diff --git a/pyproject.toml b/pyproject.toml
index b64fa8fa..595662e4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,7 +8,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "dataops-testgen"
-version = "5.70.2"
+version = "5.70.3"
 description = "DataKitchen's Data Quality DataOps TestGen"
 authors = [
     { "name" = "DataKitchen, Inc.", "email" = "info@datakitchen.io" },
@@ -92,7 +92,10 @@ dependencies = [
 
 [project.optional-dependencies]
 standalone = [
-    "pixeltable-pgserver>=0.5.1",
+    # Upper bound guards against a future release dropping a bundled PostgreSQL
+    # major that existing data dirs were initialized with (see start_server in
+    # common/standalone_postgres.py). Revisit deliberately before relaxing.
+    "pixeltable-pgserver>=0.6.0,<0.7.0",
 ]
 
 dev = [
diff --git a/testgen/common/standalone_postgres.py b/testgen/common/standalone_postgres.py
index be27e774..bc12efcf 100644
--- a/testgen/common/standalone_postgres.py
+++ b/testgen/common/standalone_postgres.py
@@ -22,6 +22,7 @@
 STANDALONE_MODE_ENV_VAR = "TG_STANDALONE_MODE"
 HOME_DIR_ENV_VAR = "TG_TESTGEN_HOME"
 STANDALONE_URI_ENV_VAR = "_TG_STANDALONE_URI"
+NEW_INSTALL_POSTGRES_VERSION = 18
 
 # Stored as ``project_host`` in the demo-DB connection row so that the actual
 # host/port — which can change across sessions on Windows (pgserver picks a
@@ -84,8 +85,13 @@ def start_server(data_dir: Path | None = None) -> None:
         data_dir = get_home_dir() / "pgdata"
     data_dir.mkdir(parents=True, exist_ok=True)
 
-    LOG.info("Starting embedded PostgreSQL (data: %s) ...", data_dir)
-    _server = pgserver.get_server(data_dir)
+    # pgserver bundles multiple PostgreSQL majors and defaults to its newest.
+    # Honor the major that initialized an existing data dir so upgrades never fail with a version mismatch.
+    existing_version = pgserver.pgdata_version(data_dir)
+    postgres_version = existing_version if existing_version is not None else NEW_INSTALL_POSTGRES_VERSION
+
+    LOG.info("Starting embedded PostgreSQL (data: %s, pg%s) ...", data_dir, postgres_version)
+    _server = pgserver.get_server(data_dir, postgres_version=postgres_version)
     LOG.info("Embedded PostgreSQL ready: %s", _server.get_uri())
 
     _reinitialize_orm_engine()
diff --git a/tests/unit/common/test_standalone_postgres.py b/tests/unit/common/test_standalone_postgres.py
new file mode 100644
index 00000000..35c106fe
--- /dev/null
+++ b/tests/unit/common/test_standalone_postgres.py
@@ -0,0 +1,48 @@
+"""Unit tests for embedded-PostgreSQL version selection in standalone mode."""
+
+import sys
+from unittest import mock
+
+import pytest
+
+from testgen.common import standalone_postgres
+
+
+@pytest.fixture
+def fake_pgserver():
+    """Mock the ``pixeltable_pgserver`` module imported inside ``start_server``."""
+    fake = mock.MagicMock()
+    fake.get_server.return_value.get_uri.return_value = "postgresql:///db?host=/tmp/sock"
+    with mock.patch.dict(sys.modules, {"pixeltable_pgserver": fake}):
+        yield fake
+
+
+@pytest.fixture(autouse=True)
+def reset_module_state():
+    """Isolate the module-level server singleton and skip real ORM/atexit side effects."""
+    standalone_postgres._server = None
+    with (
+        mock.patch.object(standalone_postgres, "_reinitialize_orm_engine"),
+        mock.patch("atexit.register"),
+    ):
+        yield
+    standalone_postgres._server = None
+
+
+def test_fresh_data_dir_uses_new_install_version(fake_pgserver, tmp_path):
+    fake_pgserver.pgdata_version.return_value = None
+
+    standalone_postgres.start_server(tmp_path / "pgdata")
+
+    assert fake_pgserver.get_server.call_args.kwargs["postgres_version"] == (
+        standalone_postgres.NEW_INSTALL_POSTGRES_VERSION
+    )
+
+
+def test_existing_data_dir_honors_on_disk_version(fake_pgserver, tmp_path):
+    # A cluster initialized by an older bundled major must keep running on it.
+    fake_pgserver.pgdata_version.return_value = 16
+
+    standalone_postgres.start_server(tmp_path / "pgdata")
+
+    assert fake_pgserver.get_server.call_args.kwargs["postgres_version"] == 16