diff --git a/probe_kv_layout.py b/probe_kv_layout.py
new file mode 100644
index 00000000..2e3dbadb
--- /dev/null
+++ b/probe_kv_layout.py
@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+"""Probe ixformer paged attention KV cache shape requirements."""
+import torch
+import ixformer
+
+num_heads = 4
+num_kv_heads = 1
+head_dim = 256
+block_size = 16
+num_blocks = 4
+context_len = num_blocks * block_size
+head_mapping = torch.zeros(num_heads, dtype=torch.int32, device="cuda")
+scale = head_dim ** -0.5
+query = torch.randn(1, num_heads, head_dim, device="cuda", dtype=torch.float16)
+context_lens = torch.tensor([context_len], device="cuda", dtype=torch.int32)
+block_tables = torch.arange(num_blocks, device="cuda", dtype=torch.int32).unsqueeze(0)
+
+# Read the ixformer vllm source for the correct layout
+import inspect
+src_file = "/usr/local/corex/lib64/python3/dist-packages/ixformer/functions/vllm.py"
+try:
+    with open(src_file) as f:
+        print(f"=== {src_file} ===")
+        print(f.read())
+except:
+    print(f"Cannot read {src_file}")
+
+# Try different 5D layouts
+print("\n=== Testing 5D KV cache layouts ===")
+for x in [1, 2, 4, 8, 16]:
+    if head_dim % x != 0:
+        continue
+    # Layout: (num_blocks, num_kv_heads, head_dim//x, block_size, x)
+    kc = torch.randn(num_blocks, num_kv_heads, head_dim // x, block_size, x,
+                       device="cuda", dtype=torch.float16)
+    vc = torch.randn(num_blocks, num_kv_heads, head_dim // x, block_size, x,
+                       device="cuda", dtype=torch.float16)
+    out = torch.empty(1, num_heads, head_dim, device="cuda", dtype=torch.float16)
+    try:
+        ixformer.vllm_single_query_cached_kv_attention(
+            out, query, kc, vc, head_mapping, scale,
+            block_tables, context_lens, block_size, context_len)
+        print(f"  x={x:2d} shape={kc.shape}: OK nan={out.isnan().any().item()}")
+    except Exception as e:
+        err = str(e)[:80]
+        print(f"  x={x:2d} shape={kc.shape}: {err}")