diff --git a/packages/k8s/src/k8s/index.ts b/packages/k8s/src/k8s/index.ts
index ae773da3..f8629dba 100644
--- a/packages/k8s/src/k8s/index.ts
+++ b/packages/k8s/src/k8s/index.ts
@@ -430,7 +430,7 @@ export async function execCpToPod(
       attempt++
       if (attempt >= 30) {
         throw new Error(
-          `cpToPod failed after ${attempt} attempts: ${JSON.stringify(error)}`
+          `cpToPod failed after ${attempt} attempts: ${error instanceof Error ? error.message : String(error)}`
         )
       }
       await sleep(1000)
@@ -527,7 +527,7 @@ export async function execCpFromPod(
       attempt++
       if (attempt >= 30) {
         throw new Error(
-          `execCpFromPod failed after ${attempt} attempts: ${JSON.stringify(error)}`
+          `execCpFromPod failed after ${attempt} attempts: ${error instanceof Error ? error.message : String(error)}`
         )
       }
       await sleep(1000)
@@ -574,7 +574,7 @@ export async function waitForJobToComplete(jobName: string): Promise<void> {
         return
       }
     } catch (error) {
-      throw new Error(`job ${jobName} has failed: ${JSON.stringify(error)}`)
+      throw new Error(`job ${jobName} has failed: ${error instanceof Error ? error.message : String(error)}`)
     }
     await backOffManager.backOff()
   }
@@ -697,7 +697,7 @@ export async function waitForPodPhases(
     }
   } catch (error) {
     throw new Error(
-      `Pod ${podName} is unhealthy with phase status ${phase}: ${JSON.stringify(error)}`
+      `Pod ${podName} is unhealthy with phase status ${phase}: ${error instanceof Error ? error.message : String(error)}`
     )
   }
 }
diff --git a/packages/k8s/tests/error-serialization-test.ts b/packages/k8s/tests/error-serialization-test.ts
new file mode 100644
index 00000000..875d9386
--- /dev/null
+++ b/packages/k8s/tests/error-serialization-test.ts
@@ -0,0 +1,198 @@
+const mockExec = jest.fn()
+const mockReadNamespacedPod = jest.fn()
+const mockReadNamespacedJob = jest.fn()
+
+jest.mock('@kubernetes/client-node', () => {
+  return {
+    KubeConfig: jest.fn().mockImplementation(() => ({
+      loadFromDefault: jest.fn(),
+      makeApiClient: jest.fn().mockImplementation(ApiClass => {
+        const name = ApiClass?.name || ApiClass?.toString() || ''
+        if (name.includes('Batch')) {
+          return { readNamespacedJob: mockReadNamespacedJob }
+        }
+        if (name.includes('Authorization')) {
+          return { createSelfSubjectAccessReview: jest.fn() }
+        }
+        return { readNamespacedPod: mockReadNamespacedPod }
+      }),
+      getContexts: jest
+        .fn()
+        .mockReturnValue([{ namespace: 'test-namespace' }])
+    })),
+    Exec: jest.fn().mockImplementation(() => ({ exec: mockExec })),
+    CoreV1Api: class CoreV1Api {},
+    BatchV1Api: class BatchV1Api {},
+    AuthorizationV1Api: class AuthorizationV1Api {},
+    Log: jest.fn()
+  }
+})
+
+jest.mock('tar-fs', () => ({
+  default: {
+    pack: jest.fn().mockReturnValue({ pipe: jest.fn() }),
+    extract: jest.fn().mockReturnValue({
+      on: jest.fn(),
+      pipe: jest.fn()
+    })
+  },
+  __esModule: true
+}))
+
+jest.mock('../src/k8s/utils', () => {
+  const actual = jest.requireActual('../src/k8s/utils')
+  return {
+    ...actual,
+    sleep: jest.fn().mockResolvedValue(undefined)
+  }
+})
+
+import {
+  execCpToPod,
+  execCpFromPod,
+  waitForJobToComplete,
+  waitForPodPhases
+} from '../src/k8s'
+import { PodPhase } from '../src/k8s/utils'
+
+describe('error serialization', () => {
+  beforeEach(() => {
+    jest.clearAllMocks()
+    process.env['ACTIONS_RUNNER_KUBERNETES_NAMESPACE'] = 'test-namespace'
+  })
+
+  afterEach(() => {
+    delete process.env['ACTIONS_RUNNER_KUBERNETES_NAMESPACE']
+  })
+
+  describe('execCpToPod', () => {
+    it('should include Error.message in thrown error after retries', async () => {
+      mockExec.mockRejectedValue(new Error('connection refused'))
+
+      await expect(
+        execCpToPod('test-pod', '/tmp/src', '/workspace')
+      ).rejects.toThrow('cpToPod failed after 30 attempts: connection refused')
+    })
+
+    it('should use String() for non-Error throwables', async () => {
+      mockExec.mockRejectedValue('raw string error')
+
+      await expect(
+        execCpToPod('test-pod', '/tmp/src', '/workspace')
+      ).rejects.toThrow('cpToPod failed after 30 attempts: raw string error')
+    })
+
+    it('should not produce empty braces in error message', async () => {
+      mockExec.mockRejectedValue(new Error('ETIMEOUT'))
+
+      await expect(
+        execCpToPod('test-pod', '/tmp/src', '/workspace')
+      ).rejects.toMatchObject({
+        message: expect.not.stringContaining('{}')
+      })
+    })
+  })
+
+  describe('execCpFromPod', () => {
+    it('should include Error.message in thrown error after retries', async () => {
+      mockExec.mockRejectedValue(new Error('container not found'))
+
+      await expect(
+        execCpFromPod('test-pod', '/workspace/output', '/tmp/dst')
+      ).rejects.toThrow(
+        'execCpFromPod failed after 30 attempts: container not found'
+      )
+    })
+
+    it('should use String() for non-Error throwables', async () => {
+      mockExec.mockRejectedValue(42)
+
+      await expect(
+        execCpFromPod('test-pod', '/workspace/output', '/tmp/dst')
+      ).rejects.toThrow('execCpFromPod failed after 30 attempts: 42')
+    })
+  })
+
+  describe('waitForJobToComplete', () => {
+    it('should include Error.message when job fails', async () => {
+      mockReadNamespacedJob.mockResolvedValue({
+        status: { failed: 1 }
+      })
+
+      await expect(waitForJobToComplete('my-job')).rejects.toThrow(
+        'job my-job has failed: job my-job has failed'
+      )
+    })
+
+    it('should include Error.message when API call throws', async () => {
+      mockReadNamespacedJob.mockRejectedValue(
+        new Error('403 Forbidden')
+      )
+
+      await expect(waitForJobToComplete('my-job')).rejects.toThrow(
+        'job my-job has failed: 403 Forbidden'
+      )
+    })
+
+    it('should use String() for non-Error throwables from API', async () => {
+      mockReadNamespacedJob.mockRejectedValue('unexpected API failure')
+
+      await expect(waitForJobToComplete('my-job')).rejects.toThrow(
+        'job my-job has failed: unexpected API failure'
+      )
+    })
+  })
+
+  describe('waitForPodPhases', () => {
+    it('should include error message when pod enters unhealthy phase', async () => {
+      mockReadNamespacedPod.mockResolvedValue({
+        status: { phase: 'Failed' }
+      })
+
+      await expect(
+        waitForPodPhases(
+          'test-pod',
+          new Set([PodPhase.RUNNING]),
+          new Set([PodPhase.PENDING])
+        )
+      ).rejects.toThrow(
+        /Pod test-pod is unhealthy with phase status Failed/
+      )
+    })
+
+    it('should include Error.message when API call throws', async () => {
+      mockReadNamespacedPod.mockRejectedValue(
+        new Error('network timeout')
+      )
+
+      await expect(
+        waitForPodPhases(
+          'test-pod',
+          new Set([PodPhase.RUNNING]),
+          new Set([PodPhase.PENDING])
+        )
+      ).rejects.toThrow(
+        'Pod test-pod is unhealthy with phase status Unknown: network timeout'
+      )
+    })
+
+    it('should not produce empty braces from Error objects', async () => {
+      mockReadNamespacedPod.mockRejectedValue(
+        new Error('socket hang up')
+      )
+
+      try {
+        await waitForPodPhases(
+          'test-pod',
+          new Set([PodPhase.RUNNING]),
+          new Set([PodPhase.PENDING])
+        )
+        fail('Expected waitForPodPhases to throw')
+      } catch (error) {
+        const msg = (error as Error).message
+        expect(msg).not.toContain('{}')
+        expect(msg).toContain('socket hang up')
+      }
+    })
+  })
+})