diff --git a/src/aituner/worker.py b/src/aituner/worker.py
index fef9901..a4df42c 100644
--- a/src/aituner/worker.py
+++ b/src/aituner/worker.py
@@ -2,6 +2,8 @@ from __future__ import annotations
 
 import json
 import math
+import os
+import signal
 import subprocess
 import threading
 import time
@@ -218,6 +220,29 @@ def _wait_for_server_or_exit(
     raise HttpClientError(f"Timed out waiting for {base_url}{healthcheck_path}: {last_error}")
 
 
+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
+    try:
+        os.killpg(pgid, signal.SIGTERM)
+    except ProcessLookupError:
+        return
+    deadline = time.monotonic() + timeout_s
+    while time.monotonic() < deadline:
+        if process.poll() is not None:
+            return
+        time.sleep(0.1)
+    try:
+        os.killpg(pgid, signal.SIGKILL)
+    except ProcessLookupError:
+        return
+    process.wait(timeout=timeout_s)
+
+
 def run_trial(trial_spec_path: Path) -> dict[str, Any]:
     from .store import StudyStore
 
@@ -237,6 +262,7 @@ def run_trial(trial_spec_path: Path) -> dict[str, Any]:
             stdout=engine_log,
             stderr=subprocess.STDOUT,
             text=True,
+            start_new_session=True,
         )
         probe_history: list[dict[str, Any]] = []
         try:
@@ -352,10 +378,4 @@ def run_trial(trial_spec_path: Path) -> dict[str, Any]:
             StudyStore.write_json(Path(trial.result_path), result)
             return result
         finally:
-            if process.poll() is None:
-                process.terminate()
-                try:
-                    process.wait(timeout=30)
-                except subprocess.TimeoutExpired:
-                    process.kill()
-                    process.wait(timeout=30)
+            _terminate_process_tree(process, timeout_s=30.0)
diff --git a/tests/test_core_flow.py b/tests/test_core_flow.py
index 9e45ede..f2ee9ea 100644
--- a/tests/test_core_flow.py
+++ b/tests/test_core_flow.py
@@ -16,7 +16,7 @@ from aituner.slo import RequestOutcome, summarize_evaluations
 from aituner.spec import Proposal, StudyState, TrialSummary, load_study_spec
 from aituner.store import StudyStore
 from aituner.trace import load_trace_requests, summarize_window
-from aituner.worker import _replay_requests, _wait_for_server_or_exit
+from aituner.worker import _replay_requests, _terminate_process_tree, _wait_for_server_or_exit
 from aituner.trace import TraceRequest
 
 
@@ -910,6 +910,17 @@ class CoreFlowTests(unittest.TestCase):
                 ready_timeout_s=10.0,
             )
 
+    def test_terminate_process_tree_kills_process_group(self) -> None:
+        process = mock.Mock()
+        process.pid = 1234
+        process.poll.side_effect = [None, None, 0]
+        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:
+                _terminate_process_tree(process, timeout_s=1.0)
+        mock_killpg.assert_called_once()
+        self.assertEqual(mock_killpg.call_args[0][0], 1234)
+
     def test_openai_url_avoids_double_v1(self) -> None:
         self.assertEqual(
             _openai_url("http://example.com", "/v1/chat/completions"),