diff --git a/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.test.ts b/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.test.ts
index acaa5c86b77..727624abbef 100644
--- a/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.test.ts
+++ b/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.test.ts
@@ -146,6 +146,10 @@ describe('selectTabAgentTypesByTabId', () => {
     ).toEqual({ 'tab-1': 'claude' })
   })
 
+  it('treats a missing layout map as no unsafe split evidence during hydration', () => {
+    expect(selectNativeChatTabWideFallbackUnsafeTabsById()).toEqual({})
+  })
+
   it('resolves the active leaf through nested splits and ignores expanded siblings', () => {
     const layout: TerminalLayoutSnapshot = {
       root: {
diff --git a/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.ts b/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.ts
index c97e373cea2..68d78a43ab3 100644
--- a/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.ts
+++ b/src/renderer/src/components/tab-bar/tab-agent-types-by-tab-id.ts
@@ -64,8 +64,10 @@ export function selectTabAgentTypesByTabId(
 }
 
 export function selectNativeChatTabWideFallbackUnsafeTabsById(
-  terminalLayoutsByTabId: Record<string, TerminalLayoutSnapshot>
+  terminalLayoutsByTabId: Record<string, TerminalLayoutSnapshot> = {}
 ): Record<string, true> {
+  // Why: legacy and hydrating store shapes may not expose layout state yet;
+  // absence carries no unsafe split evidence and must not crash tab rendering.
   const unsafeTabs: Record<string, true> = {}
   for (const [tabId, layout] of Object.entries(terminalLayoutsByTabId)) {
     if (!isNativeChatTabWideFallbackSafe(layout)) {