diff --git a/apps/docs/docs/management/custom-widgets/troubleshooting.mdx b/apps/docs/docs/management/custom-widgets/troubleshooting.mdx
index c7fe1efdd..c103d8989 100644
--- a/apps/docs/docs/management/custom-widgets/troubleshooting.mdx
+++ b/apps/docs/docs/management/custom-widgets/troubleshooting.mdx
@@ -14,6 +14,8 @@ Use the first schema/JSX diagnostic and its path. Ensure source/request IDs are
 
 Incomplete API source and icon URLs are normal form validation errors. The workbench keeps the current draft mounted and highlights the invalid field while you type; malformed URLs must never replace the form with the error boundary.
 
+An invalid definition disables only the rendered Widget canvas. The preview shell and its Data, Options, Actions, and Diagnostics tabs remain available, and Diagnostics lists the current schema issues.
+
 ## Request fails
 
 Check the source network scope, base URL, credentials, request method/path/body, runtime parameters, and board permission. For “invalid placeholder” or an unresolved parameter on a load query, first verify that every declared parameter has an explicit `optionsBinding` option reference or literal. Matching parameter and option names are not a binding. The preview journal shows redacted method, request ID, path template, status, and duration without credentials.
diff --git a/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-form.tsx b/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-form.tsx
index aafb33492..8a9951796 100644
--- a/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-form.tsx
+++ b/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-form.tsx
@@ -314,6 +314,14 @@ export function CustomWidgetForm({ mode, initialValues, definitionId }: CustomWi
           </Paper>
           <CustomWidgetPreviewPanel
             candidate={candidate.success ? candidate.data : null}
+            validationIssues={
+              candidate.success
+                ? []
+                : candidate.error.issues.map((issue) => ({
+                    path: issue.path.map(String).join(".") || undefined,
+                    message: issue.message,
+                  }))
+            }
             preview={preview}
             size={previewSize}
             onSizeChange={setPreviewSize}
diff --git a/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-preview-panel.spec.tsx b/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-preview-panel.spec.tsx
new file mode 100644
index 000000000..b6aae3c3f
--- /dev/null
+++ b/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-preview-panel.spec.tsx
@@ -0,0 +1,94 @@
+// @vitest-environment jsdom
+
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import type { Root } from "react-dom/client";
+import { MantineProvider } from "@mantine/core";
+import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
+
+import { CustomWidgetPreviewPanel } from "./_custom-widget-preview-panel";
+
+vi.mock("@homarr/api/client", () => ({
+  clientApi: {
+    customWidget: {
+      setPreviewLiveActions: { useMutation: () => ({ isPending: false, mutate: vi.fn() }) },
+      previewJournal: { useQuery: () => ({ data: [] }) },
+    },
+  },
+}));
+vi.mock("@homarr/translation/client", () => ({ useScopedI18n: () => (key: string) => key }));
+vi.mock("@homarr/notifications", () => ({ showErrorNotification: vi.fn() }));
+vi.mock("@homarr/widgets/custom-api/custom-jsx-display", () => ({ default: () => <div>widget-renderer</div> }));
+vi.mock("./_code-editor", () => ({ CodeEditor: ({ label }: { label: string }) => <div>{label}</div> }));
+vi.mock("./_custom-widget-preview-action", () => ({ PreviewActionControl: () => <div>action-control</div> }));
+
+let root: Root;
+let host: HTMLDivElement;
+
+beforeEach(() => {
+  (globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+  vi.stubGlobal(
+    "ResizeObserver",
+    class {
+      observe() {}
+      unobserve() {}
+      disconnect() {}
+    },
+  );
+  window.matchMedia = vi.fn().mockImplementation(() => ({
+    matches: false,
+    media: "",
+    onchange: null,
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  }));
+  host = document.createElement("div");
+  document.body.append(host);
+  root = createRoot(host);
+});
+
+afterEach(async () => {
+  await act(() => root.unmount());
+  host.remove();
+  vi.unstubAllGlobals();
+});
+
+describe("Custom Widget preview panel", () => {
+  test("contains invalid definitions inside the widget canvas while keeping every inspection tab", async () => {
+    await act(async () => {
+      root.render(
+        <MantineProvider>
+          <CustomWidgetPreviewPanel
+            candidate={null}
+            validationIssues={[{ path: "sources.0.baseUrl", message: "Enter a complete URL." }]}
+            preview={{ data: { previous: true }, status: {}, session: null, outcome: "idle" }}
+            size="standard"
+            onSizeChange={vi.fn()}
+            theme="dark"
+            onThemeChange={vi.fn()}
+            optionsSnapshot={{}}
+            onOptionsChange={vi.fn()}
+            onLiveActionsChange={vi.fn()}
+          />
+        </MantineProvider>,
+      );
+    });
+
+    expect(host.textContent).toContain("invalid");
+    for (const tab of ["widget", "data", "options", "actions", "diagnostics"]) {
+      expect(host.textContent).toContain(`tab.${tab}`);
+    }
+
+    const diagnosticsTab = [...host.querySelectorAll<HTMLElement>('[role="tab"]')].find((element) =>
+      element.textContent?.includes("tab.diagnostics"),
+    );
+    expect(diagnosticsTab).toBeDefined();
+    await act(async () => diagnosticsTab?.click());
+
+    expect(host.textContent).toContain("sources.0.baseUrl");
+    expect(host.textContent).toContain("Enter a complete URL.");
+  });
+});
diff --git a/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-preview-panel.tsx b/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-preview-panel.tsx
index 707f1ef4d..3c0f82bd0 100644
--- a/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-preview-panel.tsx
+++ b/apps/nextjs/src/app/[locale]/manage/custom-widgets/_custom-widget-preview-panel.tsx
@@ -39,6 +39,7 @@ export interface PreviewState {
 
 interface PreviewPanelProps {
   candidate: HomarrCustomWidgetV2 | null;
+  validationIssues: Array<{ path?: string; message: string }>;
   preview: PreviewState;
   size: string;
   onSizeChange(value: string): void;
@@ -58,40 +59,43 @@ export function CustomWidgetPreviewPanel(props: PreviewPanelProps) {
     { enabled: Boolean(props.preview.session), refetchInterval: props.preview.session ? 2_000 : false },
   );
   const widths: Record<string, number> = { compact: 320, standard: 480, wide: 720 };
-  if (!props.candidate) return <Alert color="yellow">{t("invalid")}</Alert>;
   const candidate = props.candidate;
-  const accessibilityIssues = analyzeCustomWidgetAccessibility(candidate.template);
+  const accessibilityIssues = candidate ? analyzeCustomWidgetAccessibility(candidate.template) : [];
   const fixtureData =
-    fixture === "empty" ? Object.fromEntries(candidate.requests.map((entry) => [entry.id, []])) : props.preview.data;
+    fixture === "empty" && candidate
+      ? Object.fromEntries(candidate.requests.map((entry) => [entry.id, []]))
+      : props.preview.data;
   const fixtureStatus =
-    fixture === "loading"
+    fixture === "loading" && candidate
       ? Object.fromEntries(candidate.requests.map((entry) => [entry.id, { loading: true }]))
-      : fixture === "error"
+      : fixture === "error" && candidate
         ? Object.fromEntries(
             candidate.requests.map((entry) => [entry.id, { loading: false, ok: false, error: t("fixtureError") }]),
           )
         : props.preview.status;
-  const displayData = {
-    template: candidate.template,
-    data: fixtureData,
-    status: fixtureStatus,
-    options: props.optionsSnapshot,
-    requestCapabilities: candidate.requests.map(
-      ({ id, kind, method, trigger, minimumBoardPermission, confirmation, invalidates }) => ({
-        id,
-        kind,
-        method,
-        trigger,
-        minimumBoardPermission,
-        confirmation,
-        invalidates,
-      }),
-    ),
-    previewSessionId: props.preview.session?.id,
-    previewLiveActions: props.preview.session?.liveActions ?? false,
-    queriesDisabled: fixture !== "live",
-    isEditMode: fixture !== "live",
-  };
+  const displayData = candidate
+    ? {
+        template: candidate.template,
+        data: fixtureData,
+        status: fixtureStatus,
+        options: props.optionsSnapshot,
+        requestCapabilities: candidate.requests.map(
+          ({ id, kind, method, trigger, minimumBoardPermission, confirmation, invalidates }) => ({
+            id,
+            kind,
+            method,
+            trigger,
+            minimumBoardPermission,
+            confirmation,
+            invalidates,
+          }),
+        ),
+        previewSessionId: props.preview.session?.id,
+        previewLiveActions: props.preview.session?.liveActions ?? false,
+        queriesDisabled: fixture !== "live",
+        isEditMode: fixture !== "live",
+      }
+    : null;
   const previewSummary = getPreviewSummary(props.preview.status);
   const previewResult =
     props.preview.outcome === "success"
@@ -135,6 +139,7 @@ export function CustomWidgetPreviewPanel(props: PreviewPanelProps) {
         <SegmentedControl
           size="xs"
           value={fixture}
+          disabled={!candidate}
           onChange={(value) => setFixture(value as typeof fixture)}
           data={[
             { value: "live", label: t("fixture.live") },
@@ -143,11 +148,13 @@ export function CustomWidgetPreviewPanel(props: PreviewPanelProps) {
             { value: "error", label: t("fixture.error") },
           ]}
         />
-        <PreviewResult
-          outcome={props.preview.outcome}
-          title={previewResult.title}
-          description={previewResult.description}
-        />
+        {candidate && (
+          <PreviewResult
+            outcome={props.preview.outcome}
+            title={previewResult.title}
+            description={previewResult.description}
+          />
+        )}
         <Tabs defaultValue="widget" keepMounted={false}>
           <Tabs.List grow>
             {(["widget", "data", "options", "actions", "diagnostics"] as const).map((tab) => (
@@ -165,7 +172,7 @@ export function CustomWidgetPreviewPanel(props: PreviewPanelProps) {
             <Box className={classes.previewCanvas}>
               <MantineProvider forceColorScheme={props.theme}>
                 <Paper withBorder p="sm" h={360} w={widths[props.size] ?? 480} maw="100%" style={{ overflow: "auto" }}>
-                  <CustomJsxDisplay data={displayData} />
+                  {displayData ? <CustomJsxDisplay data={displayData} /> : <Alert color="yellow">{t("invalid")}</Alert>}
                 </Paper>
               </MantineProvider>
             </Box>
@@ -206,7 +213,7 @@ export function CustomWidgetPreviewPanel(props: PreviewPanelProps) {
                   );
                 }}
               />
-              {candidate.requests
+              {candidate?.requests
                 .filter((request) => request.kind === "action")
                 .map((request) => (
                   <PreviewActionControl
@@ -216,7 +223,8 @@ export function CustomWidgetPreviewPanel(props: PreviewPanelProps) {
                     options={props.optionsSnapshot}
                   />
                 ))}
-              {candidate.requests.every((request) => request.kind !== "action") && (
+              {!candidate && <Alert color="yellow">{t("invalid")}</Alert>}
+              {candidate?.requests.every((request) => request.kind !== "action") && (
                 <Text size="sm" c="dimmed">
                   {t("noActions")}
                 </Text>
@@ -233,10 +241,24 @@ export function CustomWidgetPreviewPanel(props: PreviewPanelProps) {
               <Text size="sm" c="dimmed">
                 {t("containedFailures")}
               </Text>
+              <Text size="sm" fw={600}>
+                {t("validation.title")}
+              </Text>
+              {props.validationIssues.length === 0 ? (
+                <Alert color="green">{t("validation.ready")}</Alert>
+              ) : (
+                props.validationIssues.map((issue, index) => (
+                  <Alert key={`${issue.path ?? "widget"}-${index}`} color="red" title={issue.path}>
+                    {issue.message}
+                  </Alert>
+                ))
+              )}
               <Text size="sm" fw={600}>
                 {t("accessibility.title")}
               </Text>
-              {accessibilityIssues.length === 0 ? (
+              {!candidate ? (
+                <Alert color="gray">{t("accessibility.pending")}</Alert>
+              ) : accessibilityIssues.length === 0 ? (
                 <Alert color="green">{t("accessibility.ready")}</Alert>
               ) : (
                 accessibilityIssues.map((issue) => (
diff --git a/packages/translation/src/lang/en.json b/packages/translation/src/lang/en.json
index 1ea4533a1..61dc0c5b3 100644
--- a/packages/translation/src/lang/en.json
+++ b/packages/translation/src/lang/en.json
@@ -934,9 +934,14 @@
         "sessionActive": "Preview session active",
         "simulationPending": "Actions are simulated until you run preview",
         "containedFailures": "Runtime failures are contained inside this widget. Request journals never include secret values.",
+        "validation": {
+          "title": "Validation",
+          "ready": "The widget definition is valid."
+        },
         "accessibility": {
           "title": "Accessibility checks",
           "ready": "No common accessibility omissions detected.",
+          "pending": "Accessibility checks will run when the widget definition is valid.",
           "imageAlt": "Add alt text to every Image, using an empty alt only for decorative images.",
           "actionIconLabel": "Add an aria-label or title to every ActionIcon.",
           "inputLabel": "Add a visible label or accessible name to every input."