diff --git a/src/lib/onboard/prompt-helpers.test.ts b/src/lib/onboard/prompt-helpers.test.ts
new file mode 100644
index 0000000000..33c6ea43bb
--- /dev/null
+++ b/src/lib/onboard/prompt-helpers.test.ts
@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+import { describe, expect, it, vi } from "vitest";
+// Import from compiled dist/ so coverage is attributed correctly.
+import { promptOrDefault } from "../../../dist/lib/onboard/prompt-helpers";
+
+function makeDeps(promptReply: string) {
+  return {
+    isNonInteractive: () => false,
+    note: vi.fn(),
+    prompt: vi.fn().mockResolvedValue(promptReply),
+  };
+}
+
+describe("promptOrDefault interactive default fallback (#4387)", () => {
+  it("returns defaultValue when the user just presses Enter (empty reply)", async () => {
+    const deps = makeDeps("");
+    expect(await promptOrDefault(deps, "  Choose [6]: ", null, "6")).toBe("6");
+  });
+
+  it("treats a whitespace-only reply as the default", async () => {
+    const deps = makeDeps("   ");
+    expect(await promptOrDefault(deps, "  Choose [6]: ", null, "6")).toBe("6");
+  });
+
+  it("returns the user's reply verbatim when non-empty", async () => {
+    const deps = makeDeps("3");
+    expect(await promptOrDefault(deps, "  Choose [6]: ", null, "6")).toBe("3");
+  });
+});
diff --git a/src/lib/onboard/prompt-helpers.ts b/src/lib/onboard/prompt-helpers.ts
index c99e92f828..4c335f2581 100644
--- a/src/lib/onboard/prompt-helpers.ts
+++ b/src/lib/onboard/prompt-helpers.ts
@@ -49,7 +49,12 @@ export async function promptOrDefault(
     deps.note(`  [non-interactive] ${question.trim()} → ${result}`);
     return result;
   }
-  return deps.prompt(question);
+  // The prompt label advertises the default in brackets (e.g. `Choose [6]:`),
+  // so an empty/whitespace reply must resolve to that default. Without this,
+  // every interactive caller had to re-implement the empty-reply fallback,
+  // and any that forgot hard-rejected the displayed default (#4387).
+  const reply = await deps.prompt(question);
+  return reply.trim() === "" ? defaultValue : reply;
 }
 
 // Yes/no prompt with a typed default. The `[Y/n]` / `[y/N]` indicator and