diff --git a/src/lib/actions/maintenance.test.ts b/src/lib/actions/maintenance.test.ts
index 1bf4fdb19d7..cba6c69c1f0 100644
--- a/src/lib/actions/maintenance.test.ts
+++ b/src/lib/actions/maintenance.test.ts
@@ -39,12 +39,10 @@ vi.mock("../credentials/store", () => ({
 vi.mock("../domain/lifecycle/options", () => ({
   normalizeGarbageCollectImagesOptions: (o: unknown) => o || {},
 }));
-vi.mock("../domain/maintenance/images", () => ({
-  findOrphanedSandboxImages: vi.fn().mockReturnValue([]),
-  parseSandboxImageRows: vi.fn().mockReturnValue([]),
-}));
+// ../domain/maintenance/images is left unmocked so the gc tests run the real
+// orphan-detection helpers and can assert on gc's actual output.
 
-import { backupAll, shouldSkipUnreachableSandboxBackup } from "./maintenance";
+import { backupAll, garbageCollectImages, shouldSkipUnreachableSandboxBackup } from "./maintenance";
 
 describe("backupAll", () => {
   beforeEach(() => {
@@ -350,3 +348,40 @@ describe("shouldSkipUnreachableSandboxBackup", () => {
     expect(shouldSkipUnreachableSandboxBackup({})).toBe(false);
   });
 });
+
+describe("garbageCollectImages", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("surfaces a local-repo orphan while preserving a registered local image (#6301)", async () => {
+    // Local repo holds an orphan (gc-test-orphan-111) plus a still-registered
+    // image (live-222); the gateway repo holds only an in-use image.
+    mocks.dockerListImagesFormat.mockImplementation((repo: string) =>
+      repo === "nemoclaw-sandbox-local"
+        ? "nemoclaw-sandbox-local:gc-test-orphan-111\t3GB\nnemoclaw-sandbox-local:live-222\t2GB"
+        : "openshell/sandbox-from:in-use\t1GB",
+    );
+    mocks.listSandboxes.mockReturnValue({
+      sandboxes: [
+        { imageTag: "nemoclaw-sandbox-local:live-222" },
+        { imageTag: "openshell/sandbox-from:in-use" },
+      ],
+      defaultSandbox: null,
+    });
+    const logSpy = vi.spyOn(console, "log").mockImplementation(() => undefined);
+
+    await garbageCollectImages({ dryRun: true });
+
+    const out = logSpy.mock.calls.flat().join("\n");
+    logSpy.mockRestore();
+
+    // The local orphan is reported, the still-registered local image is not,
+    // and both repos are scanned.
+    expect(out).toContain("nemoclaw-sandbox-local:gc-test-orphan-111");
+    expect(out).not.toContain("nemoclaw-sandbox-local:live-222");
+    const scannedRepos = mocks.dockerListImagesFormat.mock.calls.map((call) => call[0]);
+    expect(scannedRepos).toContain("openshell/sandbox-from");
+    expect(scannedRepos).toContain("nemoclaw-sandbox-local");
+  });
+});
diff --git a/src/lib/actions/maintenance.ts b/src/lib/actions/maintenance.ts
index 4daa7f0fea1..d0ef891f456 100644
--- a/src/lib/actions/maintenance.ts
+++ b/src/lib/actions/maintenance.ts
@@ -9,6 +9,7 @@ import {
   normalizeGarbageCollectImagesOptions,
 } from "../domain/lifecycle/options";
 import { findOrphanedSandboxImages, parseSandboxImageRows } from "../domain/maintenance/images";
+import { SANDBOX_IMAGE_REPOS } from "../domain/sandbox/image-tag";
 import { captureSandboxListWithGatewayPreflightOrExit } from "../openshell-sandbox-list";
 import { parseReadySandboxNames } from "../runtime-recovery";
 import * as registry from "../state/registry";
@@ -147,10 +148,11 @@ export async function garbageCollectImages(
 
   let imagesOutput = "";
   try {
-    imagesOutput = dockerListImagesFormat(
-      "openshell/sandbox-from",
-      "{{.Repository}}:{{.Tag}}\t{{.Size}}",
-    );
+    // Scan every sandbox image repo, not just sandbox-from; see
+    // SANDBOX_IMAGE_REPOS for why local prebuilds were missed (#6301).
+    imagesOutput = SANDBOX_IMAGE_REPOS.map((repo) =>
+      dockerListImagesFormat(repo, "{{.Repository}}:{{.Tag}}\t{{.Size}}"),
+    ).join("\n");
   } catch {
     console.error("  Failed to query Docker images. Is Docker running?");
     process.exit(1);
diff --git a/src/lib/domain/maintenance/images.test.ts b/src/lib/domain/maintenance/images.test.ts
index 0e4b752e19d..2c3fc8214b2 100644
--- a/src/lib/domain/maintenance/images.test.ts
+++ b/src/lib/domain/maintenance/images.test.ts
@@ -32,4 +32,23 @@ describe("maintenance image helpers", () => {
       ),
     ).toEqual([{ tag: "openshell/sandbox-from:two", size: "2GB" }]);
   });
+
+  it("orphans a local image while keeping a registered local image (#6301)", () => {
+    // A locally prebuilt sandbox left an orphan under nemoclaw-sandbox-local;
+    // the matcher must flag it by tag regardless of repo, and preserve the
+    // still-registered local image of another sandbox.
+    expect(
+      findOrphanedSandboxImages(
+        [
+          { tag: "openshell/sandbox-from:one", size: "1GB" },
+          { tag: "nemoclaw-sandbox-local:live-222", size: "2GB" },
+          { tag: "nemoclaw-sandbox-local:gc-test-111", size: "3GB" },
+        ],
+        [
+          { imageTag: "openshell/sandbox-from:one" },
+          { imageTag: "nemoclaw-sandbox-local:live-222" },
+        ],
+      ),
+    ).toEqual([{ tag: "nemoclaw-sandbox-local:gc-test-111", size: "3GB" }]);
+  });
 });
diff --git a/src/lib/domain/sandbox/image-tag.ts b/src/lib/domain/sandbox/image-tag.ts
index 9aff3b80ca0..e32fed13019 100644
--- a/src/lib/domain/sandbox/image-tag.ts
+++ b/src/lib/domain/sandbox/image-tag.ts
@@ -1,6 +1,23 @@
 // SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+/** Gateway-built sandbox images (openshell sandbox create). */
+export const SANDBOX_FROM_IMAGE_REPO = "openshell/sandbox-from";
+/**
+ * Locally prebuilt sandbox images, tagged by the docker-driver-gateway path
+ * (Linux, or macOS on Apple Silicon — see isLinuxDockerDriverGatewayEnabled).
+ */
+export const LOCAL_SANDBOX_IMAGE_REPO = "nemoclaw-sandbox-local";
+
+/**
+ * Every Docker repository that can hold a sandbox image. Any orphan sweep
+ * (`nemoclaw gc`) must enumerate all of them: locally prebuilt sandboxes are
+ * tagged under LOCAL_SANDBOX_IMAGE_REPO, not the gateway-side
+ * SANDBOX_FROM_IMAGE_REPO, so scanning only the latter left local orphans
+ * invisible to gc (#6301).
+ */
+export const SANDBOX_IMAGE_REPOS = [SANDBOX_FROM_IMAGE_REPO, LOCAL_SANDBOX_IMAGE_REPO] as const;
+
 const BUILT_SANDBOX_IMAGE_RE = /Built image (openshell\/sandbox-from:\d+)/;
 
 export function resolveSandboxImageTagFromCreateOutput(
@@ -16,5 +33,5 @@ export function resolveSandboxImageTagFromCreateOutput(
   warn(
     "  Warning: could not parse image tag from build output; imageTag may be stale. Run 'nemoclaw gc' if destroy fails.",
   );
-  return `openshell/sandbox-from:${buildId}`;
+  return `${SANDBOX_FROM_IMAGE_REPO}:${buildId}`;
 }
diff --git a/src/lib/onboard/sandbox-prebuild.ts b/src/lib/onboard/sandbox-prebuild.ts
index f4feb12d9c2..0fb2c6928c9 100644
--- a/src/lib/onboard/sandbox-prebuild.ts
+++ b/src/lib/onboard/sandbox-prebuild.ts
@@ -6,6 +6,7 @@ import os from "node:os";
 import path from "node:path";
 
 import { dockerSpawn } from "../adapters/docker/exec";
+import { LOCAL_SANDBOX_IMAGE_REPO } from "../domain/sandbox/image-tag";
 import {
   SANDBOX_BUILD_CONTEXT_PREFIX,
   type SandboxBuildContextOrigin,
@@ -14,7 +15,7 @@ import { buildSubprocessEnv } from "../subprocess-env";
 
 const TRUTHY_FLAG_VALUES = new Set(["1", "true", "yes", "on"]);
 const FALSY_FLAG_VALUES = new Set(["0", "false", "no", "off"]);
-const LOCAL_IMAGE_REPO = "nemoclaw-sandbox-local";
+const LOCAL_IMAGE_REPO = LOCAL_SANDBOX_IMAGE_REPO;
 const DOCKER_ENV_NAMES = [
   "DOCKER_API_VERSION",
   "DOCKER_CERT_PATH",