diff --git a/frontend/src/lib/components/raw_apps/RawAppCoepWarning.svelte b/frontend/src/lib/components/raw_apps/RawAppCoepWarning.svelte
new file mode 100644
index 0000000000..d5172c7232
--- /dev/null
+++ b/frontend/src/lib/components/raw_apps/RawAppCoepWarning.svelte
@@ -0,0 +1,90 @@
+<script lang="ts">
+	import Modal from '../common/modal/Modal.svelte'
+	import { sendUserToast } from '$lib/toast'
+
+	interface Props {
+		/** Inline preview iframe rendering the user's app (same-origin app-preview.html) */
+		iframe: HTMLIFrameElement | undefined
+	}
+
+	let { iframe }: Props = $props()
+
+	let open = $state(false)
+
+	// One toast per remote origin — a page pulling many assets from one host
+	// should not stack toasts.
+	const warnedOrigins = new Set<string>()
+
+	const RESOURCE_TAGS = new Set(['IMG', 'SCRIPT', 'LINK', 'AUDIO', 'VIDEO', 'SOURCE', 'IFRAME'])
+
+	// The editor page is cross-origin isolated (COEP require-corp — required for
+	// the SharedArrayBuffer-based TS workers and in-browser bundling), and COEP
+	// forces every nested document, including the preview, to be isolated too. So
+	// cross-origin resources without CORS/CORP headers are blocked in the preview
+	// while loading fine on the deployed app: surface that as a warning instead
+	// of a silently broken image.
+	function onResourceError(e: Event) {
+		// Elements belong to the preview window's realm, so no instanceof checks —
+		// duck-type instead. Resource load errors don't bubble but do capture
+		// through the window; runtime ErrorEvents target the window itself and are
+		// skipped by the tagName check.
+		const el = e.target as (Element & { currentSrc?: string; src?: string; href?: string }) | null
+		if (!el?.tagName || !RESOURCE_TAGS.has(el.tagName)) return
+		const raw = el.currentSrc || el.src || el.href
+		if (typeof raw !== 'string' || !raw) return
+		let url: URL
+		try {
+			url = new URL(raw)
+		} catch {
+			return
+		}
+		// Same-origin failures (plain 404s) and non-http(s) schemes are not COEP blocks.
+		if (!/^https?:$/.test(url.protocol) || url.origin === window.location.origin) return
+		if (warnedOrigins.has(url.origin)) return
+		warnedOrigins.add(url.origin)
+		sendUserToast(
+			`The editor preview blocked a cross-origin resource (${url.host}) due to COEP/CORS isolation. It will load normally on the deployed app.`,
+			'warning',
+			[{ label: 'Read more', callback: () => (open = true) }],
+			undefined,
+			10000
+		)
+	}
+
+	export function attachTo(win: Window | null | undefined) {
+		win?.addEventListener('error', onResourceError, true)
+	}
+
+	$effect(() => {
+		const el = iframe
+		if (!el) return
+		const attach = () => attachTo(el.contentWindow)
+		el.addEventListener('load', attach)
+		// The iframe may already be loaded when this mounts; addEventListener
+		// dedupes the (handler, capture) pair, so the load event re-firing on the
+		// same window cannot double-attach.
+		if (el.contentDocument?.readyState === 'complete') attach()
+		return () => el.removeEventListener('load', attach)
+	})
+</script>
+
+<Modal bind:open title="Cross-origin resources in the editor preview" kind="X">
+	<div class="flex flex-col gap-3">
+		<p>
+			The app editor runs in a <b>cross-origin isolated</b> context (COOP/COEP headers). This is
+			required for <code>SharedArrayBuffer</code>, which powers the TypeScript language workers and
+			lets the editor build and preview your frontend live in the browser.
+		</p>
+		<p>
+			A side effect is that the browser refuses to load cross-origin resources (images, scripts,
+			stylesheets, media…) unless the remote server explicitly opts in with CORS or a
+			<code>Cross-Origin-Resource-Policy</code> header. Resources from servers that don't are
+			blocked <b>in the editor preview only</b>.
+		</p>
+		<p>
+			The deployed app is served without these headers, so the same resources load normally there —
+			open the deployed app link to verify. If you control the remote server, sending
+			<code>Cross-Origin-Resource-Policy: cross-origin</code> makes the resource load in the editor too.
+		</p>
+	</div>
+</Modal>
diff --git a/frontend/src/lib/components/raw_apps/RawAppEditor.svelte b/frontend/src/lib/components/raw_apps/RawAppEditor.svelte
index 0ef8ac5a58..3e10f5e06d 100644
--- a/frontend/src/lib/components/raw_apps/RawAppEditor.svelte
+++ b/frontend/src/lib/components/raw_apps/RawAppEditor.svelte
@@ -28,6 +28,7 @@
 	} from './utils'
 	import { runDomQueryOnHtml, type RawAppDomQuery, type RawAppDomRequester } from './rawAppDom'
 	import InlineElementPrompt from './InlineElementPrompt.svelte'
+	import RawAppCoepWarning from './RawAppCoepWarning.svelte'
 	import DarkModeObserver from '../DarkModeObserver.svelte'
 	import RawAppSidebar from './RawAppSidebar.svelte'
 	import type { Modules } from './RawAppModules.svelte'
@@ -299,6 +300,7 @@
 
 	let iframe: HTMLIFrameElement | undefined = $state(undefined)
 	let previewIframe: HTMLIFrameElement | undefined = $state(undefined)
+	let coepWarning: RawAppCoepWarning | undefined = $state(undefined)
 	let previewIframeLoaded = $state(false)
 	let lastBuild: { css: string; js: string } | undefined = undefined
 	// Detached preview tab/window rendering the same app-preview bundle as the
@@ -1104,6 +1106,9 @@
 			e.origin === window.location.origin
 		) {
 			feedExternalPreview()
+			// The detached window is cross-origin isolated like the inline preview,
+			// so blocked external resources warrant the same COEP warning.
+			coepWarning?.attachTo(externalPreviewWindow)
 			return
 		}
 
@@ -2324,6 +2329,7 @@
 									src="/ui_builder/app-preview.html"
 									class="w-full flex-1 block"
 								></iframe>
+								<RawAppCoepWarning bind:this={coepWarning} iframe={previewIframe} />
 								{#if buildError}
 									<!-- top-12 clears the tab bar; `before:bg-surface` backs the
 									     Alert's translucent red; `isolate` pins the pseudo's stacking context. -->