diff --git a/tests/test_docker_helper.py b/tests/test_docker_helper.py
index 22f4e9c..45d03c8 100644
--- a/tests/test_docker_helper.py
+++ b/tests/test_docker_helper.py
@@ -119,6 +119,27 @@ def test_stop_all_scoped_to_run_label(monkeypatch):
     mine.stop.assert_called_once()
 
 
+def test_stop_prunes_only_this_runs_containers(monkeypatch):
+    # Other projects share the Docker host: never prune their stopped containers.
+    monkeypatch.setattr(docker_helper.time, "sleep", lambda s: None)
+    helper = _helper()
+    helper.server.containers.list.return_value = []
+    helper.stop()
+    helper.server.containers.prune.assert_called_once_with(
+        filters={"label": "ssgberk.run=abc12345-0000"})
+
+
+def test_clean_removes_only_ssgberk_test_images():
+    helper = _helper()
+    ours = types.SimpleNamespace(id="i1", tags=["ssgberk/test.hugo:latest"])
+    theirs = types.SimpleNamespace(id="i2", tags=["postgres:16"])
+    untagged = types.SimpleNamespace(id="i3", tags=[])
+    helper.server.images.list.return_value = [ours, theirs, untagged]
+    helper.clean()
+    helper.server.images.remove.assert_called_once_with("i1", force=True)
+    helper.server.images.prune.assert_not_called()
+
+
 def test_stop_all_tolerates_not_found(monkeypatch):
     monkeypatch.setattr(docker_helper.time, "sleep", lambda s: None)
     helper = _helper()
diff --git a/toolset/utils/docker_helper.py b/toolset/utils/docker_helper.py
index 55dbecb..8ebfdb8 100644
--- a/toolset/utils/docker_helper.py
+++ b/toolset/utils/docker_helper.py
@@ -104,15 +104,15 @@ def __build(self, base_url, path, build_log_file, log_prefix, dockerfile,
 
     def clean(self):
         '''
-        Cleans all the docker images from the system
+        Removes the ssgberk/test.* generator images. Other images (and the dangling
+        images of other projects on a shared Docker host) are left alone; removing a
+        tagged image already deletes its unused layers.
         '''
 
-        self.server.images.prune()
         for image in self.server.images.list():
             if len(image.tags) > 0:
                 if DockerHelper.is_ssgberk_test_image(image.tags[0]):
                     self.server.images.remove(image.id, force=True)
-        self.server.images.prune()
 
     def build(self, test, build_log_dir=os.devnull):
         '''
@@ -315,7 +315,9 @@ def stop(self, containers=None):
         else:
             DockerHelper.__stop_all(self.server, self.benchmarker.config.run_id)
 
-        self.server.containers.prune()
+        # Only this run's stopped containers: the Docker host is shared with other projects.
+        self.server.containers.prune(
+            filters={"label": "%s=%s" % (RUN_LABEL, self.benchmarker.config.run_id)})
 
     def server_container_exists(self, container_id_or_name):
         '''