diff --git a/app/src/providers/CoreStateProvider.tsx b/app/src/providers/CoreStateProvider.tsx
index 37dde8035..ad0578b2e 100644
--- a/app/src/providers/CoreStateProvider.tsx
+++ b/app/src/providers/CoreStateProvider.tsx
@@ -75,6 +75,31 @@ export function coreStatePollFailureWarningMessage(failureCount: number): string
   return null;
 }
 
+function decodeJwtPayload(token: string): Record<string, unknown> | null {
+  const [, payload] = token.split('.');
+  if (!payload) return null;
+
+  try {
+    const base64 = payload.replace(/-/g, '+').replace(/_/g, '/');
+    const padded = base64.padEnd(base64.length + ((4 - (base64.length % 4)) % 4), '=');
+    const decoded = window.atob(padded);
+    return JSON.parse(decoded) as Record<string, unknown>;
+  } catch {
+    return null;
+  }
+}
+
+function isPlausibleSessionToken(token: unknown): token is string {
+  if (typeof token !== 'string') return false;
+  if (token.trim() !== token || token.length === 0) return false;
+  if (token.split('.').length !== 3) return false;
+
+  const payload = decodeJwtPayload(token);
+  if (!payload || typeof payload.exp !== 'number') return false;
+
+  return payload.exp * 1000 > Date.now();
+}
+
 interface CoreStateContextValue extends CoreState {
   refresh: () => Promise<void>;
   refreshTeams: () => Promise<void>;
@@ -458,7 +483,7 @@ export default function CoreStateProvider({ children }: { children: ReactNode })
     const onSessionTokenUpdated = (event: Event) => {
       const customEvent = event as CustomEvent<{ sessionToken?: string | null }>;
       const token = customEvent.detail?.sessionToken;
-      if (typeof token !== 'string' || token.length === 0) {
+      if (!isPlausibleSessionToken(token)) {
         return;
       }
 
diff --git a/app/src/providers/__tests__/CoreStateProvider.test.tsx b/app/src/providers/__tests__/CoreStateProvider.test.tsx
index 515f156db..26da31e29 100644
--- a/app/src/providers/__tests__/CoreStateProvider.test.tsx
+++ b/app/src/providers/__tests__/CoreStateProvider.test.tsx
@@ -45,6 +45,13 @@ function makeSnapshot(overrides: {
   };
 }
 
+function makeJwt(payload: Record<string, unknown>): string {
+  const encode = (value: Record<string, unknown>) =>
+    window.btoa(JSON.stringify(value)).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+
+  return `${encode({ alg: 'none', typ: 'JWT' })}.${encode(payload)}.signature`;
+}
+
 type CoreStateContextValue = ReturnType<typeof useCoreState>;
 
 function Consumer({ captureCtx }: { captureCtx?: (ctx: CoreStateContextValue) => void }) {
@@ -297,6 +304,81 @@ describe('CoreStateProvider — identity-change cache clearing', () => {
     );
   });
 
+  it('ignores malformed session-token-updated events (#1937)', async () => {
+    fetchSnapshot.mockResolvedValue(makeSnapshot({ userId: null, sessionToken: null }));
+    listTeams.mockResolvedValue([]);
+
+    render(
+      <CoreStateProvider>
+        <Consumer />
+      </CoreStateProvider>
+    );
+
+    await waitFor(() => expect(screen.getByTestId('ready').textContent).toBe('ready'));
+
+    await act(async () => {
+      window.dispatchEvent(
+        new CustomEvent('core-state:session-token-updated', {
+          detail: { sessionToken: 'not-a-jwt' },
+        })
+      );
+    });
+
+    expect(screen.getByTestId('token').textContent).toBe('none');
+    expect(fetchSnapshot).toHaveBeenCalledTimes(1);
+  });
+
+  it('ignores expired JWT-shaped session-token-updated events (#1937)', async () => {
+    const expiredToken = makeJwt({ exp: Math.floor(Date.now() / 1000) - 60 });
+    fetchSnapshot.mockResolvedValue(makeSnapshot({ userId: null, sessionToken: null }));
+    listTeams.mockResolvedValue([]);
+
+    render(
+      <CoreStateProvider>
+        <Consumer />
+      </CoreStateProvider>
+    );
+
+    await waitFor(() => expect(screen.getByTestId('ready').textContent).toBe('ready'));
+
+    await act(async () => {
+      window.dispatchEvent(
+        new CustomEvent('core-state:session-token-updated', {
+          detail: { sessionToken: expiredToken },
+        })
+      );
+    });
+
+    expect(screen.getByTestId('token').textContent).toBe('none');
+    expect(fetchSnapshot).toHaveBeenCalledTimes(1);
+  });
+
+  it('accepts unexpired JWT-shaped session-token-updated events (#1937)', async () => {
+    const token = makeJwt({ exp: Math.floor(Date.now() / 1000) + 60 });
+    fetchSnapshot
+      .mockResolvedValueOnce(makeSnapshot({ userId: null, sessionToken: null }))
+      .mockResolvedValueOnce(
+        makeSnapshot({ userId: null, sessionToken: token, isAuthenticated: true })
+      );
+    listTeams.mockResolvedValue([]);
+
+    render(
+      <CoreStateProvider>
+        <Consumer />
+      </CoreStateProvider>
+    );
+
+    await waitFor(() => expect(screen.getByTestId('ready').textContent).toBe('ready'));
+
+    await act(async () => {
+      window.dispatchEvent(
+        new CustomEvent('core-state:session-token-updated', { detail: { sessionToken: token } })
+      );
+    });
+
+    expect(screen.getByTestId('token').textContent).toBe(token);
+  });
+
   it('setMeetAutoOrchestratorHandoff(true) calls update RPC + flips snapshot optimistically (#1299)', async () => {
     fetchSnapshot.mockResolvedValue(makeSnapshot({ userId: 'u1', sessionToken: 'tok1' }));
     listTeams.mockResolvedValue([]);