diff --git a/cli/src/commands/app/dev.ts b/cli/src/commands/app/dev.ts
index fdf26fa7da..fa8b8fceb8 100644
--- a/cli/src/commands/app/dev.ts
+++ b/cli/src/commands/app/dev.ts
@@ -67,7 +67,7 @@ const DEFAULT_PORT = 4000;
 const DEFAULT_HOST = "localhost";
 
 // HTML template with live reload and SQL migration modal
-const createHTML = (jsPath: string, cssPath: string) => `
+const createHTML = (jsPath: string, cssPath: string, appCtx: unknown) => `
 <!DOCTYPE html>
 <html lang="en">
 <head>
@@ -228,6 +228,7 @@ const createHTML = (jsPath: string, cssPath: string) => `
       </div>
     </div>
   </div>
+  <script>window.ctx = ${JSON.stringify(appCtx).replace(/</g, "\\u003c")};</script>
   <script src="${jsPath}"></script>
   <script>
     // Live reload via EventSource
@@ -406,6 +407,18 @@ async function dev(opts: DevOptions, appFolder?: string) {
   await requireLogin(opts);
   const workspaceId = workspace.workspaceId;
 
+  // What the deployed app receives as `window.ctx`: the viewer's membership in this
+  // workspace, shaped as the frontend's UserExt, or no viewer when it can't be looked up.
+  const viewer = await wmill.whoami({ workspace: workspaceId }).catch(() => undefined);
+  const appCtx = {
+    ctx: viewer && {
+      ...viewer,
+      workspace_id: workspaceId,
+      pgroups: (viewer.groups ?? []).map((g) => `g/${g}`),
+    },
+    workspace: workspaceId,
+  };
+
   // Resolve relative imports in app inline scripts from local (not-yet-deployed)
   // content so previews use locally-edited workspace libs instead of deployed.
   // Computed here as a startup snapshot; re-run `wmill app dev` to pick up
@@ -966,7 +979,7 @@ async function dev(opts: DevOptions, appFolder?: string) {
 
     // Serve injected HTML for root and any other path
     res.writeHead(200, { "Content-Type": "text/html" });
-    res.end(createHTML("/dist/bundle.js", "/dist/bundle.css"));
+    res.end(createHTML("/dist/bundle.js", "/dist/bundle.css", appCtx));
   });
 
   // Create WebSocket server on the same HTTP server
diff --git a/frontend/scripts/ui_builder_artifact.json b/frontend/scripts/ui_builder_artifact.json
index c82e327126..82e53ad1a5 100644
--- a/frontend/scripts/ui_builder_artifact.json
+++ b/frontend/scripts/ui_builder_artifact.json
@@ -1,5 +1,5 @@
 {
 	"baseUrl": "https://pub-06154ed168a24e73a86ab84db6bf15d8.r2.dev",
-	"version": "d3fdb70",
-	"sha256": "8c894b3f02d5d1872a966c37d43314f05ce42900f7330bdcf45a18a772deb2b9"
+	"version": "6880a7f",
+	"sha256": "05d49a6ed4790b444058574ccb650c8d3882161bbf004c0f8c0370964e853367"
 }
diff --git a/frontend/src/lib/components/raw_apps/RawAppEditor.svelte b/frontend/src/lib/components/raw_apps/RawAppEditor.svelte
index 454030009b..8e02faf291 100644
--- a/frontend/src/lib/components/raw_apps/RawAppEditor.svelte
+++ b/frontend/src/lib/components/raw_apps/RawAppEditor.svelte
@@ -52,6 +52,7 @@
 	import { captureScale, MAX_IMAGE_EDGE } from '../copilot/chat/imageUtils'
 	import { rawAppLintStore } from './lintStore'
 	import { dbSchemas, userStore } from '$lib/stores'
+	import { useActingUser } from '$lib/actingUser.svelte'
 	import RawAppSdkConsent from './RawAppSdkConsent.svelte'
 	import {
 		hasStoredSdkConsent,
@@ -258,6 +259,7 @@
 	const opWorkspace = $derived(autosaveWorkspace ?? $operatingWorkspace)
 	// Everything under the editor acts on it too (see operatingWorkspace.svelte.ts).
 	setOperatingWorkspace(() => opWorkspace)
+	const operatingUser = useActingUser(() => opWorkspace)
 
 	// The path autosaves land on, which is what the session preview loads the app by.
 	const draftStoragePath = $derived(autosavePath ?? liveEditorDraftStoragePath)
@@ -508,6 +510,8 @@ addEventListener('message', function (e) {
 		'The preview is waiting for the user to approve the permissions the app declares in policy.frontend_sdk_scopes, so the app code is not running and the page is empty. This is not a bug in the app: do not change the code. Ask the user to approve the permissions in the preview, then retry.'
 	const SDK_STARTING_TEXT =
 		'The preview is still loading the permissions the app declares in policy.frontend_sdk_scopes, so the app code has not started yet. Retry in a moment.'
+	const VIEWER_PENDING_TEXT =
+		'The preview is still looking up the viewer the app receives in window.ctx, so the app code has not started yet. This is not a bug in the app: do not change the code. Retry in a moment.'
 	const PREVIEW_SETTLE_MS = 1000
 	let editorDestroyed = false
 	// Latest uncaught runtime error thrown by the rendered app; cleared on next build.
@@ -1813,12 +1817,13 @@ addEventListener('message', function (e) {
 	}
 
 	function syncExternalPreview() {
-		if (previewSdkPending || !externalPreviewReady) return
+		if (previewSdkPending || previewUserPending || !externalPreviewReady) return
 		if (lastBuild) {
 			postToExternalPreview({
 				type: 'preview',
 				css: lastBuild.css,
 				js: lastBuild.js,
+				ctx: previewCtx(),
 				sdk: previewSdk,
 				storage: previewStorage()
 			})
@@ -1832,18 +1837,41 @@ addEventListener('message', function (e) {
 		// Between dropping a credential and settling its replacement the shell stays
 		// blank; whichever settles last — the mint or the shell's own `load` — starts
 		// the app. Same for a shell still reloading: it replays once ready.
-		if (previewSdkPending || !previewIframeLoaded) return
+		if (previewSdkPending || previewUserPending || !previewIframeLoaded) return
 		runtimeError = undefined
 		emptyRender = false
 		postToPreview({
 			type: 'preview',
 			css: build.css,
 			js: build.js,
+			ctx: previewCtx(),
 			sdk: previewSdk,
 			storage: previewStorage()
 		})
 	}
 
+	// The deployed app's `window.ctx`: the viewer's membership in the app's workspace.
+	function previewCtx() {
+		return $state.snapshot({ ctx: operatingUser.current, workspace: opWorkspace })
+	}
+
+	// A build fed before the viewer is known keeps that viewer until the next one. Outside
+	// the navigation workspace the viewer is looked up; in it, `$userStore` still holds the
+	// previous workspace's membership right after a switch. A failed lookup also settles
+	// this: the app then runs with no viewer, as we don't know who it is.
+	const previewUserPending = $derived(
+		!!opWorkspace &&
+			(!operatingUser.resolved(opWorkspace) ||
+				(!!operatingUser.current && operatingUser.current.workspace_id !== opWorkspace))
+	)
+	$effect(() => {
+		if (previewUserPending) return
+		untrack(() => {
+			if (lastBuild) feedPreviewIframe(lastBuild)
+			syncExternalPreview()
+		})
+	})
+
 	// The shell seeds a fresh document's storage shim from this; unsandboxed
 	// previews use real storage and get none.
 	function previewStorage() {
@@ -2005,6 +2033,7 @@ addEventListener('message', function (e) {
 		if (buildTracker.pending) return { text: BUILD_PENDING_TEXT }
 		if (sdkPrompt) return { text: SDK_CONSENT_PENDING_TEXT }
 		if (previewSdkPending) return { text: SDK_STARTING_TEXT }
+		if (previewUserPending) return { text: VIEWER_PENDING_TEXT }
 		const res = await readPreviewDom(query)
 		return res && buildError !== undefined
 			? { text: `${buildFailedText(buildError)}\n\n${res.text}` }
@@ -2183,6 +2212,7 @@ addEventListener('message', function (e) {
 		if (buildTracker.pending) throw new Error(BUILD_PENDING_TEXT)
 		if (sdkPrompt) throw new Error(SDK_CONSENT_PENDING_TEXT)
 		if (previewSdkPending) throw new Error(SDK_STARTING_TEXT)
+		if (previewUserPending) throw new Error(VIEWER_PENDING_TEXT)
 		if (buildError !== undefined) throw new Error(buildFailedText(buildError))
 		const body = await requestFromPreview('elementRect', { selector: 'body' })
 		if (!body?.rect) throw new Error('App preview is not ready')
@@ -2860,6 +2890,8 @@ addEventListener('message', function (e) {
 												title="Replay the last build into the preview"
 												aria-label="Rebuild"
 												onClick={() => {
+													// Retries a viewer lookup that failed; the hold replays the build once it answers.
+													operatingUser.forgetFailures()
 													if (lastBuild) {
 														feedPreviewIframe(lastBuild)
 													}
diff --git a/frontend/src/lib/components/raw_apps/templates.ts b/frontend/src/lib/components/raw_apps/templates.ts
index 00d7594386..e54f89fdfa 100644
--- a/frontend/src/lib/components/raw_apps/templates.ts
+++ b/frontend/src/lib/components/raw_apps/templates.ts
@@ -1074,20 +1074,14 @@ const App = () => {
           description="Windmill passes the workspace and the viewer to the app."
           code={${JSON.stringify(BUILDER_CONTEXT_CODE)}}
         >
-          {ctx.workspace ? (
-            <>
-              <div>
-                <span className="muted">Workspace </span>
-                <strong>{ctx.workspace}</strong>
-              </div>
-              <div>
-                <span className="muted">Viewer </span>
-                <strong>{viewer ? viewer.name || viewer.username : 'Anonymous visitor'}</strong>
-              </div>
-            </>
-          ) : (
-            <span className="muted">The editor preview has no viewer. Deploy the app to see yours here.</span>
-          )}
+          <div>
+            <span className="muted">Workspace </span>
+            <strong>{ctx.workspace}</strong>
+          </div>
+          <div>
+            <span className="muted">Viewer </span>
+            <strong>{viewer ? viewer.name || viewer.username : 'Anonymous visitor'}</strong>
+          </div>
         </Card>
       </div>
     </main>
@@ -1166,15 +1160,11 @@ const builderAppSvelte = `<script lang="ts">
     <section class="card">
       {@render header(steps.length + 1, 'Know who is viewing', 'Windmill passes the workspace and the viewer to the app.')}
       <div class="card-body">
-        {#if ctx.workspace}
-          <div><span class="muted">Workspace </span><strong>{ctx.workspace}</strong></div>
-          <div>
-            <span class="muted">Viewer </span>
-            <strong>{viewer ? viewer.name || viewer.username : 'Anonymous visitor'}</strong>
-          </div>
-        {:else}
-          <span class="muted">The editor preview has no viewer. Deploy the app to see yours here.</span>
-        {/if}
+        <div><span class="muted">Workspace </span><strong>{ctx.workspace}</strong></div>
+        <div>
+          <span class="muted">Viewer </span>
+          <strong>{viewer ? viewer.name || viewer.username : 'Anonymous visitor'}</strong>
+        </div>
       </div>
       <pre class="snippet"><code>{contextCode}</code></pre>
     </section>