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feat(web): honor _meta.ui.domain with a dedicated app origin

Browse source 
An MCP App is rendered by handing its HTML to the sandbox proxy as
`srcdoc`, in a frame sandboxed without `allow-same-origin` — the #1565
isolation model. That gives the app document an opaque origin, so every
request it makes carries `Origin: null` and no CORS / OAuth-callback /
API-key allowlist can admit it. `_meta.ui.domain` is the spec field a
server uses to ask its host for a stable, dedicated origin, and it was
being dropped on the floor.

The field is explicitly host-dependent and the Inspector owns no domain
infrastructure, so it treats any non-empty value as a request rather than
an address, and answers with a real loopback origin on its own listener
(MCP_APP_ORIGIN_PORT, default 6276). One shared origin, path-keyed per
document: that delivers what the field is for without minting a port or
DNS name per app, and is therefore not a per-app isolation boundary.

The inner frame is granted `allow-same-origin` on this path only, which
is what makes the origin real. It is not a #1565 regression: the listener
is on its own port, so the app stays cross-origin to both the proxy and
the Inspector. The proxy refuses the grant if the URL's origin equals its
own, and the grant is never reachable from the server-supplied `sandbox`
string, which is still stripped. The per-app CSP is delivered as a real
response header there rather than only as a <meta>.

Every failure path falls back to the default srcdoc render with a console
warning naming the field, rather than blanking the app.

Also fixes a pre-existing `tsc -b` break on v2/main (useServers.test.tsx
passed `metadata: []` where `RequestMetadata` is expected), which has had
CI red since #2087 and blocks the pre-push gate.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>
This commit is contained in:
cliffhall 2026-08-23 23:03:55 -04:00 • committed by GitHub
commit ec40cf5531
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1 changed files with 1554 additions and 18 deletions
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16
clients/web/server/sandbox-controller.ts
View file

@ -70,7 +70,21 @@ const CSP_HOST_SOURCE = /^[a-z][a-z0-9+.-]*:\/\/[^\s;,'"[\]*]+$/i;
* header can't be corrupted.
*/
export function sandboxFrameAncestors(allowedOrigins?: string[]): string {
const valid = (allowedOrigins ?? []).filter((o) => CSP_HOST_SOURCE.test(o));
return frameAncestorsDirective(allowedOrigins);
}
/**
* Build a `frame-ancestors` directive from a caller-supplied origin list,
* dropping malformed entries ({@link CSP_HOST_SOURCE}) and falling back to
* {@link LOOPBACK_FRAME_ANCESTORS} when nothing valid remains.
*
* Shared with the app-origin server (#2056), which restricts a published app
* document to being framed by the sandbox proxy. Same grammar, same failure
* mode — a directive whose only source is invalid degrades to `'none'` and
* blocks the frame — so the two derive it here rather than each rolling one.
*/
export function frameAncestorsDirective(origins?: string[]): string {
const valid = (origins ?? []).filter((o) => CSP_HOST_SOURCE.test(o));
const sources = valid.length > 0 ? valid : LOOPBACK_FRAME_ANCESTORS;
return `frame-ancestors ${sources.join(" ")}`;
}