diff --git a/scripts/loopx b/scripts/loopx
index b5090097c..faeb5b36f 100755
--- a/scripts/loopx
+++ b/scripts/loopx
@@ -63,17 +63,33 @@ if [[ "$loopx_command" == "quota" \
   exec node --no-warnings --experimental-strip-types "$native_scheduler_entry" "$@"
 fi
 
-python_bin="${LOOPX_PYTHON:-}"
-if [[ -z "$python_bin" && -f "$repo_root/.loopx-python" ]]; then
-  IFS= read -r python_bin <"$repo_root/.loopx-python"
-fi
-if [[ -z "$python_bin" ]]; then
-  python_bin="python3"
-fi
-if ! command -v "$python_bin" >/dev/null 2>&1; then
-  echo "loopx runtime error: configured Python executable not found: $python_bin" >&2
-  echo "Set LOOPX_PYTHON to Python 3.11+ and reinstall LoopX." >&2
-  exit 2
+python_selector="$repo_root/scripts/loopx-python.sh"
+if [[ -n "${LOOPX_PYTHON:-}" ]]; then
+  python_bin="$LOOPX_PYTHON"
+  if ! command -v "$python_bin" >/dev/null 2>&1; then
+    echo "loopx runtime error: configured Python executable not found: $python_bin" >&2
+    echo "Set LOOPX_PYTHON to Python 3.11+ and reinstall LoopX." >&2
+    exit 2
+  fi
+elif [[ -f "$python_selector" ]]; then
+  if ! python_bin="$(bash "$python_selector")"; then
+    echo "Set LOOPX_PYTHON to Python 3.11+ and reinstall LoopX." >&2
+    exit 2
+  fi
+else
+  # Keep old release snapshots runnable if the companion selector is absent.
+  python_bin="${LOOPX_PYTHON:-}"
+  if [[ -z "$python_bin" && -f "$repo_root/.loopx-python" ]]; then
+    IFS= read -r python_bin <"$repo_root/.loopx-python"
+  fi
+  if [[ -z "$python_bin" ]]; then
+    python_bin="python3"
+  fi
+  if ! command -v "$python_bin" >/dev/null 2>&1; then
+    echo "loopx runtime error: configured Python executable not found: $python_bin" >&2
+    echo "Set LOOPX_PYTHON to Python 3.11+ and reinstall LoopX." >&2
+    exit 2
+  fi
 fi
 
 if [[ "$loopx_command" == "canary" && "$loopx_subcommand" == "premerge" ]]; then
diff --git a/tests/test_loopx_launcher_python_selection.py b/tests/test_loopx_launcher_python_selection.py
new file mode 100644
index 000000000..4800747bd
--- /dev/null
+++ b/tests/test_loopx_launcher_python_selection.py
@@ -0,0 +1,116 @@
+from __future__ import annotations
+
+import os
+from pathlib import Path
+import shlex
+import shutil
+import subprocess
+import sys
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+def _write_stub(path: Path, body: str) -> None:
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(f"#!/usr/bin/env bash\n{body}\n", encoding="utf-8")
+    path.chmod(0o755)
+
+
+def test_source_launcher_uses_project_venv_without_path_activation(tmp_path: Path) -> None:
+    release_root = tmp_path / "release"
+    scripts = release_root / "scripts"
+    scripts.mkdir(parents=True)
+    package = release_root / "loopx"
+    package.mkdir()
+    (package / "cli.py").write_text("print('launcher selected the project interpreter')\n", encoding="utf-8")
+    shutil.copy2(REPO_ROOT / "scripts/loopx", scripts / "loopx")
+    shutil.copy2(REPO_ROOT / "scripts/loopx-python.sh", scripts / "loopx-python.sh")
+
+    selected = tmp_path / "selected-python.log"
+    _write_stub(
+        release_root / ".venv/bin/python",
+        "printf selected > "
+        f"{shlex.quote(str(selected))}\n"
+        f"exec {shlex.quote(sys.executable)} \"$@\"",
+    )
+
+    completed = subprocess.run(
+        ["bash", str(scripts / "loopx"), "version"],
+        cwd=release_root,
+        env={
+            **{key: value for key, value in os.environ.items() if key != "LOOPX_PYTHON"},
+            "PATH": "/usr/bin:/bin",
+        },
+        text=True,
+        capture_output=True,
+        check=False,
+    )
+
+    assert completed.returncode == 0, completed.stderr
+    assert selected.read_text(encoding="utf-8") == "selected"
+    assert "Python 3.11+ is required" not in completed.stderr
+
+
+def test_source_launcher_preserves_explicit_missing_python_diagnostic(
+    tmp_path: Path,
+) -> None:
+    release_root = tmp_path / "release"
+    scripts = release_root / "scripts"
+    scripts.mkdir(parents=True)
+    (release_root / "loopx").mkdir()
+    shutil.copy2(REPO_ROOT / "scripts/loopx", scripts / "loopx")
+    shutil.copy2(REPO_ROOT / "scripts/loopx-python.sh", scripts / "loopx-python.sh")
+
+    missing = tmp_path / "missing-python"
+    completed = subprocess.run(
+        ["bash", str(scripts / "loopx"), "version"],
+        cwd=release_root,
+        env={
+            **{key: value for key, value in os.environ.items() if key != "LOOPX_PYTHON"},
+            "LOOPX_PYTHON": str(missing),
+            "PATH": "/usr/bin:/bin",
+        },
+        text=True,
+        capture_output=True,
+        check=False,
+    )
+
+    assert completed.returncode == 2
+    assert "configured Python executable not found" in completed.stderr
+    assert str(missing) in completed.stderr
+
+
+def test_source_launcher_preserves_explicit_old_python_diagnostic(
+    tmp_path: Path,
+) -> None:
+    release_root = tmp_path / "release"
+    scripts = release_root / "scripts"
+    scripts.mkdir(parents=True)
+    (release_root / "loopx").mkdir()
+    shutil.copy2(REPO_ROOT / "scripts/loopx", scripts / "loopx")
+    shutil.copy2(REPO_ROOT / "scripts/loopx-python.sh", scripts / "loopx-python.sh")
+
+    legacy = tmp_path / "python-legacy"
+    _write_stub(
+        legacy,
+        "if [[ \"$1\" == \"-c\" ]]; then\n"
+        "  printf '%s\\n' 'loopx runtime error: Python 3.11+ is required; selected Python is 3.9. Set LOOPX_PYTHON to Python 3.11+ and reinstall LoopX.' >&2\n"
+        "fi\n"
+        "exit 2",
+    )
+    clean_env = {
+        key: value for key, value in os.environ.items() if key != "LOOPX_PYTHON"
+    }
+
+    completed = subprocess.run(
+        ["bash", str(scripts / "loopx"), "version"],
+        cwd=release_root,
+        env={**clean_env, "LOOPX_PYTHON": str(legacy), "PATH": "/usr/bin:/bin"},
+        text=True,
+        capture_output=True,
+        check=False,
+    )
+
+    assert completed.returncode == 2
+    assert "selected Python is 3.9" in completed.stderr