diff --git a/packages/clerk-js/src/core/__tests__/state.test.ts b/packages/clerk-js/src/core/__tests__/state.test.ts
new file mode 100644
index 00000000000..85c79bb94f8
--- /dev/null
+++ b/packages/clerk-js/src/core/__tests__/state.test.ts
@@ -0,0 +1,266 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { eventBus } from '../events';
+import { SignIn } from '../resources/SignIn';
+import { SignUp } from '../resources/SignUp';
+import { signInResourceSignal, signUpResourceSignal } from '../signals';
+import { State } from '../state';
+
+describe('State', () => {
+  let state: State;
+
+  // Capture original static clerk references to restore after tests
+  const originalSignUpClerk = SignUp.clerk;
+  const originalSignInClerk = SignIn.clerk;
+
+  beforeEach(() => {
+    // Reset signals to initial state
+    signUpResourceSignal({ resource: null });
+    signInResourceSignal({ resource: null });
+    // Create a new State instance which registers event handlers
+    state = new State();
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+    // Reset signals after each test
+    signUpResourceSignal({ resource: null });
+    signInResourceSignal({ resource: null });
+    // Restore original clerk references to prevent global state leakage
+    SignUp.clerk = originalSignUpClerk;
+    SignIn.clerk = originalSignInClerk;
+  });
+
+  describe('shouldIgnoreNullUpdate behavior', () => {
+    describe('SignUp', () => {
+      it('should allow first resource update when previous resource is null', () => {
+        // Arrange: Signal starts with null
+        expect(signUpResourceSignal().resource).toBeNull();
+
+        // Act: Emit a resource update with a SignUp that has an id
+        const signUp = new SignUp({ id: 'signup_123', status: 'missing_requirements' } as any);
+
+        // Assert: Signal should be updated
+        expect(signUpResourceSignal().resource).toBe(signUp);
+        expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+      });
+
+      it('should ignore null resource update when previous resource exists and canBeDiscarded is false', () => {
+        // Arrange: Set up a SignUp with id and canBeDiscarded = false (default)
+        const existingSignUp = new SignUp({ id: 'signup_123', status: 'missing_requirements' } as any);
+        expect(signUpResourceSignal().resource).toBe(existingSignUp);
+        expect(existingSignUp.__internal_future.canBeDiscarded).toBe(false);
+
+        // Act: Emit a resource update with a null SignUp (simulating client refresh with null sign_up)
+        const nullSignUp = new SignUp(null);
+
+        // Assert: Signal should NOT be updated - should still have the existing SignUp
+        expect(signUpResourceSignal().resource).toBe(existingSignUp);
+        expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+      });
+
+      it('should allow null resource update when previous resource exists and canBeDiscarded is true', async () => {
+        // Arrange: Set up a SignUp with id and mock setActive
+        const mockSetActive = vi.fn().mockResolvedValue({});
+        SignUp.clerk = { setActive: mockSetActive } as any;
+
+        const existingSignUp = new SignUp({
+          id: 'signup_123',
+          status: 'complete',
+          created_session_id: 'session_123',
+        } as any);
+        expect(signUpResourceSignal().resource).toBe(existingSignUp);
+        expect(existingSignUp.__internal_future.canBeDiscarded).toBe(false);
+
+        // Act: Call finalize() which sets canBeDiscarded to true
+        await existingSignUp.__internal_future.finalize();
+
+        // Verify canBeDiscarded is now true
+        expect(existingSignUp.__internal_future.canBeDiscarded).toBe(true);
+        expect(mockSetActive).toHaveBeenCalledWith({ session: 'session_123', navigate: undefined });
+
+        // Now emit a null resource update (simulating client refresh with null sign_up)
+        const nullSignUp = new SignUp(null);
+
+        // Assert: The null update SHOULD be allowed because canBeDiscarded is true
+        // shouldIgnoreNullUpdate returns false when canBeDiscarded is true
+        expect(signUpResourceSignal().resource).toBe(nullSignUp);
+        expect(signUpResourceSignal().resource?.id).toBeUndefined();
+      });
+
+      it('should allow null resource update after reset() is called', async () => {
+        // Arrange: Set up mock client that tracks the new SignUp created during reset
+        let newSignUpFromReset: SignUp | null = null;
+        const mockClient = {
+          signUp: new SignUp(null),
+          resetSignUp: vi.fn().mockImplementation(function (this: typeof mockClient) {
+            newSignUpFromReset = new SignUp(null);
+            this.signUp = newSignUpFromReset;
+            // reset() emits resource:error to clear errors, but the signal update
+            // happens via resource:update when the new SignUp is created
+            eventBus.emit('resource:error', { resource: newSignUpFromReset, error: null });
+            // Emit resource:update to update the signal (simulating what happens in real flow)
+            eventBus.emit('resource:update', { resource: newSignUpFromReset });
+          }),
+        };
+        SignUp.clerk = { client: mockClient } as any;
+
+        // Create a SignUp with id
+        const existingSignUp = new SignUp({ id: 'signup_123', status: 'missing_requirements' } as any);
+        expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+        expect(existingSignUp.__internal_future.canBeDiscarded).toBe(false);
+
+        // Act: Call reset() - this sets canBeDiscarded to true before resetting
+        await existingSignUp.__internal_future.reset();
+
+        // Assert: Verify reset was called
+        expect(mockClient.resetSignUp).toHaveBeenCalled();
+
+        // Assert: Verify canBeDiscarded was set to true on the original SignUp
+        expect(existingSignUp.__internal_future.canBeDiscarded).toBe(true);
+
+        // Assert: Verify the signal was updated with a new SignUp that has no id
+        // The previous id 'signup_123' should be gone
+        expect(signUpResourceSignal().resource).toBe(newSignUpFromReset);
+        expect(signUpResourceSignal().resource?.id).toBeUndefined();
+        expect(signUpResourceSignal().resource).not.toBe(existingSignUp);
+      });
+
+      it('should allow resource update when new resource has an id (not a null update)', () => {
+        // Arrange: Set up a SignUp with id
+        const existingSignUp = new SignUp({ id: 'signup_123', status: 'missing_requirements' } as any);
+        expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+
+        // Act: Emit a resource update with a different SignUp that also has an id
+        const newSignUp = new SignUp({ id: 'signup_456', status: 'complete' } as any);
+
+        // Assert: Signal should be updated with the new SignUp
+        expect(signUpResourceSignal().resource).toBe(newSignUp);
+        expect(signUpResourceSignal().resource?.id).toBe('signup_456');
+      });
+    });
+
+    describe('SignIn', () => {
+      it('should allow first resource update when previous resource is null', () => {
+        // Arrange: Signal starts with null
+        expect(signInResourceSignal().resource).toBeNull();
+
+        // Act: Emit a resource update with a SignIn that has an id
+        const signIn = new SignIn({ id: 'signin_123', status: 'needs_identifier' } as any);
+
+        // Assert: Signal should be updated
+        expect(signInResourceSignal().resource).toBe(signIn);
+        expect(signInResourceSignal().resource?.id).toBe('signin_123');
+      });
+
+      it('should ignore null resource update when previous resource exists and canBeDiscarded is false', () => {
+        // Arrange: Set up a SignIn with id and canBeDiscarded = false (default)
+        const existingSignIn = new SignIn({ id: 'signin_123', status: 'needs_identifier' } as any);
+        expect(signInResourceSignal().resource).toBe(existingSignIn);
+        expect(existingSignIn.__internal_future.canBeDiscarded).toBe(false);
+
+        // Act: Emit a resource update with a null SignIn (simulating client refresh with null sign_in)
+        const nullSignIn = new SignIn(null);
+
+        // Assert: Signal should NOT be updated - should still have the existing SignIn
+        expect(signInResourceSignal().resource).toBe(existingSignIn);
+        expect(signInResourceSignal().resource?.id).toBe('signin_123');
+      });
+    });
+  });
+
+  describe('Simulated client refresh scenarios', () => {
+    it('should protect against null update during email verification (before finalize)', () => {
+      // This test simulates the scenario described in USER-4372:
+      // 1. User is in the middle of sign up with verified email
+      // 2. Client refresh happens (e.g., router.refresh in Next.js)
+      // 3. Server returns sign_up: null because sign up is complete on server side
+      // 4. The null update should be IGNORED to prevent flash of default form
+
+      // Arrange: User has completed email verification but hasn't called finalize yet
+      const signUp = new SignUp({
+        id: 'signup_123',
+        status: 'complete',
+        created_session_id: 'session_123',
+        verifications: {
+          email_address: { status: 'verified' },
+        },
+      } as any);
+
+      expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+      expect(signUp.__internal_future.canBeDiscarded).toBe(false);
+
+      // Act: Simulate client refresh returning null sign_up
+      // This is what happens in Client.fromJSON when server returns sign_up: null
+      const nullSignUp = new SignUp(null);
+
+      // Assert: The null update should be IGNORED
+      expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+      expect(signUpResourceSignal().resource).toBe(signUp);
+    });
+
+    it('should allow null update after finalize sets canBeDiscarded to true', async () => {
+      // This test verifies that after finalize() is called,
+      // the null update IS allowed (because canBeDiscarded is true)
+
+      // Arrange
+      const mockSetActive = vi.fn().mockResolvedValue({});
+      SignUp.clerk = { setActive: mockSetActive } as any;
+
+      const signUp = new SignUp({
+        id: 'signup_123',
+        status: 'complete',
+        created_session_id: 'session_123',
+      } as any);
+
+      expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+      expect(signUp.__internal_future.canBeDiscarded).toBe(false);
+
+      // Act: Call finalize - this sets canBeDiscarded to true
+      await signUp.__internal_future.finalize();
+
+      // Verify canBeDiscarded is now true
+      expect(signUp.__internal_future.canBeDiscarded).toBe(true);
+
+      // Now simulate a null update
+      const nullSignUp = new SignUp(null);
+
+      // Assert: The null update SHOULD be allowed now
+      // Because canBeDiscarded is true, shouldIgnoreNullUpdate returns false
+      expect(signUpResourceSignal().resource).toBe(nullSignUp);
+      expect(signUpResourceSignal().resource?.id).toBeUndefined();
+    });
+  });
+
+  describe('Edge cases', () => {
+    it('should handle rapid successive updates correctly', () => {
+      // First update with valid SignUp
+      const signUp1 = new SignUp({ id: 'signup_1', status: 'missing_requirements' } as any);
+      expect(signUpResourceSignal().resource?.id).toBe('signup_1');
+
+      // Second update with another valid SignUp
+      const signUp2 = new SignUp({ id: 'signup_2', status: 'missing_requirements' } as any);
+      expect(signUpResourceSignal().resource?.id).toBe('signup_2');
+
+      // Null update should be ignored
+      const nullSignUp = new SignUp(null);
+      expect(signUpResourceSignal().resource?.id).toBe('signup_2');
+
+      // Another valid update should work
+      const signUp3 = new SignUp({ id: 'signup_3', status: 'complete' } as any);
+      expect(signUpResourceSignal().resource?.id).toBe('signup_3');
+    });
+
+    it('should handle update with same instance correctly', () => {
+      // Create a SignUp
+      const signUp = new SignUp({ id: 'signup_123', status: 'missing_requirements' } as any);
+      expect(signUpResourceSignal().resource?.id).toBe('signup_123');
+
+      // Manually emit update with the same instance (simulating fromJSON on same instance)
+      eventBus.emit('resource:update', { resource: signUp });
+
+      // Signal should still have the same instance
+      expect(signUpResourceSignal().resource).toBe(signUp);
+    });
+  });
+});