diff --git a/tools/test_build_system/conftest.py b/tools/test_build_system/conftest.py
index 0fb2ec88673..ad789dcb82d 100644
--- a/tools/test_build_system/conftest.py
+++ b/tools/test_build_system/conftest.py
@@ -67,10 +67,13 @@ 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 source submodule exists and has content
-        if src_submodule.exists() and any(src_submodule.iterdir()):
+        # 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()):
             logging.debug(f'copying submodule {submodule_rel_path}')
-            # Worktree submodule paths may be gitlink files; rmtree() does not remove those.
+            # 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():
diff --git a/tools/test_build_system/test_build_system_helpers/idf_utils.py b/tools/test_build_system/test_build_system_helpers/idf_utils.py
index e0f222ee4d3..357c0c5faa9 100644
--- a/tools/test_build_system/test_build_system_helpers/idf_utils.py
+++ b/tools/test_build_system/test_build_system_helpers/idf_utils.py
@@ -19,6 +19,55 @@ EnvDict = dict[str, str]
 IdfPyFunc = typing.Callable[..., subprocess.CompletedProcess]
 
 
+_LOG_ERROR_MARKERS = (
+    'CMake Error',
+    'FAILED:',
+    'fatal error',
+    'ninja: build stopped',
+    'HINT:',
+)
+
+
+def _clip_log_output(text: str | None, max_lines: int = 80, max_line_len: int = 400) -> str:
+    """Last ``max_lines`` of process output for logging, plus failure lines.
+
+    pytest.ini enables ``log_cli``, so ``logging.error(full_stdout)`` after a
+    failed build is one record. On Windows CI that live-log can stall for hours
+    even when the line count is small: CMake's ``-- Component paths:`` line is
+    a single multi-KB (sometimes multi-MB) string.
+    """
+    if not text:
+        return ''
+    lines = text.splitlines()
+
+    def _short(line: str) -> str:
+        if len(line) <= max_line_len:
+            return line
+        return line[:max_line_len] + f'... [{len(line) - max_line_len} chars omitted]'
+
+    omitted = max(0, len(lines) - max_lines)
+    tail_start = len(lines) - max_lines if omitted else 0
+    tail = lines[tail_start:]
+
+    failures: list[str] = []
+    for idx, line in enumerate(lines):
+        if idx >= tail_start:
+            break
+        if any(marker in line for marker in _LOG_ERROR_MARKERS):
+            failures.append(line)
+            if len(failures) >= 40:
+                break
+
+    parts: list[str] = []
+    if failures:
+        parts.append('[... failure lines ...]')
+        parts.extend(_short(line) for line in failures)
+    if omitted:
+        parts.append(f'[... {omitted} lines omitted ...]')
+    parts.extend(_short(line) for line in tail)
+    return '\n'.join(parts)
+
+
 def normalize_output(text: str) -> str:
     """Collapse all whitespace runs to a single space.
 
@@ -108,8 +157,8 @@ def run_idf_py(
     except subprocess.CalledProcessError as e:
         logging.error('The following idf.py command has failed: {}'.format(' '.join(cmd)))
         logging.error(f'Working directory: {workdir}')
-        logging.error(f'Stdout: {e.stdout}')
-        logging.error(f'Stderr: {e.stderr}')
+        logging.error(f'Stdout: {_clip_log_output(e.stdout)}')
+        logging.error(f'Stderr: {_clip_log_output(e.stderr)}')
         raise
 
 
@@ -153,8 +202,8 @@ def run_cmake(
     except subprocess.CalledProcessError as e:
         logging.error('The following cmake command has failed: {}'.format(' '.join(cmd)))
         logging.error(f'Working directory: {workdir}')
-        logging.error(f'Stdout: {e.stdout}')
-        logging.error(f'Stderr: {e.stderr}')
+        logging.error(f'Stdout: {_clip_log_output(e.stdout)}')
+        logging.error(f'Stderr: {_clip_log_output(e.stderr)}')
         raise