diff --git a/spec/symbols.spec.ts b/spec/symbols.spec.ts
new file mode 100644
index 0000000..842a364
--- /dev/null
+++ b/spec/symbols.spec.ts
@@ -0,0 +1,120 @@
+/*
+ *                            *** MIT LICENSE ***
+ * -------------------------------------------------------------------------
+ * This code may be modified and distributed under the MIT license.
+ * See the LICENSE file for details.
+ * -------------------------------------------------------------------------
+ *
+ * @summary   Tests on state symbols shared across module copies
+ *
+ * @author    Alvis HT Tang <alvis@hilbert.space>
+ * @license   MIT
+ * @copyright Copyright (c) 2026 - All Rights Reserved.
+ * -------------------------------------------------------------------------
+ */
+
+import { describe, expect, it, vi } from 'vitest';
+
+type Copy = typeof import('#index');
+
+/**
+ * load an independent instance of the whole module graph, as a second bundled
+ * copy of xception in the same process would be
+ * @returns the freshly evaluated module namespace
+ */
+async function loadCopy(): Promise<Copy> {
+  vi.resetModules();
+
+  return import('#index');
+}
+
+const copyA = await loadCopy();
+const copyB = await loadCopy();
+
+describe('two module copies', () => {
+  it('should evaluate each copy into its own class', () => {
+    expect(copyB.Xception).not.toBe(copyA.Xception);
+  });
+
+  it('should resolve every state symbol to the same registry-global key', () => {
+    const pick = ({
+      $cause,
+      $code,
+      $meta,
+      $namespace,
+      $severity,
+      $tags,
+    }: Copy): Record<string, symbol> => ({
+      $cause,
+      $code,
+      $meta,
+      $namespace,
+      $severity,
+      $tags,
+    });
+
+    expect(pick(copyB)).toEqual(pick(copyA));
+  });
+
+  it('should treat an instance from another copy as an Xception', () => {
+    const error = new copyA.Xception('from copy A');
+
+    expect(error).toBeInstanceOf(copyB.Xception);
+  });
+
+  it('should treat a named subclass instance from another copy as that subclass', () => {
+    const NotFoundA = copyA.createXceptionClass('NotFoundError');
+    const NotFoundB = copyB.createXceptionClass('NotFoundError');
+
+    expect(new NotFoundA('missing')).toBeInstanceOf(NotFoundB);
+  });
+
+  it('should inherit severity and merge tags when wrapping a cause from another copy', () => {
+    const cause = new copyA.Xception('inner', {
+      severity: 'warning',
+      tags: ['inner'],
+    });
+
+    const error = new copyB.Xception('outer', { cause, tags: ['outer'] });
+
+    expect(error).toEqual(
+      expect.objectContaining({
+        cause,
+        severity: 'warning',
+        tags: ['inner', 'outer'],
+      }),
+    );
+  });
+
+  it('should carry namespace, meta and tags when converting an error from another copy', () => {
+    const cause = new copyA.Xception('inner', {
+      namespace: 'copy:a',
+      meta: { id: 1 },
+      tags: ['inner'],
+      severity: 'warning',
+    });
+
+    const error = copyB.xception(cause, { tags: ['outer'] });
+
+    expect(error).toEqual(
+      expect.objectContaining({
+        namespace: 'copy:a',
+        meta: { id: 1 },
+        tags: ['inner', 'outer'],
+        severity: 'warning',
+      }),
+    );
+  });
+
+  it('should serialize an error whose cause comes from another copy', () => {
+    const cause = new copyA.Xception('inner', { code: 'E_INNER' });
+
+    const error = new copyB.Xception('outer', { cause });
+
+    expect(error.toJSON()).toEqual(
+      expect.objectContaining({
+        cause: expect.objectContaining({ code: 'E_INNER', message: 'inner' }),
+      }),
+    );
+  });
+});
diff --git a/src/symbols.ts b/src/symbols.ts
index fb92037..eecc655 100644
--- a/src/symbols.ts
+++ b/src/symbols.ts
@@ -12,10 +12,13 @@
  * -------------------------------------------------------------------------
  */
 
-export const $namespace = Symbol('namespace');
-export const $tags = Symbol('tags');
-export const $cause = Symbol('cause');
-export const $meta = Symbol('meta');
-export const $severity = Symbol('severity');
-export const $code = Symbol('code');
-export const $brands = Symbol('xception.brands');
+// NOTE: every key is registry-global so that an instance created by one copy of
+// this module (e.g. a second bundled copy in the same process) keeps its state
+// readable by another copy, which a module-local Symbol() would hide
+export const $namespace: unique symbol = Symbol.for('xception.namespace');
+export const $tags: unique symbol = Symbol.for('xception.tags');
+export const $cause: unique symbol = Symbol.for('xception.cause');
+export const $meta: unique symbol = Symbol.for('xception.meta');
+export const $severity: unique symbol = Symbol.for('xception.severity');
+export const $code: unique symbol = Symbol.for('xception.code');
+export const $brands: unique symbol = Symbol.for('xception.brands');