diff --git a/packages/errors/package.json b/packages/errors/package.json
index 913b8693..f2adac91 100644
--- a/packages/errors/package.json
+++ b/packages/errors/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@bytelyst/errors",
-  "version": "0.1.11",
+  "version": "0.1.13",
   "type": "module",
   "exports": {
     ".": {
diff --git a/packages/errors/src/__tests__/errors.test.ts b/packages/errors/src/__tests__/errors.test.ts
index b9aa6a04..1724c422 100644
--- a/packages/errors/src/__tests__/errors.test.ts
+++ b/packages/errors/src/__tests__/errors.test.ts
@@ -54,3 +54,41 @@ describe('HTTP error classes', () => {
     expect(err.details).toEqual({ retryAfter: 60 });
   });
 });
+
+describe('cross-instance detection (duplicated module copy)', () => {
+  // Simulate a SECOND physical copy of @bytelyst/errors: a structurally
+  // identical class that brands itself with the same global symbol but has a
+  // different prototype chain (so plain prototype-based instanceof would fail).
+  const BRAND = Symbol.for('@bytelyst/errors.ServiceError');
+  class DuplicatedServiceError extends Error {
+    constructor(
+      public statusCode: number,
+      message: string
+    ) {
+      super(message);
+      this.name = 'ServiceError';
+      Object.defineProperty(this, BRAND, { value: true, enumerable: false });
+    }
+  }
+
+  it('recognizes a ServiceError created by a duplicated copy', () => {
+    const err = new DuplicatedServiceError(409, 'conflict');
+    expect(err).toBeInstanceOf(ServiceError);
+    // Sanity: prototype chain genuinely differs (this is the bug scenario).
+    expect(Object.getPrototypeOf(err)).not.toBe(ServiceError.prototype);
+  });
+
+  it('does not treat plain objects or non-branded errors as ServiceError', () => {
+    expect(new Error('plain') instanceof ServiceError).toBe(false);
+    expect({ statusCode: 500 } instanceof ServiceError).toBe(false);
+    expect((null as unknown) instanceof ServiceError).toBe(false);
+  });
+
+  it('preserves subclass instanceof semantics', () => {
+    const notFound = new NotFoundError();
+    expect(notFound instanceof NotFoundError).toBe(true);
+    expect(notFound instanceof ServiceError).toBe(true);
+    // A NotFoundError must NOT be an instance of an unrelated sibling subclass.
+    expect(notFound instanceof BadRequestError).toBe(false);
+  });
+});
diff --git a/packages/errors/src/service-error.ts b/packages/errors/src/service-error.ts
index 1bc957f4..0b9fc35f 100644
--- a/packages/errors/src/service-error.ts
+++ b/packages/errors/src/service-error.ts
@@ -1,3 +1,12 @@
+/**
+ * Cross-realm brand for ServiceError instances.
+ *
+ * `Symbol.for()` returns the *same* symbol across every copy of this module,
+ * so even if a bundler/packer (e.g. docker-prep tarballs) produces a second
+ * physical copy of `@bytelyst/errors`, branded instances stay recognizable.
+ */
+const SERVICE_ERROR_BRAND = Symbol.for('@bytelyst/errors.ServiceError');
+
 /**
  * Base error class for typed HTTP service errors.
  * All specific error types extend this class.
@@ -10,5 +19,30 @@ export class ServiceError extends Error {
   ) {
     super(message);
     this.name = 'ServiceError';
+    // Brand the instance so `instanceof ServiceError` works even when this
+    // module has been duplicated and prototype identity no longer matches.
+    Object.defineProperty(this, SERVICE_ERROR_BRAND, {
+      value: true,
+      enumerable: false,
+      writable: false,
+      configurable: false,
+    });
+  }
+
+  /**
+   * Custom `instanceof` that is resilient to duplicated module copies.
+   *
+   * - For the base `ServiceError`, recognize any brand-carrying instance
+   *   regardless of which physical copy of this module created it.
+   * - For subclasses, fall back to the standard prototype-chain check so that
+   *   semantics like `notFound instanceof BadRequestError === false` hold.
+   */
+  static [Symbol.hasInstance](instance: unknown): boolean {
+    if (typeof instance !== 'object' || instance === null) return false;
+    if (this === ServiceError) {
+      return (instance as Record<symbol, unknown>)[SERVICE_ERROR_BRAND] === true;
+    }
+    const proto = (this as unknown as { prototype: object }).prototype;
+    return Object.prototype.isPrototypeOf.call(proto, instance);
   }
 }
diff --git a/packages/fastify-core/src/__tests__/fastify-core.test.ts b/packages/fastify-core/src/__tests__/fastify-core.test.ts
index 77e6233d..f95ac930 100644
--- a/packages/fastify-core/src/__tests__/fastify-core.test.ts
+++ b/packages/fastify-core/src/__tests__/fastify-core.test.ts
@@ -99,6 +99,37 @@ describe('createServiceApp', () => {
     await app.close();
   });
 
+  it('handles a ServiceError from a DUPLICATED module copy (docker-prep tarball scenario)', async () => {
+    // Simulate a second physical copy of @bytelyst/errors: a structurally
+    // identical, brand-carrying class with a different prototype chain. The
+    // handler must still map it to its statusCode instead of falling to 500.
+    const BRAND = Symbol.for('@bytelyst/errors.ServiceError');
+    class DuplicatedConflictError extends Error {
+      statusCode = 409;
+      details?: Record<string, unknown>;
+      constructor(message: string) {
+        super(message);
+        this.name = 'ServiceError';
+        Object.defineProperty(this, BRAND, { value: true, enumerable: false });
+      }
+    }
+    const app = await createServiceApp({
+      name: 'test',
+      version: '0.1.0',
+      logger: false,
+    });
+
+    app.get('/conflict', async () => {
+      throw new DuplicatedConflictError('Version conflict');
+    });
+
+    const res = await app.inject({ method: 'GET', url: '/conflict' });
+    expect(res.statusCode).toBe(409);
+    expect(JSON.parse(res.payload).error).toBe('Version conflict');
+
+    await app.close();
+  });
+
   it('handles ServiceError with details', async () => {
     const { BadRequestError } = await import('@bytelyst/errors');
     const app = await createServiceApp({
diff --git a/scripts/gitea/.publish-manifest.json b/scripts/gitea/.publish-manifest.json
index fd9de96d..b7c831fc 100644
--- a/scripts/gitea/.publish-manifest.json
+++ b/scripts/gitea/.publish-manifest.json
@@ -140,9 +140,9 @@
     "publishedAt": "2026-05-27T08:43:49.944Z"
   },
   "@bytelyst/errors": {
-    "version": "0.1.11",
-    "contentHash": "76443c39ff29478a483cb82439dcc44c5a25a1e87cf53eba3c1ba1e30b7947cf",
-    "publishedAt": "2026-05-27T08:43:51.483Z"
+    "version": "0.1.13",
+    "contentHash": "73720df322fdc53a53a27a680f1c4765ad81f611c95c9ede48808da7183a7ccc",
+    "publishedAt": "2026-05-29T05:02:21.329Z"
   },
   "@bytelyst/event-store": {
     "version": "0.1.6",