diff --git a/plugins/magpie-setup/hooks/check-upgrade.sh b/plugins/magpie-setup/hooks/check-upgrade.sh
index adef0acab..b2f4cbb85 100755
--- a/plugins/magpie-setup/hooks/check-upgrade.sh
+++ b/plugins/magpie-setup/hooks/check-upgrade.sh
@@ -74,14 +74,30 @@ fi
 
 # Once per session, the isolated-setup warning: `setup_preflight` decides
 # whether it applies and prints the notice with it, as hook JSON whose
-# `systemMessage` the user sees. Without python3 the notice goes out alone.
-# `-P` keeps the working directory, which is the project being opened, off
-# `sys.path`: `python3 -m` would otherwise put it ahead of `PYTHONPATH`, and
-# a `setup_preflight/` package committed to that project would run in place
-# of the plugin's own.
+# `systemMessage` the user sees. Without a usable Python the notice goes out
+# alone.
+#
+# The hook runs outside the sandbox, so the interpreter is never `python3`
+# from PATH, which project settings can set: only fixed system and Homebrew
+# locations, which no sandbox grant may cover (the container-gateway hook
+# refuses to run at all when one does). uv-managed interpreters are left out:
+# trusting them needs the gateway hook's full writable-root checks, and this
+# hook only prints a notice, so without a system Python it prints its own.
+# It runs with `-I -S`, so no PYTHON* variable, site-packages or working
+# directory (the project being opened, which could carry its own
+# `setup_preflight/`) reaches `sys.path`; a one-line bootstrap puts the
+# plugin's own source there. `setup_preflight` needs only the stdlib.
+python=""
+for candidate in /opt/homebrew/bin/python3 /usr/local/bin/python3 /usr/bin/python3; do
+  [ -x "$candidate" ] || continue
+  "$candidate" -I -S -c 'import sys; sys.exit(sys.version_info < (3, 11))' >/dev/null 2>&1 || continue
+  python="$candidate"
+  break
+done
 output="$notice"
-if command -v python3 >/dev/null 2>&1; then
-  output="$(PYTHONPATH="$root/skills/setup" python3 -P -m setup_preflight.isolated session-start \
+if [ -n "$python" ]; then
+  bootstrap='import runpy, sys; sys.path.insert(0, sys.argv.pop(1)); runpy.run_module("setup_preflight.isolated", run_name="__main__", alter_sys=True)'
+  output="$("$python" -I -S -c "$bootstrap" "$root/skills/setup" session-start \
     --project-root "${CLAUDE_PROJECT_DIR:-$PWD}" --notice "$notice" 2>/dev/null)" || output="$notice"
 fi
 
diff --git a/tools/setup-preflight/tests/test_session_hook.py b/tools/setup-preflight/tests/test_session_hook.py
index edd0e2dee..bf07a058f 100644
--- a/tools/setup-preflight/tests/test_session_hook.py
+++ b/tools/setup-preflight/tests/test_session_hook.py
@@ -54,3 +54,54 @@ def test_a_package_in_the_project_does_not_shadow_the_plugin(project: Path, tmp_
     assert not marker.exists()
     assert "PLANTED" not in done.stdout
     assert "Apache Magpie plugin" in done.stdout
+
+
+def test_a_python3_on_path_is_never_run(project: Path, tmp_path: Path) -> None:
+    """The hook runs outside the sandbox; PATH comes from settings a project can set."""
+    bin_dir = tmp_path / "bin"
+    bin_dir.mkdir()
+    marker = tmp_path / "path-python-ran"
+    fake = bin_dir / "python3"
+    fake.write_text(f"#!/bin/sh\ntouch {marker}\nexit 0\n")
+    fake.chmod(0o755)
+    env_path = f"{bin_dir}:{os.environ['PATH']}"
+    done = subprocess.run(
+        ["bash", str(HOOK)],
+        input="{}",
+        capture_output=True,
+        text=True,
+        env={
+            **os.environ,
+            "PATH": env_path,
+            "CLAUDE_PLUGIN_ROOT": str(PLUGIN_ROOT),
+            "CLAUDE_PLUGIN_DATA": str(tmp_path / "data"),
+            "CLAUDE_PROJECT_DIR": str(project),
+        },
+        cwd=project,
+        check=False,
+    )
+    assert done.returncode == 0, done.stderr
+    assert not marker.exists()
+    assert "Apache Magpie plugin" in done.stdout
+
+
+def test_pythonpath_does_not_reach_the_preflight(project: Path, tmp_path: Path) -> None:
+    """-I drops PYTHONPATH: a planted setup_preflight there never runs."""
+    planted = tmp_path / "pp" / "setup_preflight"
+    planted.mkdir(parents=True)
+    marker = tmp_path / "pp-ran"
+    code = f"open({str(marker)!r}, 'w').close()\n"
+    (planted / "__init__.py").write_text(code, encoding="utf-8")
+    (planted / "isolated.py").write_text(code, encoding="utf-8")
+    env = {
+        **os.environ,
+        "PYTHONPATH": str(tmp_path / "pp"),
+        "CLAUDE_PLUGIN_ROOT": str(PLUGIN_ROOT),
+        "CLAUDE_PLUGIN_DATA": str(tmp_path / "data"),
+        "CLAUDE_PROJECT_DIR": str(project),
+    }
+    done = subprocess.run(
+        ["bash", str(HOOK)], input="{}", capture_output=True, text=True, env=env, cwd=project, check=False
+    )
+    assert done.returncode == 0, done.stderr
+    assert not marker.exists()