diff --git a/train_second.py b/train_second.py
index ba3b989a..bbeff4e1 100644
--- a/train_second.py
+++ b/train_second.py
@@ -1,5 +1,6 @@
 # load packages
 import copy
+import os
 import random
 import yaml
 import time
@@ -7,7 +8,13 @@
 import numpy as np
 import torch
 
-torch.autograd.set_detect_anomaly(True)
+# Anomaly detection is a debugging tool — it traces every op in the autograd
+# graph to surface NaN/inf sources, but it makes backward 5–10× slower, holds
+# extra memory (causes OOMs that wouldn't otherwise happen), and on some
+# single-GPU + bf16 setups it deadlocks the first backward pass entirely.
+# Off by default; opt in via STYLETTS2_DETECT_ANOMALY=1 when diagnosing.
+if os.environ.get("STYLETTS2_DETECT_ANOMALY") == "1":
+    torch.autograd.set_detect_anomaly(True)
 
 if getattr(torch, "_original_load", None) is None:
     torch._original_load = torch.load