diff --git a/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/compose.py b/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/compose.py
index 24a7259..99441a9 100644
--- a/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/compose.py
+++ b/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/compose.py
@@ -133,6 +133,15 @@ def generate_compose(
     if mongo_dbs:
         services["dab-mongo"] = {
             "image": MONGO_IMAGE,
+            # mongo:8 intermittently SIGSEGVs (exit 139) on startup, and its
+            # WiredTiger cache auto-sizes to ~half of host RAM (≈7GB on a 15GB
+            # box), starving the agent. Cap the cache and let docker restart a
+            # crashed mongod: the data dir is already populated so the restart
+            # comes straight back up with data and the healthcheck recovers.
+            # Without this, a single crash bricks the trial — main's healthcheck
+            # fails for its whole retry window and the trial is cancelled.
+            "command": ["--wiredTigerCacheSizeGB", "1"],
+            "restart": "on-failure",
             "healthcheck": {
                 "test": ["CMD", "mongosh", "--quiet", "--eval", "db.runCommand({ping:1})"],
                 "interval": "5s",
diff --git a/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/prepare.py b/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/prepare.py
index f0d88c4..f71a174 100644
--- a/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/prepare.py
+++ b/packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/prepare.py
@@ -262,6 +262,14 @@ def _materialize_task_dir(
             dataset=dataset_meta.name,
             query_id=query_id,
         )
+    # Some upstream validators do `from common_scaffold.validate.levenshtein
+    # import levenshtein`. verify.py exec_module's validate.py inside the
+    # dab-agent container, which has no common_scaffold installed, so the
+    # import raises, verify.py exits non-zero under `set -eu`, no reward.json
+    # is written, and harbor reports RewardFileNotFoundError (verifier appears
+    # to never run). Vendor common_scaffold next to verify.py — /tests is
+    # sys.path[0] — so the import resolves. The batch path already does this.
+    _install_common_scaffold(tests_dir=tests_dir, data_root=dataset_dir.parent)
 
     write_stratum_file(
         tests_dir=tests_dir,
diff --git a/packages/razorback-plugin-dab/tests/unit/test_compose_mongo.py b/packages/razorback-plugin-dab/tests/unit/test_compose_mongo.py
index ba63cac..bc88d4f 100644
--- a/packages/razorback-plugin-dab/tests/unit/test_compose_mongo.py
+++ b/packages/razorback-plugin-dab/tests/unit/test_compose_mongo.py
@@ -32,6 +32,17 @@ def test_mongo_service_emitted(tmp_path: Path):
     assert "mongosh" in services["dab-mongo"]["healthcheck"]["test"]
 
 
+def test_mongo_has_restart_and_cache_cap(tmp_path: Path):
+    # mongo:8 intermittently SIGSEGVs on startup; on a constrained host its
+    # default WiredTiger cache (~half of RAM) also starves the agent. The
+    # service must cap the cache and restart on failure so a single crash
+    # does not brick the trial via main's healthcheck retry window.
+    text = generate_compose(db_config=_AGNEWS_LIKE, dataset_name="agnews", data_root=tmp_path)
+    mongo = yaml.safe_load(text)["services"]["dab-mongo"]
+    assert mongo["restart"] == "on-failure"
+    assert mongo["command"] == ["--wiredTigerCacheSizeGB", "1"]
+
+
 def test_main_depends_on_mongo(tmp_path: Path):
     text = generate_compose(db_config=_AGNEWS_LIKE, dataset_name="agnews", data_root=tmp_path)
     compose = yaml.safe_load(text)
diff --git a/packages/razorback-plugin-dab/tests/unit/test_prepare_per_query.py b/packages/razorback-plugin-dab/tests/unit/test_prepare_per_query.py
index 2cd651f..cbb05d0 100644
--- a/packages/razorback-plugin-dab/tests/unit/test_prepare_per_query.py
+++ b/packages/razorback-plugin-dab/tests/unit/test_prepare_per_query.py
@@ -1,6 +1,7 @@
 # ABOUTME: AC-3 — task-dir shape for one (dataset, query) emission.
 # ABOUTME: Asserts forbidden files (ground_truth.csv, validate.py) never reach workdir.
 
+import importlib.util
 from pathlib import Path
 
 import pytest
@@ -74,6 +75,65 @@ def test_task_dir_layout(tmp_path: Path):
     assert (task_dir / "steps" / "main" / "workdir" / "README.md").exists()
 
 
+def _build_common_scaffold_data_root(root: Path) -> Path:
+    """Single-query data root whose validator imports common_scaffold."""
+    data_root = root / "data"
+    scaffold_validate = data_root / "common_scaffold" / "validate"
+    scaffold_validate.mkdir(parents=True)
+    (scaffold_validate / "levenshtein.py").write_text(
+        "def levenshtein(left, right):\n"
+        "    return 0 if left == right else 1\n"
+    )
+    (scaffold_validate / "__pycache__").mkdir()
+    (scaffold_validate / "__pycache__" / "ignored.pyc").write_bytes(b"cached")
+
+    qdir = data_root / "query_PATENTS"
+    qdir.mkdir(parents=True)
+    (qdir / "db_description.txt").write_text("Synthetic affected DAB dataset.")
+    q1 = qdir / "query1"
+    q1.mkdir()
+    (q1 / "query.json").write_text('{"question": "Return abc."}')
+    (q1 / "validate.py").write_text(
+        "from common_scaffold.validate.levenshtein import levenshtein\n\n"
+        "def validate(answer):\n"
+        "    distance = levenshtein(answer, 'abc')\n"
+        "    return (distance == 0, f'distance={distance}')\n"
+    )
+    return data_root
+
+
+def test_per_query_materializes_common_scaffold_for_upstream_validators(
+    tmp_path: Path, monkeypatch
+) -> None:
+    # Regression: 9 of 54 DAB validators do `from common_scaffold.validate
+    # .levenshtein import levenshtein`. verify.py exec_module's validate.py in
+    # the dab-agent container (no common_scaffold installed), so without the
+    # vendored package the import raises, no reward.json is written, and harbor
+    # reports RewardFileNotFoundError (the verifier appears never to run). The
+    # batch path already vendored it; the per-query path must too.
+    data_root = _build_common_scaffold_data_root(tmp_path)
+    manifest = prepare_dataset_tasks(
+        data_root=data_root,
+        dataset="PATENTS",
+        tasks_root=tmp_path / "tasks",
+    )
+    tests_dir = manifest[0]["task_dir"] / "tests"
+
+    assert (tests_dir / "common_scaffold" / "validate" / "levenshtein.py").exists()
+    assert not (tests_dir / "common_scaffold" / "validate" / "__pycache__").exists()
+
+    # The copied validator must import and run with /tests on sys.path
+    # (verify.py runs as `python /tests/verify.py`, so /tests is sys.path[0]).
+    monkeypatch.syspath_prepend(str(tests_dir))
+    spec = importlib.util.spec_from_file_location(
+        "_generated_validate_per_query", tests_dir / "validate.py"
+    )
+    assert spec is not None and spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    assert module.validate("abc") == (True, "distance=0")
+
+
 def test_compose_bind_mount_sources_resolve_to_real_files(tmp_path: Path):
     """PKG-13 T1 / AC-4 + PKG-14 AC-1: bind-mount sources in the generated
     compose must point at existing files. Under PKG-14 bind mode (the default)