fix: preserve shared skill mounts across concurrent sandboxes

src/forge/sandbox/runner.py

@@ -432,7 +432,10 @@ def _build_volume_mounts(
                 )
 
         for host_path, container_path in skill_mounts:
-            mounts.append((host_path, container_path, "ro,Z"))
+            # Skill trees are shared by concurrent sandbox containers. A private
+            # SELinux label (``Z``) lets a later mount revoke an already-running
+            # container's access; ``z`` labels this read-only mount as shared.
+            mounts.append((host_path, container_path, "ro,z"))
 
         return mounts
 

tests/unit/sandbox/test_runner_mounts.py

@@ -0,0 +1,23 @@
+from pathlib import Path
+from types import SimpleNamespace
+
+from forge.sandbox.runner import ContainerRunner
+
+
+def test_skill_mounts_use_shared_selinux_label(tmp_path: Path) -> None:
+    """Concurrent sandboxes must retain access to shared skill directories."""
+    runner = object.__new__(ContainerRunner)
+    runner.settings = SimpleNamespace(llm_backend="openai")
+    workspace = tmp_path / "workspace"
+    task_file = tmp_path / "task.json"
+    skill_dir = tmp_path / "skills"
+
+    mounts = runner._build_volume_mounts(
+        workspace,
+        task_file,
+        None,
+        [(skill_dir, "/skills/skill_0")],
+    )
+
+    assert (skill_dir, "/skills/skill_0", "ro,z") in mounts
+    assert (skill_dir, "/skills/skill_0", "ro,Z") not in mounts