class InspectorClient extends InspectorClientEventTarget {
     this.elicit = options.elicit ?? true;
     this.receiverTasks = options.receiverTasks ?? false;
     this.advertisedExtensions = options.advertisedExtensions;
+    this.appElicitationRenderer = options.appElicitation;
     this.receiverTaskTtlMs = options.receiverTaskTtlMs ?? 60_000;
     this.progress = options.progress ?? true;
     this.resetTimeoutOnProgress = options.resetTimeoutOnProgress ?? true;
@@ -785,6 +814,14 @@ export class InspectorClient extends InspectorClientEventTarget {
     const advertisedExtensions = buildClientExtensions({
       enterpriseManaged: options.oauth?.enterpriseManaged ?? false,
       advertised: this.advertisedExtensions,
+      // Read off the built `capabilities.elicitation.form` rather than
+      // re-deriving from `options.elicit`: the nested MCP Apps `elicitation`
+      // setting must never be advertised without the core form capability it
+      // extends, and two derivations of the same fact can drift. Disabling form
+      // elicitation therefore drops both, as the contract requires. (#1854)
+      appElicitation:
+        this.appElicitationRenderer !== undefined &&
+        capabilities.elicitation?.form !== undefined,
     });
     if (Object.keys(advertisedExtensions).length > 0) {
       capabilities.extensions = {
@@ -1688,6 +1725,13 @@ export class InspectorClient extends InspectorClientEventTarget {
       elicitation.cancel();
     }
     this.pendingElicitations = [];
+    // App-rendered elicitations (#1854) are not in the queue above — they live
+    // in the host's renderer — so abort them here on the same teardown paths.
+    // `tryAppElicitation` removes each controller in its own `finally`.
+    for (const controller of this.activeAppElicitations) {
+      controller.abort();
+    }
+    this.activeAppElicitations.clear();
   }
 
   /**
@@ -2820,11 +2864,17 @@ export class InspectorClient extends InspectorClientEventTarget {
    * corresponding `ElicitResult` (echoed to the server on retry); only a
    * genuine failure or a `signal` abort rejects.
    */
-  private enqueuePendingElicitation(
+  private async enqueuePendingElicitation(
     request: ElicitRequest,
     origin: PendingRequestOrigin,
     signal?: AbortSignal,
   ): Promise<ElicitResult> {
+    // App-rendered form elicitation (#1854) is offered first and falls back to
+    // the native queue below on every failure. Both entry points — the inbound
+    // `elicitation/create` handler and the MRTR driver's embedded requests —
+    // funnel through here, so neither can miss the routing.
+    const appResult = await this.tryAppElicitation(request, signal);
+    if (appResult) return appResult;
     // See {@link enqueuePendingSample} — Promise settle is idempotent.
     return new Promise<ElicitResult>((resolvePromise, rejectPromise) => {
       const elicitation = new ElicitationCreateMessage(
@@ -2842,6 +2892,92 @@ export class InspectorClient extends InspectorClientEventTarget {
     });
   }
 
+  /**
+   * Attempt to resolve an `elicitation/create` request by rendering the MCP App
+   * the server attached to it (#1854), returning the app's standard
+   * `ElicitResult`.
+   *
+   * Returns `null` for "not app-rendered — use the native UI", which covers
+   * every negotiation gate and every failure mode the contract lists: either
+   * peer did not negotiate the capability, the metadata is absent or unusable,
+   * the mode is not `form`, the renderer failed (resource read, sandbox/bridge
+   * init, missing app capability, timeout), or the app returned something that
+   * is not a valid result for this request. An explicit `decline` or `cancel`
+   * is a *completed* elicitation and is returned, not fallen back on.
+   *
+   * The one case that does not fall back is an abort of the originating
+   * request: the caller is cancelling the whole elicitation, so re-opening it
+   * in the native queue would resurrect work the user just abandoned.
+   */
+  private async tryAppElicitation(
+    request: ElicitRequest,
+    signal?: AbortSignal,
+  ): Promise<ElicitResult | null> {
+    const renderer = this.appElicitationRenderer;
+    if (!renderer) return null;
+    // Gate 1-3 (client) + 4 (server), from the negotiated capabilities of this
+    // connection — `this.capabilities` is populated at initialize.
+    if (!supportsAppElicitation(this.clientCapabilities, this.capabilities)) {
+      return null;
+    }
+    // Only form mode is app-renderable; `url` keeps its existing path.
+    if (!isFormElicitation(request.params)) return null;
+    let resourceUri: string | undefined;
+    try {
+      resourceUri = getElicitationUiResourceUri(request.params);
+    } catch (error) {
+      this.logger.warn(
+        { error },
+        "Elicitation carried unusable _meta.ui.resourceUri; using the native elicitation UI",
+      );
+      return null;
+    }
+    if (!resourceUri) return null;
+
+    // Request-scoped abort: forwards the caller's signal (MRTR cancellation)
+    // and is aborted by `settleAndDropPendingPeerRequests` on disconnect, so a
+    // rendered app cannot outlive the connection that asked for it.
+    const controller = new AbortController();
+    const forwardAbort = () => controller.abort(signal?.reason);
+    if (signal) {
+      if (signal.aborted) forwardAbort();
+      else signal.addEventListener("abort", forwardAbort, { once: true });
+    }
+    this.activeAppElicitations.add(controller);
+    try {
+      const result = await renderer({
+        requestId: `app-elicitation-${++this.appElicitationSeq}`,
+        resourceUri,
+        params: request.params,
+        signal: controller.signal,
+      });
+      this.outputValidator ??= new AjvJsonSchemaValidator();
+      const invalid = validateAppElicitResult(
+        this.outputValidator,
+        request.params,
+        result,
+      );
+      if (invalid) {
+        this.logger.warn(
+          { resourceUri, reason: invalid },
+          "App-rendered elicitation returned an invalid result; using the native elicitation UI",
+        );
+        return null;
+      }
+      return result;
+    } catch (error) {
+      if (controller.signal.aborted) throw createPendingAbortError();
+      this.logger.warn(
+        { error, resourceUri },
+        "App-rendered elicitation failed; using the native elicitation UI",
+      );
+      return null;
+    } finally {
+      this.activeAppElicitations.delete(controller);
+      signal?.removeEventListener("abort", forwardAbort);
+    }
+  }
+
   /**
    * Reject a still-pending request when `signal` aborts (e.g. the user cancels
    * the tool call while its MRTR round is awaiting an answer). No-op when