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fix(web): let a malformed _meta.ui stop the carrier lookup

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uiMeta returned undefined for both "this bag declares no ui" and "its ui
is unusable", so a content item declaring `ui: null` fell through and was
granted whatever a BROADER carrier asked for — wrong precedence, wrong
direction to fail in. It now distinguishes the two: no `ui` key means
keep looking, a declared-but-malformed one decides for "no hints".

Also pin the full three-way precedence with a case carrying all three
carriers at once (no prior case would have caught the first two operands
being swapped), and state why a permission's object value need not be
empty: `{}` is the spec's extension point, so rejecting a populated one
would drop grants from servers written against a newer spec.

Signed-off-by: cliffhall <cliff@futurescale.com>
This commit is contained in:
cliffhall 2026-08-23 22:05:23 -04:00
commit 53943c2ef8
1 changed files with 117 additions and 29 deletions
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54
clients/web/src/components/elements/AppRenderer/createAppBridgeFactory.ts
View file

@ -111,21 +111,31 @@ export interface AppBridgeFactoryDeps {
}
/**
* The MCP Apps sandbox hints (`csp`, `permissions`, …) nested under a `_meta`
* bag's `ui` key. `McpUiResourceMeta` describes the value of `_meta.ui`, NOT
* `_meta` itself — reading `_meta` directly leaves every field `undefined`, so
* a spec-conforming app's declared `connectDomains` never reach the generated
* CSP and it renders under `connect-src 'none'` (#2055). This mirrors the tool
* side, where `getToolUiResourceUri` already reads `_meta.ui.resourceUri`.
* The MCP Apps sandbox hints (`csp`, `permissions`, …) a single `_meta` bag
* declares under its `ui` key, or undefined when that bag declares none.
*
* The two "no hints" cases are deliberately distinguished by the return type:
* a bag with no `ui` key at all returns undefined (this carrier is silent —
* keep looking), while a bag whose `ui` is malformed returns `{ meta:
* undefined }` (this carrier spoke, and what it said is unusable — stop). Were
* they conflated, a content item declaring `ui: null` would fall through to a
* broader carrier and be granted whatever *that* one asked for, which is both
* the wrong precedence and the wrong direction to fail in.
*
* `McpUiResourceMeta` describes the value of `_meta.ui`, NOT `_meta` itself —
* reading `_meta` directly leaves every field undefined, so a spec-conforming
* app's declared `connectDomains` never reach the generated CSP and it renders
* under `connect-src 'none'` (#2055). This mirrors the tool side, where
* `getToolUiResourceUri` already reads `_meta.ui.resourceUri`.
*/
function uiMeta(meta: object | undefined): McpUiResourceMeta | undefined {
// `_meta` is typed with an index signature, which has no properties in common
// with a `{ ui?: unknown }` parameter — hence the cast rather than a narrower
// parameter type. Reading one key off an untyped bag is safe.
const ui = (meta as { ui?: unknown } | undefined)?.ui;
return ui !== null && typeof ui === "object"
? (ui as McpUiResourceMeta)
: undefined;
function uiMeta(
bag: object | undefined,
): { meta: McpUiResourceMeta | undefined } | undefined {
if (bag === undefined || !("ui" in bag)) return undefined;
const ui: unknown = (bag as { ui: unknown }).ui;
const wellFormed =
ui !== null && typeof ui === "object" && !Array.isArray(ui);
return { meta: wellFormed ? (ui as McpUiResourceMeta) : undefined };
}
/**
@ -140,9 +150,11 @@ function uiMeta(meta: object | undefined): McpUiResourceMeta | undefined {
* documents as the static default a host reviews at connection time, and
* which a read content item explicitly takes precedence over.
*
* Whichever is found first wins outright — the levels are not merged, since a
* server that restates only `csp` at the more specific level means that to be
* the whole grant, not a patch over the broader one.
* The first carrier that *declares* a `ui` key decides, and its value is used
* outright — the levels are not merged, since a server that restates only
* `csp` at the more specific level means that to be the whole grant, not a
* patch over the broader one. A carrier declaring a malformed `ui` decides
* too, for "no hints"; see {@link uiMeta}.
*/
function extractHtmlAndMeta(
result: ReadResourceResult,
@ -154,11 +166,9 @@ function extractHtmlAndMeta(
for (const content of result.contents) {
const text = (content as { text?: unknown }).text;
if (typeof text === "string") {
return {
html: text,
meta:
uiMeta(content._meta) ?? uiMeta(result._meta) ?? uiMeta(listedMeta),
};
const carried =
uiMeta(content._meta) ?? uiMeta(result._meta) ?? uiMeta(listedMeta);
return { html: text, meta: carried?.meta };
}
}
throw new Error("UI resource has no text (HTML) content");