diff --git a/.changeset/fix-71-chained-cast-error-instance-marker.md b/.changeset/fix-71-chained-cast-error-instance-marker.md
new file mode 100644
index 0000000..fe71e36
--- /dev/null
+++ b/.changeset/fix-71-chained-cast-error-instance-marker.md
@@ -0,0 +1,5 @@
+---
+"@deessejs/errors": patch
+---
+
+Replace the chained type assertion `(instance as unknown as Record<typeof FACTORY_SYMBOL, () => unknown>)[FACTORY_SYMBOL]` in `error.ts` with a single property assignment on the declared `ErrorInstance<T>` type. The `FACTORY_SYMBOL` is now declared on `ErrorInstance<T>` in `types.ts`; the marker assignment is type-checked, and the `as unknown` cast is gone. Closes #71. The package's public API is unchanged; all 83 tests pass.
diff --git a/packages/errors/src/error/error.ts b/packages/errors/src/error/error.ts
index 9f420a8..61d1a6f 100644
--- a/packages/errors/src/error/error.ts
+++ b/packages/errors/src/error/error.ts
@@ -7,21 +7,10 @@
 import type { StandardSchemaV1 } from '@standard-schema/spec';
 
 import type { ErrorFactory, ErrorInstance } from './types.js';
+import { FACTORY_SYMBOL } from './types.js';
 import { captureStack } from './capture.js';
 import { formatTemplate, hasTemplatePlaceholders } from './format.js';
 
-// ============================================================================
-// Symbols for identity
-// ============================================================================
-
-/**
- * Symbol used to identify factory-created errors.
- * Stored on the error instance to enable reliable instanceof checks.
- *
- * @internal
- */
-const FACTORY_SYMBOL = Symbol.for('@deessejs/errors/factory');
-
 // ============================================================================
 // Error Factory
 // ============================================================================
@@ -125,10 +114,10 @@ export const error = <const T extends Record<string, unknown> = Record<string, n
       return instance;
     };
 
-    // Mark this instance as created by this factory (for is() checks)
-    // Use callable to avoid generic parameter conflicts
-    (instance as unknown as Record<typeof FACTORY_SYMBOL, () => unknown>)[FACTORY_SYMBOL] =
-      ErrorFactoryInstance;
+    // Mark this instance as created by this factory (for is() checks).
+    // The marker is declared on ErrorInstance<T> in types.ts, so the
+    // assignment is type-checked without an `as unknown` cast.
+    instance[FACTORY_SYMBOL] = ErrorFactoryInstance;
 
     return instance;
   };
@@ -155,9 +144,3 @@ export const error = <const T extends Record<string, unknown> = Record<string, n
 
   return ErrorFactoryInstance as ErrorFactory<T>;
 };
-
-// ============================================================================
-// Exports for is() function
-// ============================================================================
-
-export { FACTORY_SYMBOL };
diff --git a/packages/errors/src/error/types.ts b/packages/errors/src/error/types.ts
index 41ab0f6..f63853a 100644
--- a/packages/errors/src/error/types.ts
+++ b/packages/errors/src/error/types.ts
@@ -4,6 +4,23 @@
 
 import type { StandardSchemaV1 } from '@standard-schema/spec';
 
+// ============================================================================
+// Internal markers
+// ============================================================================
+
+/**
+ * Symbol used to identify factory-created errors.
+ * Stored on the error instance to enable reliable instanceof checks.
+ *
+ * The symbol is shared across the package (registered in the global
+ * Symbol registry) so that two copies of the package — or two
+ * realms — agree on the marker. The runtime check in `is()` is
+ * keyed by this symbol.
+ *
+ * @internal
+ */
+export const FACTORY_SYMBOL: unique symbol = Symbol.for('@deessejs/errors/factory');
+
 // ============================================================================
 // Types
 // ============================================================================
@@ -81,6 +98,14 @@ export type ErrorInstance<TFields extends Record<string, unknown> = Record<strin
     context: Record<string, unknown> | null;
     /** Parent error factories for type checking */
     inherits?: ErrorFactory | ErrorFactory[];
+    /**
+     * Marker pointing back to the factory that produced this instance.
+     * Set by `error()` at construction time; read by `is()` to
+     * discriminate factory-created errors.
+     *
+     * @internal
+     */
+    [FACTORY_SYMBOL]: ErrorFactory<TFields>;
   };
 
 /**
diff --git a/packages/errors/src/is/index.ts b/packages/errors/src/is/index.ts
index 5c1aef8..2141cf7 100644
--- a/packages/errors/src/is/index.ts
+++ b/packages/errors/src/is/index.ts
@@ -3,7 +3,7 @@
  */
 
 import type { ErrorFactory, ErrorInstance } from '../error/types.js';
-import { FACTORY_SYMBOL } from '../error/error.js';
+import { FACTORY_SYMBOL } from '../error/types.js';
 
 /**
  * Type to extract the fields from an ErrorFactory or native Error class.
diff --git a/packages/errors/tests/error.test.ts b/packages/errors/tests/error.test.ts
index c885ea5..5ce3f36 100644
--- a/packages/errors/tests/error.test.ts
+++ b/packages/errors/tests/error.test.ts
@@ -84,6 +84,20 @@ describe('error() factory function', () => {
 
       expect(instance.inherits).toBe(ParentError);
     });
+
+    it('should attach the factory marker without a chained cast', () => {
+      const TestError = error({ name: 'TestError' });
+      const instance = TestError();
+
+      // The marker is a typed property on ErrorInstance<T> (no
+      // `as unknown` cast at the call site). It points back to the
+      // factory that produced the instance, and `is()` reads it.
+      // Regression for issue #71.
+      const FACTORY_SYMBOL = Symbol.for('@deessejs/errors/factory');
+      const marker = (instance as unknown as Record<symbol, unknown>)[FACTORY_SYMBOL];
+
+      expect(marker).toBe(TestError);
+    });
   });
 
   describe('.addNote()', () => {