diff --git a/testgen/common/standalone_postgres.py b/testgen/common/standalone_postgres.py
index bc12efcf..c25cb8c5 100644
--- a/testgen/common/standalone_postgres.py
+++ b/testgen/common/standalone_postgres.py
@@ -22,7 +22,10 @@
 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
+# pgserver also bundles PostgreSQL 18, but its Windows build links against libwinpthread-1.dll
+# without shipping it, so initdb cannot start postgres on a machine that has no MinGW toolchain.
+# Raise this once that build runs on a stock Windows box.
+NEW_INSTALL_POSTGRES_VERSION = 16
 
 # 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
diff --git a/tests/unit/common/test_standalone_postgres.py b/tests/unit/common/test_standalone_postgres.py
index 35c106fe..2e7599e1 100644
--- a/tests/unit/common/test_standalone_postgres.py
+++ b/tests/unit/common/test_standalone_postgres.py
@@ -39,10 +39,17 @@ def test_fresh_data_dir_uses_new_install_version(fake_pgserver, tmp_path):
     )
 
 
-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
+def test_new_install_version_is_16():
+    # The bundled PostgreSQL 18 Windows build needs a MinGW runtime it does not ship,
+    # so a fresh cluster must be initialized on 16.
+    assert standalone_postgres.NEW_INSTALL_POSTGRES_VERSION == 16
+
+
+@pytest.mark.parametrize("on_disk_version", [16, 18])
+def test_existing_data_dir_honors_on_disk_version(fake_pgserver, tmp_path, on_disk_version):
+    # A cluster keeps running on the major that initialized it, whichever that was.
+    fake_pgserver.pgdata_version.return_value = on_disk_version
 
     standalone_postgres.start_server(tmp_path / "pgdata")
 
-    assert fake_pgserver.get_server.call_args.kwargs["postgres_version"] == 16
+    assert fake_pgserver.get_server.call_args.kwargs["postgres_version"] == on_disk_version