diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 2e2ae3bb..87f8522a 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -34,7 +34,7 @@ jobs:
         with:
           python-version: "3.12"
 
-      - run: pip install pyyaml
+      - run: pip install pyyaml jsonschema==4.23.0
 
       - name: Run tests
         run: python -m unittest discover tests
diff --git a/scripts/pipeline.py b/scripts/pipeline.py
index 3ab01267..aa58dd2c 100644
--- a/scripts/pipeline.py
+++ b/scripts/pipeline.py
@@ -235,6 +235,17 @@ def main():
     )
     args = parser.parse_args()
 
+    # A second run on the same output directory is refused before any work:
+    # the database rebuild alone takes minutes, and the reader holding the
+    # directory would otherwise see the answer only after all of it.
+    if not args.skip_packs and Path(args.output_dir).is_dir():
+        try:
+            with artifact_lock(args.output_dir):
+                pass
+        except ArtifactLockBusy as exc:
+            print(f"ERROR: {exc}")
+            sys.exit(1)
+
     results = {}
     all_ok = True
     total_start = time.monotonic()