Keep shard output leaf exclusive

src/format_bench/shards.py

@@ -138,7 +138,7 @@ def _hardlink_tree(source: Path, destination: Path) -> None:
 
 def _mkdir_private_tree(destination: Path) -> None:
     missing: list[Path] = []
-    current = destination
+    current = destination.parent
     while not current.exists():
         missing.append(current)
         parent = current.parent
@@ -152,6 +152,10 @@ def _mkdir_private_tree(destination: Path) -> None:
     # parent creation before the private output leaf exists.
     for directory in reversed(missing):
         directory.mkdir(mode=0o700)
+    # LLM contract: OUTPUT_ABSENT -> OUTPUT_CREATED | COLLISION_REJECTED.
+    # The final leaf stays exclusive even if another process creates it after
+    # merge validation but before publication begins.
+    destination.mkdir(mode=0o700)
 
 
 def _copy_run_files(base_run: Path, output_run: Path) -> None:

tests/test_shards.py

@@ -7,7 +7,7 @@
 
 import pytest
 
-from format_bench.shards import merge_equivalence_shards
+from format_bench.shards import _mkdir_private_tree, merge_equivalence_shards
 
 
 def _write(path: Path, value: object) -> None:
@@ -111,6 +111,16 @@ def _shard_results(pair: str, name: str) -> dict:
     }
 
 
+def test_private_output_tree_rejects_race_created_leaf(tmp_path: Path) -> None:
+    output = tmp_path / "missing" / "nested" / "merged"
+    output.mkdir(parents=True)
+
+    with pytest.raises(FileExistsError):
+        _mkdir_private_tree(output)
+
+    assert list(output.iterdir()) == []
+
+
 @pytest.mark.skipif(os.name != "posix", reason="POSIX directory-mode contract")
 @pytest.mark.parametrize("mask", (0o002, 0o000))
 def test_merge_equivalence_shards_reuses_artifacts_and_unions_results(