diff --git a/tests/conftest.py b/tests/conftest.py
index 6b1798c..51f5d55 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -4,9 +4,27 @@ import shutil
 import json
 import uuid
 import sqlite3
+import subprocess as _subprocess
 from pathlib import Path
 import pytest
 
+# Issue #19: the sandbox reaper runs in fixture teardown, before `monkeypatch`
+# undoes a test's patches. Bind the real Popen class once at import so a test
+# that spies on or forbids subprocess.run/Popen (G5) cannot break the reaper.
+# (subprocess.run resolves `Popen` through the module at call time, so binding
+# `run` alone is not enough.)
+_REAL_POPEN = _subprocess.Popen
+
+
+def _run_real(cmd):
+    """stdout of `cmd` via the import-time Popen; '' on any OS error."""
+    try:
+        proc = _REAL_POPEN(cmd, stdout=_subprocess.PIPE, stderr=_subprocess.DEVNULL, text=True)
+        out, _ = proc.communicate(timeout=10)
+        return out
+    except (OSError, _subprocess.SubprocessError):
+        return ""
+
 SKILLS_DIR = str(Path(__file__).resolve().parent.parent / ".agents" / "skills")
 if SKILLS_DIR not in sys.path:
     sys.path.insert(0, SKILLS_DIR)
@@ -19,29 +37,14 @@ def _subscriber_daemon_pids(root):
     detached into its own session with `--idle-timeout 0`. A sandbox that ran
     it must reap it, or every suite run leaves ~30 daemons holding wildcard
     subscriptions on the default public broker behind."""
-    import subprocess as _sp
     root = str(root)
-    try:
-        out = _sp.run(["pgrep", "-f", "reconcile.sh --subscribe"],
-                      capture_output=True, text=True).stdout.split()
-    except OSError:
-        return []
     found = []
-    for pid in out:
-        try:
-            args = _sp.run(["ps", "-o", "args=", "-p", pid],
-                           capture_output=True, text=True).stdout
-        except OSError:
-            continue
-        if root in args:
+    for pid in _run_real(["pgrep", "-f", "reconcile.sh --subscribe"]).split():
+        if root in _run_real(["ps", "-o", "args=", "-p", pid]):
             found.append(int(pid))
     kids = []
     for pid in found:
-        try:
-            kids += [int(k) for k in _sp.run(["pgrep", "-P", str(pid)],
-                                             capture_output=True, text=True).stdout.split()]
-        except (OSError, ValueError):
-            pass
+        kids += [int(k) for k in _run_real(["pgrep", "-P", str(pid)]).split() if k.isdigit()]
     return found + kids
 
 
@@ -73,12 +76,7 @@ def _pid_alive(pid):
     except PermissionError:
         return True
     # a zombie still answers kill(0); ps state Z means it is gone for our purposes
-    import subprocess as _sp
-    try:
-        state = _sp.run(["ps", "-o", "stat=", "-p", str(pid)],
-                        capture_output=True, text=True).stdout.strip()
-    except OSError:
-        return True
+    state = _run_real(["ps", "-o", "stat=", "-p", str(pid)]).strip()
     return bool(state) and not state.startswith("Z")