diff --git a/apps/desktop/src/renderer/src/App.test.ts b/apps/desktop/src/renderer/src/App.test.ts
new file mode 100644
index 00000000..e8d56538
--- /dev/null
+++ b/apps/desktop/src/renderer/src/App.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest';
+import { formatIframeError } from './components/PreviewPane';
+
+describe('formatIframeError', () => {
+  it('omits location when source or lineno is missing', () => {
+    expect(formatIframeError('error', 'something broke')).toBe('error: something broke');
+    expect(formatIframeError('error', 'something broke', 'app.js')).toBe('error: something broke');
+    expect(formatIframeError('error', 'something broke', undefined, 10)).toBe(
+      'error: something broke',
+    );
+  });
+
+  it('appends source and lineno when both are present', () => {
+    expect(formatIframeError('unhandledrejection', 'promise failed', 'app.js', 42)).toBe(
+      'unhandledrejection: promise failed (app.js:42)',
+    );
+  });
+});
diff --git a/apps/desktop/src/renderer/src/components/PreviewPane.tsx b/apps/desktop/src/renderer/src/components/PreviewPane.tsx
index ad971602..f0168ef3 100644
--- a/apps/desktop/src/renderer/src/components/PreviewPane.tsx
+++ b/apps/desktop/src/renderer/src/components/PreviewPane.tsx
@@ -13,6 +13,16 @@ export interface PreviewPaneProps {
   onPickStarter: (prompt: string) => void;
 }
 
+export function formatIframeError(
+  kind: string,
+  message: string,
+  source?: string,
+  lineno?: number,
+): string {
+  const location = source && lineno ? ` (${source}:${lineno})` : '';
+  return `${kind}: ${message}${location}`;
+}
+
 export function isTrustedPreviewMessageSource(
   source: MessageEventSource | null,
   previewWindow: Window | null | undefined,
@@ -46,11 +56,14 @@ export function PreviewPane({ onPickStarter }: PreviewPaneProps) {
       }
 
       if (isIframeErrorMessage(event.data)) {
-        const location =
-          event.data.source && event.data.lineno
-            ? ` (${event.data.source}:${event.data.lineno})`
-            : '';
-        pushIframeError(`${event.data.kind}: ${event.data.message}${location}`);
+        pushIframeError(
+          formatIframeError(
+            event.data.kind,
+            event.data.message,
+            event.data.source,
+            event.data.lineno,
+          ),
+        );
       }
     }
 
diff --git a/apps/desktop/src/renderer/src/store.test.ts b/apps/desktop/src/renderer/src/store.test.ts
new file mode 100644
index 00000000..b27c33a6
--- /dev/null
+++ b/apps/desktop/src/renderer/src/store.test.ts
@@ -0,0 +1,97 @@
+import type { OnboardingState } from '@open-codesign/shared';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { useCodesignStore } from './store';
+
+const READY_CONFIG: OnboardingState = {
+  hasKey: true,
+  provider: 'anthropic',
+  modelPrimary: 'claude-sonnet-4-6',
+  modelFast: 'claude-haiku-3',
+  baseUrl: null,
+  designSystem: null,
+};
+
+const initialState = useCodesignStore.getState();
+
+function resetStore() {
+  useCodesignStore.setState({
+    ...initialState,
+    messages: [],
+    previewHtml: null,
+    isGenerating: false,
+    errorMessage: null,
+    lastError: null,
+    config: READY_CONFIG,
+    configLoaded: true,
+    toastMessage: null,
+    iframeErrors: [],
+    toasts: [],
+  });
+}
+
+beforeEach(() => {
+  resetStore();
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+describe('useCodesignStore iframe error handling', () => {
+  it('clears stale iframe errors when starting a new generation', async () => {
+    let resolveGenerate: ((value: unknown) => void) | undefined;
+    const generate = vi.fn(
+      () =>
+        new Promise((resolve) => {
+          resolveGenerate = resolve;
+        }),
+    );
+
+    vi.stubGlobal('window', {
+      codesign: {
+        generate,
+      },
+    });
+
+    useCodesignStore.setState({ iframeErrors: ['old iframe error'] });
+
+    const sendPromise = useCodesignStore.getState().sendPrompt({ prompt: 'make a landing page' });
+
+    expect(useCodesignStore.getState().iframeErrors).toEqual([]);
+    expect(useCodesignStore.getState().isGenerating).toBe(true);
+
+    resolveGenerate?.({
+      artifacts: [{ content: '<html></html>' }],
+      message: 'Done.',
+    });
+    await sendPromise;
+
+    expect(generate).toHaveBeenCalledOnce();
+  });
+
+  it('deduplicates consecutive identical iframe errors', () => {
+    const { pushIframeError } = useCodesignStore.getState();
+
+    pushIframeError('first');
+    pushIframeError('first'); // duplicate — should be skipped
+    pushIframeError('second');
+    pushIframeError('second'); // duplicate — should be skipped
+    pushIframeError('third');
+
+    expect(useCodesignStore.getState().iframeErrors).toEqual(['first', 'second', 'third']);
+  });
+
+  it('caps iframeErrors at 50 entries and drops the oldest when exceeded', () => {
+    const { pushIframeError } = useCodesignStore.getState();
+
+    for (let i = 0; i < 55; i++) {
+      pushIframeError(`error-${i}`);
+    }
+
+    const errors = useCodesignStore.getState().iframeErrors;
+    expect(errors).toHaveLength(50);
+    // oldest (0-4) should have been shifted out; newest (5-54) remain
+    expect(errors[0]).toBe('error-5');
+    expect(errors[49]).toBe('error-54');
+  });
+});
diff --git a/apps/desktop/src/renderer/src/store.ts b/apps/desktop/src/renderer/src/store.ts
index 0856fbfe..d8e528d9 100644
--- a/apps/desktop/src/renderer/src/store.ts
+++ b/apps/desktop/src/renderer/src/store.ts
@@ -172,9 +172,12 @@ export const useCodesignStore = create<CodesignState>((set, get) => ({
   },
 
   pushIframeError(message) {
-    set((s) => ({
-      iframeErrors: [...s.iframeErrors.slice(-9), message],
-    }));
+    set((s) => {
+      const last = s.iframeErrors[s.iframeErrors.length - 1];
+      if (last === message) return {};
+      const next = [...s.iframeErrors, message];
+      return { iframeErrors: next.length > 50 ? next.slice(1) : next };
+    });
   },
 
   async loadConfig() {