diff --git a/backend/agents.py b/backend/agents.py
index 139fce0..9db9f97 100644
--- a/backend/agents.py
+++ b/backend/agents.py
@@ -9,6 +9,7 @@ import logging
 import os
 import re
 import shutil
+import signal
 import tempfile
 import time
 import urllib.request
@@ -84,6 +85,18 @@ def provider_available(provider: str) -> bool:
     return True
 
 
+def _kill(process) -> None:
+    """Killt den Agenten samt Kindprozessen über die Prozess-Gruppe (sonst überleben die
+    von der CLI gestarteten Kinder, halten die Pipes offen und blockieren communicate())."""
+    try:
+        os.killpg(os.getpgid(process.pid), signal.SIGKILL)
+    except (ProcessLookupError, PermissionError):
+        try:
+            process.kill()
+        except ProcessLookupError:
+            pass
+
+
 def kill_process(agent_key_prefix: str) -> None:
     """Killt alle aktiven Prozesse, deren Key mit dem Prefix beginnt (deckt -plan/-w1… ab)."""
     for key, process in list(_active_processes.items()):
@@ -92,7 +105,7 @@ def kill_process(agent_key_prefix: str) -> None:
             continue
         if key.startswith(agent_key_prefix):
             log.debug("kill agent %s", key)
-            process.kill()
+            _kill(process)
 
 
 async def run_agent(
@@ -128,6 +141,7 @@ async def _communicate(agent_key: str, cmd: list[str], stdin_data: bytes | None,
             stdin=asyncio.subprocess.PIPE if stdin_data is not None else asyncio.subprocess.DEVNULL,
             stdout=asyncio.subprocess.PIPE,
             stderr=asyncio.subprocess.PIPE,
+            start_new_session=True,  # eigene Prozess-Gruppe → killpg killt auch Kindprozesse
         )
 
     if stagger:
@@ -144,7 +158,7 @@ async def _communicate(agent_key: str, cmd: list[str], stdin_data: bytes | None,
                 timeout=timeout,
             )
         except asyncio.TimeoutError:
-            process.kill()
+            _kill(process)
             try:
                 await asyncio.wait_for(process.wait(), timeout=5)
             except asyncio.TimeoutError: