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feat(apps): negotiated app-rendered form elicitations

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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>
This commit is contained in:
cliffhall 2026-08-22 19:05:21 -04:00
commit 08fc20a22d
1 changed files with 3718 additions and 93 deletions
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21
core/mcp/extensions.ts
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@ -98,6 +98,19 @@ export interface BuildClientExtensionsInput {
* over the registry's `defaultAdvertised`; an absent key falls back to it.
*/
advertised?: Record<string, boolean>;
/**
* True when this client can render an MCP App and resolve an
* `elicitation/create` request through its bridge (#1854). Adds the nested
* `elicitation` setting to the UI extension's advertisement, which is half of
* the negotiation a server checks before attaching an App to an elicitation.
*
* Deliberately an input rather than a registry default: the shared
* `InspectorClient` knowing the MCP Apps MIME type says nothing about whether
* the *client* has a sandbox renderer, so CLI and TUI must never advertise it.
* Ignored when the UI extension itself is not advertised — a nested setting on
* an extension we did not declare would be meaningless.
*/
appElicitation?: boolean;
}
/**
@ -126,6 +139,14 @@ export function buildClientExtensions(
: {};
}
}
// Nested app-rendered-elicitation opt-in (#1854), layered onto the UI
// extension's own advertisement rather than added as a second extension.
// Guarded on the UI entry actually being present so turning the Apps
// extension off in Server Settings also turns this off.
const uiAdvertisement = map[UI_EXTENSION_KEY];
if (input.appElicitation && uiAdvertisement) {
map[UI_EXTENSION_KEY] = { ...uiAdvertisement, elicitation: {} };
}
if (input.enterpriseManaged) {
map[EMA_EXTENSION_KEY] = {};
}