diff --git a/src/commands/backup.atomic.test.ts b/src/commands/backup.atomic.test.ts
index 192631bc8f93..e914e553e985 100644
--- a/src/commands/backup.atomic.test.ts
+++ b/src/commands/backup.atomic.test.ts
@@ -6,6 +6,7 @@ import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it, vi }
 import { createTempHomeEnv, type TempHomeEnv } from "../test-utils/temp-home.js";
 import {
   backupVerifyCommandMock,
+  createMockTarStream,
   createBackupTestRuntime,
   mockStateOnlyBackupPlan,
   resetBackupTempHome,
@@ -70,7 +71,7 @@ describe("backupCreateCommand atomic archive write", () => {
       archivePrefix: "openclaw-backup-failure-",
     });
     try {
-      tarCreateMock.mockRejectedValueOnce(new Error("disk full"));
+      tarCreateMock.mockReturnValueOnce(createMockTarStream({ error: new Error("disk full") }));
 
       await expect(
         backupCreateCommand(runtime, {
@@ -93,9 +94,7 @@ describe("backupCreateCommand atomic archive write", () => {
     const realLink = fs.link.bind(fs);
     const linkSpy = vi.spyOn(fs, "link");
     try {
-      tarCreateMock.mockImplementationOnce(async ({ file }: { file: string }) => {
-        await fs.writeFile(file, "archive-bytes", "utf8");
-      });
+      tarCreateMock.mockReturnValueOnce(createMockTarStream());
       linkSpy.mockImplementationOnce(async (existingPath, newPath) => {
         await fs.writeFile(newPath, "concurrent-archive", "utf8");
         return await realLink(existingPath, newPath);
@@ -120,9 +119,7 @@ describe("backupCreateCommand atomic archive write", () => {
     });
     const linkSpy = vi.spyOn(fs, "link");
     try {
-      tarCreateMock.mockImplementationOnce(async ({ file }: { file: string }) => {
-        await fs.writeFile(file, "archive-bytes", "utf8");
-      });
+      tarCreateMock.mockReturnValueOnce(createMockTarStream());
       linkSpy.mockRejectedValueOnce(
         Object.assign(new Error("hard links not supported"), { code: "EOPNOTSUPP" }),
       );
diff --git a/src/commands/backup.test-support.ts b/src/commands/backup.test-support.ts
index 50aaa4e50ec2..fd2335d87b92 100644
--- a/src/commands/backup.test-support.ts
+++ b/src/commands/backup.test-support.ts
@@ -1,6 +1,7 @@
 // Backup test support provides temp config/state fixtures and mocked backup runtime helpers.
 import fs from "node:fs/promises";
 import path from "node:path";
+import { Readable } from "node:stream";
 import { vi } from "vitest";
 import type { RuntimeEnv } from "../runtime.js";
 import { deleteTestEnvValue } from "../test-utils/env.js";
@@ -14,6 +15,24 @@ const backupTestMocks = vi.hoisted(() => ({
 
 export const { backupVerifyCommandMock, tarCreateMock } = backupTestMocks;
 
+export function createMockTarStream(
+  params: {
+    beforeRead?: () => Promise<void> | void;
+    contents?: string;
+    error?: Error;
+  } = {},
+): Readable {
+  return Readable.from(
+    (async function* () {
+      await params.beforeRead?.();
+      if (params.error) {
+        throw params.error;
+      }
+      yield params.contents ?? "archive-bytes";
+    })(),
+  );
+}
+
 vi.mock("tar", () => ({
   c: backupTestMocks.tarCreateMock,
 }));
diff --git a/src/commands/backup.test.ts b/src/commands/backup.test.ts
index 7506a4428a94..ceef61de0b36 100644
--- a/src/commands/backup.test.ts
+++ b/src/commands/backup.test.ts
@@ -17,6 +17,7 @@ import {
 } from "./backup-shared.js";
 import {
   backupVerifyCommandMock,
+  createMockTarStream,
   createBackupTestRuntime,
   mockStateOnlyBackupPlan,
   resetBackupTempHome,
@@ -78,9 +79,7 @@ describe("backup commands", () => {
   beforeEach(async () => {
     await resetBackupTempHome(tempHome);
     tarCreateMock.mockReset();
-    tarCreateMock.mockImplementation(async ({ file }: { file: string }) => {
-      await fs.writeFile(file, "archive-bytes", "utf8");
-    });
+    tarCreateMock.mockImplementation(() => createMockTarStream());
     backupVerifyCommandMock.mockReset();
     backupVerifyCommandMock.mockResolvedValue({
       ok: true,
@@ -268,17 +267,16 @@ describe("backup commands", () => {
         }),
       );
       tarCreateMock.mockImplementationOnce(
-        async (
-          options: { file: string; onWriteEntry?: (entry: { path: string }) => void },
-          entryPaths: string[],
-        ) => {
-          capturedManifest = JSON.parse(
-            await fs.readFile(entryPaths[0], "utf8"),
-          ) as CapturedBackupManifest;
-          capturedEntryPaths = entryPaths;
-          capturedOnWriteEntry = options.onWriteEntry ?? null;
-          await fs.writeFile(options.file, "archive-bytes", "utf8");
-        },
+        (options: { onWriteEntry?: (entry: { path: string }) => void }, entryPaths: string[]) =>
+          createMockTarStream({
+            beforeRead: async () => {
+              capturedManifest = JSON.parse(
+                await fs.readFile(entryPaths[0], "utf8"),
+              ) as CapturedBackupManifest;
+              capturedEntryPaths = entryPaths;
+              capturedOnWriteEntry = options.onWriteEntry ?? null;
+            },
+          }),
       );
       const result = await backupCreateCommand(runtime, {
         output: backupDir,
@@ -367,21 +365,20 @@ describe("backup commands", () => {
       const runtime = createBackupTestRuntime();
       await mockStateOnlyBackupPlan(stateDir);
       tarCreateMock.mockImplementationOnce(
-        async (
-          options: { file: string; filter?: (entryPath: string) => boolean },
-          entryPaths: string[],
-        ) => {
-          const manifestPath = entryPaths[0];
-          const stateRoot = entryPaths[1];
-          if (!manifestPath || !stateRoot) {
-            throw new Error("backup test expected manifest and state entries");
-          }
-          expect(options.filter?.(manifestPath)).toBe(true);
-          expect(
-            options.filter?.(path.join(stateRoot, "agents", "main", "sessions", "s.jsonl")),
-          ).toBe(false);
-          await fs.writeFile(options.file, "archive-bytes", "utf8");
-        },
+        (options: { filter?: (entryPath: string) => boolean }, entryPaths: string[]) =>
+          createMockTarStream({
+            beforeRead: () => {
+              const manifestPath = entryPaths[0];
+              const stateRoot = entryPaths[1];
+              if (!manifestPath || !stateRoot) {
+                throw new Error("backup test expected manifest and state entries");
+              }
+              expect(options.filter?.(manifestPath)).toBe(true);
+              expect(
+                options.filter?.(path.join(stateRoot, "agents", "main", "sessions", "s.jsonl")),
+              ).toBe(false);
+            },
+          }),
       );
 
       const result = await backupCreateCommand(runtime, {