diff --git a/packages/core/src/errors.ts b/packages/core/src/errors.ts
index b74a92cc..9cfc65b7 100644
--- a/packages/core/src/errors.ts
+++ b/packages/core/src/errors.ts
@@ -1,5 +1,14 @@
 import { log } from './logger.js';
 
+/**
+ * Brand recognised across copies of this module. `@releasekit/release` bundles version, notes and
+ * publish from their own builds, and each of those inlines its own copy of core — so one process can
+ * hold several distinct `ReleaseKitError` classes, and `instanceof` against any one of them misses an
+ * error thrown through another (its code then degrades to GENERAL_ERROR). `Symbol.for` keys a
+ * process-wide registry, so every copy stamps and recognises the same brand.
+ */
+const RELEASEKIT_ERROR_BRAND = Symbol.for('@releasekit/core:ReleaseKitError');
+
 /**
  * Base error class that all releasekit errors should extend.
  * Provides consistent error handling with codes and suggestions.
@@ -11,6 +20,8 @@ export abstract class ReleaseKitError extends Error {
   constructor(message: string) {
     super(message);
     this.name = this.constructor.name;
+    // Non-enumerable, so the brand never shows up in a spread, JSON output, or a test's toEqual.
+    Object.defineProperty(this, RELEASEKIT_ERROR_BRAND, { value: true });
   }
 
   logError(): void {
@@ -24,8 +35,16 @@ export abstract class ReleaseKitError extends Error {
     }
   }
 
+  /** True for an error from any copy of core (see {@link RELEASEKIT_ERROR_BRAND}), not just this one. */
   static isReleaseKitError(error: unknown): error is ReleaseKitError {
-    return error instanceof ReleaseKitError;
+    if (error instanceof ReleaseKitError) {
+      return true;
+    }
+    return (
+      typeof error === 'object' &&
+      error !== null &&
+      (error as { [RELEASEKIT_ERROR_BRAND]?: unknown })[RELEASEKIT_ERROR_BRAND] === true
+    );
   }
 }
 
diff --git a/packages/core/test/unit/errors.spec.ts b/packages/core/test/unit/errors.spec.ts
index c2e89a23..c41d5074 100644
--- a/packages/core/test/unit/errors.spec.ts
+++ b/packages/core/test/unit/errors.spec.ts
@@ -93,6 +93,25 @@ describe('ReleaseKitError', () => {
       expect(ReleaseKitError.isReleaseKitError('error')).toBe(false);
       expect(ReleaseKitError.isReleaseKitError({})).toBe(false);
     });
+
+    it('should recognise an error from another copy of this module', async () => {
+      vi.resetModules();
+      const other = await import('../../src/errors.js');
+      class OtherCopyError extends other.ReleaseKitError {
+        readonly code = 'CONFIG_ERROR';
+        readonly suggestions: string[] = [];
+      }
+      const error = new OtherCopyError('from the other copy');
+
+      expect(other.ReleaseKitError).not.toBe(ReleaseKitError);
+      expect(error instanceof ReleaseKitError).toBe(false);
+      expect(ReleaseKitError.isReleaseKitError(error)).toBe(true);
+    });
+
+    it('should keep the brand out of enumerable properties', () => {
+      const error = new TestError('Test');
+      expect(Object.getOwnPropertySymbols({ ...error })).toEqual([]);
+    });
   });
 });
 
diff --git a/packages/release/test/unit/commands/cross-bundle-errors.spec.ts b/packages/release/test/unit/commands/cross-bundle-errors.spec.ts
new file mode 100644
index 00000000..cff03f3f
--- /dev/null
+++ b/packages/release/test/unit/commands/cross-bundle-errors.spec.ts
@@ -0,0 +1,61 @@
+import { EXIT_CODES, ReleaseKitError } from '@releasekit/core';
+import { LLMError } from '@releasekit/notes';
+import { BasePublishError, PublishErrorCode } from '@releasekit/publish';
+import { BaseVersionError } from '@releasekit/version';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('../../../src/release.js');
+
+import { createReleaseCommand } from '../../../src/commands/release-command.js';
+import { runRelease } from '../../../src/release.js';
+
+describe('release --json when a stage fails', () => {
+  let exitSpy: ReturnType<typeof vi.spyOn>;
+  let logSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    exitSpy = vi.spyOn(process, 'exit').mockImplementation(() => undefined as never);
+    logSpy = vi.spyOn(console, 'log').mockImplementation(() => undefined);
+    vi.spyOn(console, 'error').mockImplementation(() => undefined);
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it.each([
+    {
+      stage: 'version',
+      error: new BaseVersionError('git tag failed', 'GIT_ERROR'),
+      code: 'GIT_ERROR',
+      retryable: false,
+      exit: EXIT_CODES.GIT_ERROR,
+    },
+    {
+      stage: 'notes',
+      error: new LLMError('provider timed out', { retryable: true }),
+      code: 'LLM_ERROR',
+      retryable: true,
+      exit: EXIT_CODES.LLM_ERROR,
+    },
+    {
+      stage: 'publish',
+      error: new BasePublishError('npm auth is not configured', PublishErrorCode.CONFIG_ERROR),
+      code: 'CONFIG_ERROR',
+      retryable: false,
+      exit: EXIT_CODES.CONFIG_ERROR,
+    },
+  ])('should report $code from the $stage stage and exit $exit', async ({ stage, error, code, retryable, exit }) => {
+    expect(
+      error,
+      `the ${stage} build shares release's ReleaseKitError, so this case no longer tests a separate copy of core`,
+    ).not.toBeInstanceOf(ReleaseKitError);
+    vi.mocked(runRelease).mockRejectedValue(error);
+
+    await createReleaseCommand().parseAsync(['node', 'release', '--json']);
+
+    const envelope = JSON.parse(logSpy.mock.calls[0]?.[0] as string);
+    expect(envelope.errors).toMatchObject([{ code, retryable, message: error.message }]);
+    expect(exitSpy).toHaveBeenCalledWith(exit);
+  });
+});