diff --git a/ops/devlane/dispatch/tests/launch_support.py b/ops/devlane/dispatch/tests/launch_support.py
index 14a2df2..8042bdb 100644
--- a/ops/devlane/dispatch/tests/launch_support.py
+++ b/ops/devlane/dispatch/tests/launch_support.py
@@ -663,6 +663,18 @@ def sha256_file(path) -> str:
 
 
 def pid_is_alive(pid: int) -> bool:
+    # A group SIGKILL can leave a descendant as a zombie until the host's
+    # init process reaps it.  ``kill(pid, 0)`` still succeeds for zombies,
+    # even though they cannot execute and therefore are not survivors of the
+    # isolation boundary.  Check procfs first so the process-group assertion
+    # measures live workers rather than the reaping behaviour of PID 1 (which
+    # is notably delayed in some CI containers).
+    stat = Path(f"/proc/{pid}/stat")
+    try:
+        if stat.read_text(encoding="utf-8").split()[2] == "Z":
+            return False
+    except (FileNotFoundError, IndexError, OSError):
+        pass
     try:
         os.kill(pid, 0)
     except ProcessLookupError:
diff --git a/ops/devlane/task/tests/test_run.py b/ops/devlane/task/tests/test_run.py
index f3f48d6..c2925be 100644
--- a/ops/devlane/task/tests/test_run.py
+++ b/ops/devlane/task/tests/test_run.py
@@ -1307,6 +1307,14 @@ def _orphan_maker(self):
 
     @staticmethod
     def _alive(pid):
+        # kill(0) also reports zombies as present.  A killed orphan can stay
+        # zombied until PID 1 reaps it in a container, but it is no longer a
+        # runnable descendant and must not make this isolation check flaky.
+        try:
+            if Path(f"/proc/{pid}/stat").read_text().split()[2] == "Z":
+                return False
+        except (FileNotFoundError, IndexError, OSError):
+            pass
         try:
             os.kill(pid, 0)
             return True