diff --git a/delphi/polismath/conversation/conversation.py b/delphi/polismath/conversation/conversation.py
index 46a17745a..efae4236c 100644
--- a/delphi/polismath/conversation/conversation.py
+++ b/delphi/polismath/conversation/conversation.py
@@ -1440,10 +1440,10 @@ def recompute(self) -> 'Conversation':
         # has ONLY graph-node keys — :last-mod-timestamp is not one
         # (conversation.clj:780-820), so every votes recompute DROPS the mod
         # watermark; blobs carry lastModTimestamp only when the tick's last
-        # write was a mod-update. Improved mode keeps the persistent watermark
-        # (documented divergence). tests/test_mod_update_parity.py.
-        if resolve_engine_mode() == ENGINE_MODE_LEGACY:
-            result.last_mod_timestamp = None
+        # write was a mod-update. tests/test_mod_update_parity.py. (The
+        # former improved-mode persistent watermark is parked:
+        # POST_CUTOVER_IMPROVEMENTS.md item 4.)
+        result.last_mod_timestamp = None
 
         # Compute PCA and projections
         result._compute_pca(prev_pca=prev_pca)
diff --git a/delphi/tests/test_mod_update_parity.py b/delphi/tests/test_mod_update_parity.py
index d28edc7f7..22be4be02 100644
--- a/delphi/tests/test_mod_update_parity.py
+++ b/delphi/tests/test_mod_update_parity.py
@@ -28,11 +28,6 @@ def legacy_mode(monkeypatch):
     monkeypatch.setenv(ENGINE_MODE_ENV_VAR, 'clojure-legacy')
 
 
-@pytest.fixture
-def improved_mode(monkeypatch):
-    monkeypatch.setenv(ENGINE_MODE_ENV_VAR, 'improved')
-
-
 def _conv(**sets):
     conv = Conversation("mod-parity-probe", last_updated=1)
     for attr, val in sets.items():
@@ -150,14 +145,6 @@ def test_votes_recompute_drops_watermark_in_legacy_mode(self, legacy_mode):
         conv2 = conv.update_votes(dict(self.BATCH), recompute=True)
         assert conv2.last_mod_timestamp is None
 
-    def test_watermark_persists_in_improved_mode(self, improved_mode):
-        # Documented divergence: improved mode keeps the sane persistent
-        # watermark instead of Clojure's graph-drop.
-        conv = Conversation("wm-keep", last_updated=1)
-        conv = conv.mod_update([_row(0, mod=-1, modified=777)])
-        conv2 = conv.update_votes(dict(self.BATCH), recompute=True)
-        assert conv2.last_mod_timestamp == 777
-
 
 class TestGroupVotesTallyRawMatrix:
     """Clojure's group-votes aggregates votes-base, whose fnk reads