diff --git a/onnxruntime/contrib_ops/webgpu/bert/multihead_attention.cc b/onnxruntime/contrib_ops/webgpu/bert/multihead_attention.cc
index 4934845ce1cfd..0af6d2d4e907a 100644
--- a/onnxruntime/contrib_ops/webgpu/bert/multihead_attention.cc
+++ b/onnxruntime/contrib_ops/webgpu/bert/multihead_attention.cc
@@ -673,14 +673,15 @@ fn computeO(q_idx: u32, sg_id:u32, enabled:bool)
 
 )HELPER_FN";
 
-// Shader is designed to be dispatched as Dispatch(num_heads, present_seq_length / TILE_SIZE, 1)
+// Shader is designed to be dispatched as Dispatch(num_heads, new_sequence_length / TILE_SIZE, 1)
+// Each workgroup is responsible for a range of q values (TILE_SIZE) and visits all Ks for those q's.
   shader.MainFunctionBody() << R"MAIN_FN(
 let head_idx = workgroup_id.x;
 
-let wave_x = (local_id.x / 4);
-let wave_y = (local_id.y / 4);
+let wave_x = u32(local_id.x / 4);
+let wave_y = u32(local_id.y / 4);
 // It is always the case that 0 <= wave_id < TILE_SIZE
-let wave_id = wave_x + wave_y * 4;
+let wave_id:u32 = wave_x + wave_y * 4;
 
 let q_idx_start = workgroup_id.y * TILE_SIZE;
 let q_idx_global = q_idx_start + wave_id;
@@ -712,7 +713,7 @@ for(var k_start = 0u; k_start < uniforms.present_sequence_length; k_start+=TILE_
     workgroupBarrier();
     if (k_idx_global_using_wave_valid)
     {
-      for (var q_idx = 0u; q_idx < TILE_SIZE && q_idx_start + q_idx < uniforms.present_sequence_length; q_idx++)
+      for (var q_idx = 0u; q_idx < TILE_SIZE && q_idx_start + q_idx < uniforms.new_sequence_length; q_idx++)
       {
           // Leveraging the subgroups for parallelism, compute dot product of QK.
           // Because for the case of new_seq 1, there is a single query and context length of K
@@ -824,6 +825,7 @@ Status MultiHeadAttention::ComputeInternal(onnxruntime::webgpu::ComputeContext&
 
   if (parameters.batch_size == 1 &&
     bias == nullptr &&
+    past_key != nullptr && past_value != nullptr && past_key->SizeInBytes() > 0 &&
     present_key != nullptr && present_value != nullptr && present_key->SizeInBytes() > 0 &&
     present_value->SizeInBytes() > 0 && parameters.head_size % 4 == 0) {
     return ApplyFlashAttention(query, key, value, attention_bias, output, past_key, present_key, past_value,