Fix FFM-1689: enable cold-start session gate by default

src/config.py

@@ -51,10 +51,10 @@ class Config(BaseSettings):
     DEEPSEEK_BASE_URL: str = "https://api.deepseek.com"
     CHAT_AGENT_ENABLED: bool = False
 
-    # FFM-1161: gate cold_start engines so they only serve genuinely-new users
-    # (nsessions <= 1). Dormant returners with nmemes_sent < 30 but multiple
-    # sessions fall through to the growing-user blender instead.
-    COLD_START_NSESSIONS_GATE_ENABLED: bool = False
+    # FFM-1161/FFM-1689: gate cold_start engines so they only serve genuinely-new
+    # users (nsessions <= 1). Dormant returners with nmemes_sent < 30 but
+    # multiple sessions fall through to the growing-user blender instead.
+    COLD_START_NSESSIONS_GATE_ENABLED: bool = True
 
     # Recommendation batch diagnostics are realtime operational data, not
     # durable product facts. Keep compact logs/spans always on and sample full

tests/recommendations/test_meme_queue.py

@@ -702,7 +702,7 @@ class TestRetriever(CandidatesRetriever):
 
 @pytest.mark.asyncio
 async def test_gate_off_dormant_returner_still_uses_cold_start():
-    """Default (gate disabled): nsessions is ignored — cold_start still routes by nmemes_sent."""
+    """Emergency override off: nsessions is ignored and cold_start routes by nmemes_sent."""
     retriever = _growing_retriever_class()()
     with (
         _patch_user_info(nsessions=5, nmemes_sent=8),
@@ -714,6 +714,21 @@ async def test_gate_off_dormant_returner_still_uses_cold_start():
     assert any(c["recommended_by"] == "cold_start_adapt" for c in candidates)
 
 
+@pytest.mark.asyncio
+async def test_gate_default_blocks_dormant_returner_from_cold_start():
+    """Default config blocks nsessions>=2 low-sent users from cold_start engines."""
+    retriever = _growing_retriever_class()()
+    with _patch_user_info(nsessions=3, nmemes_sent=8):
+        candidates = await generate_recommendations(
+            TEST_USER_ID, 10, nmemes_sent=8, retriever=retriever, random_seed=42
+        )
+
+    sources = {c["recommended_by"] for c in candidates}
+    assert "cold_start_explore" not in sources
+    assert "cold_start_adapt" not in sources
+    assert candidates[0]["recommended_by"] == "lr_smoothed"
+
+
 @pytest.mark.asyncio
 async def test_gate_on_first_session_routes_to_cold_start_explore():
     """Gate on + nsessions<=1 + nmemes_sent<6 → cold_start_explore (Phase 1)."""