diff --git a/debug_topk.py b/debug_topk.py
new file mode 100644
index 00000000..dd6fdf88
--- /dev/null
+++ b/debug_topk.py
@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+"""Debug topk_softmax CUDA kernel mismatch."""
+import torch
+import os
+from torch.utils.cpp_extension import load
+
+ext = load(name="moe_topk_softmax_v3",
+           sources=[os.path.join(os.path.dirname(os.path.abspath(__file__)),
+                                 "ex_engine/csrc/moe_topk_softmax_v3.cu")],
+           extra_cuda_cflags=["-O3"], verbose=False)
+
+torch.manual_seed(123)
+gating = torch.randn(8, 64, device='cuda', dtype=torch.float32)
+
+# CUDA kernel
+results = ext.moe_topk_softmax(gating, 8, False)
+tw_cuda, ti_cuda = results[0], results[1]
+
+# PyTorch reference
+probs = torch.softmax(gating, dim=-1)
+tw_ref, ti_ref = torch.topk(probs, 8, dim=-1)
+
+print("=== Per-row comparison ===")
+for r in range(8):
+    ids_match = set(ti_cuda[r].tolist()) == set(ti_ref[r].tolist())
+    w_diff = (tw_cuda[r].sort()[0] - tw_ref[r].sort()[0]).abs().max().item()
+    print(f"Row {r}: CUDA ids={ti_cuda[r].tolist()[:4]}...  "
+          f"Ref ids={ti_ref[r].tolist()[:4]}...  "
+          f"ids_match={ids_match}  w_diff={w_diff:.6e}  "
+          f"cuda_sum={tw_cuda[r].sum():.4f}  ref_sum={tw_ref[r].sum():.4f}")
+
+# Check if consecutive rows are identical
+print("\n=== Row duplication check ===")
+for r in range(0, 8, 2):
+    same = (ti_cuda[r] == ti_cuda[r+1]).all().item()
+    print(f"Row {r} == Row {r+1}: {same}")
+
+# Minimal 2-row test
+print("\n=== Minimal 2-row test ===")
+g2 = torch.tensor([[1.0, 2.0, 3.0] + [0.0]*61,
+                    [3.0, 2.0, 1.0] + [0.0]*61], device='cuda', dtype=torch.float32)
+r2 = ext.moe_topk_softmax(g2, 3, False)
+p2 = torch.softmax(g2, dim=-1)
+t2w, t2i = torch.topk(p2, 3, dim=-1)
+print(f"CUDA row0 ids: {r2[1][0].tolist()[:3]}  weights: {r2[0][0].tolist()[:3]}")
+print(f"CUDA row1 ids: {r2[1][1].tolist()[:3]}  weights: {r2[0][1].tolist()[:3]}")
+print(f"Ref  row0 ids: {t2i[0].tolist()[:3]}  weights: {t2w[0].tolist()[:3]}")
+print(f"Ref  row1 ids: {t2i[1].tolist()[:3]}  weights: {t2w[1].tolist()[:3]}")