diff --git a/src/format_bench/workflow.py b/src/format_bench/workflow.py
index 16077f3..025305a 100644
--- a/src/format_bench/workflow.py
+++ b/src/format_bench/workflow.py
@@ -2,6 +2,7 @@
 
 import hashlib
 import shutil
+import stat
 from datetime import datetime, timezone
 from pathlib import Path
 from typing import Iterable
@@ -58,9 +59,19 @@ def _create_missing_private_parents(destination: Path) -> None:
 
     # LLM contract: EXISTING_ANCESTOR -> PRIVATE_ANCESTORS_READY | FAILED.
     # Create shallow-to-deep without parents=True so every new level requests
-    # 0700; any mkdir failure stops before the private run leaf is created.
+    # 0700. A concurrent private creator is safe; every other mkdir failure
+    # stops before the private run leaf is created.
     for directory in reversed(missing):
-        directory.mkdir(mode=0o700)
+        try:
+            directory.mkdir(mode=0o700)
+        except FileExistsError as error:
+            existing_mode = directory.stat(follow_symlinks=False).st_mode
+            if not stat.S_ISDIR(existing_mode):
+                raise
+            if stat.S_IMODE(existing_mode) & 0o077:
+                raise PermissionError(
+                    f"concurrently created run ancestor is not private: {directory}"
+                ) from error
 
 
 def _safe_format_component(
diff --git a/tests/test_workflow.py b/tests/test_workflow.py
index 017a4c9..2c371c7 100644
--- a/tests/test_workflow.py
+++ b/tests/test_workflow.py
@@ -259,6 +259,49 @@ def record_mode(
     assert (run_dir / "manifest.json").is_file()
 
 
+@pytest.mark.skipif(os.name != "posix", reason="POSIX directory-mode contract")
+@pytest.mark.parametrize(
+    ("raced_mode", "expected_error"),
+    ((0o700, None), (0o755, PermissionError)),
+)
+def test_missing_private_parent_validates_concurrent_creator(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+    raced_mode: int,
+    expected_error: type[Exception] | None,
+) -> None:
+    existing_parent = tmp_path / "existing"
+    existing_parent.mkdir(mode=0o700)
+    raced_parent = existing_parent / "runs"
+    destination = raced_parent / "run"
+    real_mkdir = Path.mkdir
+    raced = False
+
+    def race_mkdir(
+        path: Path,
+        mode: int = 0o777,
+        parents: bool = False,
+        exist_ok: bool = False,
+    ) -> None:
+        nonlocal raced
+        if path == raced_parent and not raced:
+            real_mkdir(path, mode=raced_mode)
+            path.chmod(raced_mode)
+            raced = True
+        real_mkdir(path, mode=mode, parents=parents, exist_ok=exist_ok)
+
+    monkeypatch.setattr(Path, "mkdir", race_mkdir)
+
+    if expected_error is None:
+        workflow._create_missing_private_parents(destination)
+    else:
+        with pytest.raises(expected_error, match="not private"):
+            workflow._create_missing_private_parents(destination)
+
+    assert raced is True
+    assert stat.S_IMODE(raced_parent.stat().st_mode) == raced_mode
+
+
 @pytest.mark.skipif(os.name != "posix", reason="POSIX directory-mode contract")
 @pytest.mark.parametrize("mask", (0o002, 0o000))
 def test_prepare_run_creates_missing_default_parent_privately(