diff --git a/tools/test_build_system/conftest.py b/tools/test_build_system/conftest.py
index ad789dcb82d..93409099b11 100644
--- a/tools/test_build_system/conftest.py
+++ b/tools/test_build_system/conftest.py
@@ -36,7 +36,31 @@ def _get_git_submodule_paths(repo_path: Path) -> list[str]:
     return submodule_paths
 
 
-def _create_idf_copy_via_worktree(path_from: Path, path_to: Path) -> str:
+def _is_materialized_submodule(path: Path) -> bool:
+    """Whether path holds submodule content rather than a placeholder.
+
+    ``git worktree add`` leaves a submodule as an empty directory or as a
+    gitlink file, and both are unusable as component sources.
+    """
+    try:
+        return path.is_dir() and any(path.iterdir())
+    except OSError:
+        return False
+
+
+def _copy_submodule(src_submodule: Path, dst_submodule: Path) -> None:
+    """Replace the worktree placeholder with the submodule content of the source repo."""
+    # rmtree() cannot remove a gitlink file (even with ignore_errors=True), and
+    # copytree() would then leave a file where CMake expects a directory
+    # (e.g. mbedtls/include).
+    if dst_submodule.is_file() or dst_submodule.is_symlink():
+        dst_submodule.unlink()
+    elif dst_submodule.exists():
+        shutil.rmtree(dst_submodule)
+    shutil.copytree(src_submodule, dst_submodule, symlinks=True, ignore=shutil.ignore_patterns('.git'))
+
+
+def _create_idf_copy_via_worktree(path_from: Path, path_to: Path) -> str | None:
     """
     Create IDF copy using git worktree (fast) + copying submodule directories.
 
@@ -45,6 +69,12 @@ def _create_idf_copy_via_worktree(path_from: Path, path_to: Path) -> str:
     repo (which has them already checked out) instead of running git submodule
     update (which can fail due to auth issues on CI).
 
+    Return the worktree branch name, or None if a submodule could not be
+    materialized. In that case the worktree is removed again and the caller
+    creates the copy with shutil.copytree instead. Leaving a placeholder behind
+    would produce a copy that only fails once a test builds it, as a missing
+    include directory or source file of the affected component.
+
     After copying submodules, remove the worktree's top-level ``.git`` file so
     the result matches the old ``shutil.copytree`` behavior (no git repo at
     ``IDF_PATH``). Otherwise CMake's ``git_submodule_check`` runs inside the
@@ -67,18 +97,15 @@ def _create_idf_copy_via_worktree(path_from: Path, path_to: Path) -> str:
         src_submodule = path_from / submodule_rel_path
         dst_submodule = path_to / submodule_rel_path
 
-        # Only copy if the source submodule is a populated directory. A gitlink
-        # file or empty dir means the source checkout did not materialize it.
-        if src_submodule.is_dir() and any(src_submodule.iterdir()):
+        # Nothing to copy when the source checkout did not materialize the submodule.
+        if _is_materialized_submodule(src_submodule):
             logging.debug(f'copying submodule {submodule_rel_path}')
-            # Worktree submodule paths are often gitlink files; rmtree() cannot
-            # remove those (even with ignore_errors=True), and copytree() then
-            # leaves a file where CMake expects a directory (e.g. mbedtls/include).
-            if dst_submodule.is_file() or dst_submodule.is_symlink():
-                dst_submodule.unlink()
-            elif dst_submodule.exists():
-                shutil.rmtree(dst_submodule)
-            shutil.copytree(src_submodule, dst_submodule, symlinks=True, ignore=shutil.ignore_patterns('.git'))
+            _copy_submodule(src_submodule, dst_submodule)
+
+        if not _is_materialized_submodule(dst_submodule):
+            logging.warning(f'submodule {submodule_rel_path} could not be copied into {path_to}')
+            _cleanup_worktree(path_from, path_to, branch_name)
+            return None
 
     # Match old shutil-based idf_copy: no top-level .git (see docstring above).
     (path_to / '.git').unlink(missing_ok=True)
@@ -310,6 +337,8 @@ def idf_copy(func_work_dir: Path, request: FixtureRequest) -> typing.Generator[P
         # Clean up any partial worktree before fallback
         if path_to.exists():
             shutil.rmtree(path_to, ignore_errors=True)
+
+    if branch_name is None:
         _create_idf_copy_via_shutil(path_from, path_to)
 
     os.environ['IDF_PATH'] = str(path_to)