diff --git a/studio/install_llama_prebuilt.py b/studio/install_llama_prebuilt.py
index 6c3be0e09f..64ea255803 100644
--- a/studio/install_llama_prebuilt.py
+++ b/studio/install_llama_prebuilt.py
@@ -4870,6 +4870,35 @@ def confirm_install_tree(install_dir: Path, host: HostInfo) -> None:
         raise RuntimeError("activated install was missing expected files: " + ", ".join(missing))
 
 
+def activate_staged_dir(staging_dir: Path, dst: Path) -> None:
+    """Move a freshly extracted ``staging_dir`` onto ``dst``.
+
+    ``os.replace`` is attempted first as the fast path. On Windows ARM64 the
+    antivirus scanner can transiently hold a freshly extracted DLL open at the
+    moment ``MoveFileEx`` runs, surfacing as ``[WinError 5] Access is denied``;
+    a file-by-file copy bypasses the rename entirely.
+
+    This fallback is intentionally limited to staging trees we just extracted.
+    It must not be used to move an existing/active install aside: there an
+    ``os.replace`` failure means the directory is genuinely in use, and a
+    silent copy + ``rmtree`` could partially delete a live install.
+
+    Only busy/lock errors (``is_busy_lock_error``) trigger the copy; anything
+    else (disk full, cross-device, missing path) re-raises so it cannot leave
+    a partially copied install behind. A copy is preferred over retrying the
+    rename because antivirus scans of large DLLs can outlast any reasonable
+    retry window.
+    """
+    try:
+        os.replace(staging_dir, dst)
+    except OSError as exc:
+        if not is_busy_lock_error(exc):
+            raise
+        log(f"os.replace failed ({exc!r}); falling back to file-by-file copy of staging tree")
+        shutil.copytree(staging_dir, dst, dirs_exist_ok = True)
+        remove_tree(staging_dir)
+
+
 def activate_install_tree(staging_dir: Path, install_dir: Path, host: HostInfo) -> None:
     rollback_dir: Path | None = None
     failed_dir: Path | None = None
@@ -4881,7 +4910,7 @@ def activate_install_tree(staging_dir: Path, install_dir: Path, host: HostInfo)
             log(f"moved existing install to rollback path {rollback_dir.name}")
 
         log(f"activating staged install {staging_dir} -> {install_dir}")
-        os.replace(staging_dir, install_dir)
+        activate_staged_dir(staging_dir, install_dir)
         log(f"activated staged install at {install_dir}")
         log(f"confirming activated install tree at {install_dir}")
         confirm_install_tree(install_dir, host)
diff --git a/tests/studio/install/test_install_llama_prebuilt_logic.py b/tests/studio/install/test_install_llama_prebuilt_logic.py
index 7ca87aa8c4..000c5a7ace 100644
--- a/tests/studio/install/test_install_llama_prebuilt_logic.py
+++ b/tests/studio/install/test_install_llama_prebuilt_logic.py
@@ -1,3 +1,4 @@
+import errno
 import importlib.util
 import io
 import json
@@ -28,6 +29,7 @@ ApprovedReleaseChecksums = INSTALL_LLAMA_PREBUILT.ApprovedReleaseChecksums
 hydrate_source_tree = INSTALL_LLAMA_PREBUILT.hydrate_source_tree
 validate_prebuilt_choice = INSTALL_LLAMA_PREBUILT.validate_prebuilt_choice
 activate_install_tree = INSTALL_LLAMA_PREBUILT.activate_install_tree
+activate_staged_dir = INSTALL_LLAMA_PREBUILT.activate_staged_dir
 create_install_staging_dir = INSTALL_LLAMA_PREBUILT.create_install_staging_dir
 sha256_file = INSTALL_LLAMA_PREBUILT.sha256_file
 source_archive_logical_name = INSTALL_LLAMA_PREBUILT.source_archive_logical_name
@@ -666,6 +668,48 @@ def test_activate_install_tree_cleans_all_paths_when_rollback_restore_fails(
     assert "removing rollback path" in output
 
 
+def test_activate_staged_dir_copies_when_replace_hits_busy_lock(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+):
+    staging_dir = tmp_path / "llama.cpp.staging-test"
+    (staging_dir / "bin").mkdir(parents = True)
+    (staging_dir / "bin" / "ggml-base.dll").write_bytes(b"fake dll")
+    dst = tmp_path / "llama.cpp"
+
+    def denied_replace(src, dst_arg):
+        raise PermissionError(errno.EACCES, "Access is denied", str(src))
+
+    monkeypatch.setattr(INSTALL_LLAMA_PREBUILT.os, "replace", denied_replace)
+
+    activate_staged_dir(staging_dir, dst)
+
+    assert (dst / "bin" / "ggml-base.dll").read_bytes() == b"fake dll"
+    assert not staging_dir.exists()
+
+    captured = capsys.readouterr()
+    assert "falling back to file-by-file copy" in captured.out + captured.err
+
+
+def test_activate_staged_dir_reraises_non_busy_errors(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+):
+    staging_dir = tmp_path / "llama.cpp.staging-test"
+    staging_dir.mkdir()
+    (staging_dir / "new.txt").write_text("new install\n")
+    dst = tmp_path / "llama.cpp"
+
+    def out_of_space_replace(src, dst_arg):
+        raise OSError(errno.ENOSPC, "No space left on device", str(src))
+
+    monkeypatch.setattr(INSTALL_LLAMA_PREBUILT.os, "replace", out_of_space_replace)
+
+    with pytest.raises(OSError, match = "No space left on device"):
+        activate_staged_dir(staging_dir, dst)
+
+    assert not dst.exists()
+    assert (staging_dir / "new.txt").read_text() == "new install\n"
+
+
 def test_binary_env_linux_includes_binary_parent_in_ld_library_path(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ):