diff --git a/tests/test_ops_training_accuracy.py b/tests/test_ops_training_accuracy.py
index 3dfa2426..3138539c 100644
--- a/tests/test_ops_training_accuracy.py
+++ b/tests/test_ops_training_accuracy.py
@@ -1,4 +1,5 @@
 from web.services.ops_api import _build_training_accuracy_payload
+from datetime import date, timedelta
 
 
 def test_training_accuracy_payload_includes_recent_deb_summary():
@@ -36,3 +37,34 @@ def test_training_accuracy_payload_includes_recent_deb_summary():
     assert payload["deb_summary"]["recent_14d"]["samples"] == 5
     assert "deb_v1_raw" in payload["deb_summary"]["versions"]
     assert "deb_v2_bucket_calibrated" in payload["deb_summary"]["versions"]
+
+
+def test_training_accuracy_payload_caps_version_backtest_samples(monkeypatch):
+    start = date(2025, 1, 1)
+    history = {
+        "alpha": {
+            (start + timedelta(days=idx)).isoformat(): {
+                "actual_high": 20.0 + (idx % 3),
+                "deb_prediction": 20.0 + (idx % 3),
+            }
+            for idx in range(405)
+        }
+    }
+    captured = {}
+
+    def fake_backtest(rows, **_kwargs):
+        row_list = list(rows)
+        captured["count"] = len(row_list)
+        captured["first_date"] = row_list[0]["target_date"]
+        return {"versions": {}}
+
+    monkeypatch.setattr("src.analysis.deb_evaluation.backtest_deb_versions", fake_backtest)
+
+    _build_training_accuracy_payload(
+        history,
+        {"alpha": {"name": "Alpha"}},
+        today_str="2026-03-01",
+    )
+
+    assert captured["count"] == 400
+    assert captured["first_date"] == "2025-01-06"
diff --git a/web/services/ops_api.py b/web/services/ops_api.py
index 3d4e42f5..e1734db6 100644
--- a/web/services/ops_api.py
+++ b/web/services/ops_api.py
@@ -18,6 +18,8 @@ from web.services.observation_freshness import (
 from web.core import GrantPointsRequest
 import web.routes as legacy_routes
 
+_DEB_VERSION_BACKTEST_SAMPLE_LIMIT = 400
+
 
 def _sf(value: Any) -> Optional[float]:
     try:
@@ -2455,8 +2457,9 @@ def _build_training_accuracy_payload(
     recent_7_start = (today_date - timedelta(days=7)).isoformat()
     recent_14_start = (today_date - timedelta(days=14)).isoformat()
     end_date = (today_date - timedelta(days=1)).isoformat()
+    version_rows = all_rows[-_DEB_VERSION_BACKTEST_SAMPLE_LIMIT:]
     versions = backtest_deb_versions(
-        all_rows,
+        version_rows,
         min_train_samples=2,
     ).get("versions", {})