diff --git a/src/aituner/worker.py b/src/aituner/worker.py
index f22ce71..7931f12 100644
--- a/src/aituner/worker.py
+++ b/src/aituner/worker.py
@@ -401,27 +401,37 @@ def _wait_for_server_or_exit(
     raise HttpClientError(f"Timed out waiting for {base_url}{healthcheck_path}: {last_error}")
 
 
+def _process_group_exists(pgid: int) -> bool:
+    try:
+        os.killpg(pgid, 0)
+        return True
+    except ProcessLookupError:
+        return False
+
+
 def _terminate_process_tree(process: subprocess.Popen[str], *, timeout_s: float = 30.0) -> None:
-    if process.poll() is not None:
-        return
     try:
         pgid = os.getpgid(process.pid)
     except ProcessLookupError:
-        return
+        # Children can keep the session/process group alive after the vLLM API
+        # server exits.  In that case the group id is still the original pid
+        # because the process was launched with start_new_session=True.
+        pgid = process.pid
     try:
         os.killpg(pgid, signal.SIGTERM)
     except ProcessLookupError:
         return
     deadline = time.monotonic() + timeout_s
     while time.monotonic() < deadline:
-        if process.poll() is not None:
+        if not _process_group_exists(pgid):
             return
         time.sleep(0.1)
     try:
         os.killpg(pgid, signal.SIGKILL)
     except ProcessLookupError:
         return
-    process.wait(timeout=timeout_s)
+    if process.poll() is None:
+        process.wait(timeout=timeout_s)
 
 
 def run_trial(trial_spec_path: Path) -> dict[str, Any]:
diff --git a/tests/test_core_flow.py b/tests/test_core_flow.py
index 08854ba..c4da979 100644
--- a/tests/test_core_flow.py
+++ b/tests/test_core_flow.py
@@ -4549,13 +4549,29 @@ class CoreFlowTests(unittest.TestCase):
     def test_terminate_process_tree_kills_process_group(self) -> None:
         process = mock.Mock()
         process.pid = 1234
-        process.poll.side_effect = [None, None, 0]
+        process.poll.return_value = None
         process.wait.return_value = 0
         with mock.patch("aituner.worker.os.getpgid", return_value=1234):
-            with mock.patch("aituner.worker.os.killpg") as mock_killpg:
+            with mock.patch(
+                "aituner.worker.os.killpg",
+                side_effect=[None, ProcessLookupError],
+            ) as mock_killpg:
                 _terminate_process_tree(process, timeout_s=1.0)
-        mock_killpg.assert_called_once()
-        self.assertEqual(mock_killpg.call_args[0][0], 1234)
+        self.assertEqual(mock_killpg.call_args_list[0].args[0], 1234)
+        self.assertEqual(mock_killpg.call_args_list[0].args[1], 15)
+
+    def test_terminate_process_tree_kills_group_when_parent_already_exited(self) -> None:
+        process = mock.Mock()
+        process.pid = 1234
+        process.poll.return_value = 0
+        with mock.patch("aituner.worker.os.getpgid", side_effect=ProcessLookupError):
+            with mock.patch(
+                "aituner.worker.os.killpg",
+                side_effect=[None, ProcessLookupError],
+            ) as mock_killpg:
+                _terminate_process_tree(process, timeout_s=1.0)
+        self.assertEqual(mock_killpg.call_args_list[0].args[0], 1234)
+        process.wait.assert_not_called()
 
     def test_openai_url_avoids_double_v1(self) -> None:
         self.assertEqual(