diff --git a/dsv4/kernels/cuda/sampler.cu b/dsv4/kernels/cuda/sampler.cu
index 565a2a7c..06cfeb88 100644
--- a/dsv4/kernels/cuda/sampler.cu
+++ b/dsv4/kernels/cuda/sampler.cu
@@ -99,10 +99,10 @@ __global__ void fused_sampler_kernel(
 
     if (tid == 0) {
         // Merge: find global top-k from BDIM * LK = 8192 candidates
-        int eff_k = min(top_k, 256);  // kernel max
-        if (eff_k <= 0) eff_k = 256;
+        int eff_k = min(top_k, 128);  // kernel max (stack limit: 128 * 8 = 1KB)
+        if (eff_k <= 0) eff_k = 128;
 
-        float gsc[256]; int gid[256]; int gn = 0;
+        float gsc[128]; int gid[128]; int gn = 0;
         for (int t = 0; t < BDIM; t++) {
             for (int i = 0; i < LK; i++) {
                 float s = s_sc[t*LK+i];
@@ -115,7 +115,7 @@ __global__ void fused_sampler_kernel(
 
         // ---------- Phase 3: softmax + top-p + sample ----------
         float mx = gsc[0];  // sorted desc, first is max
-        float probs[256]; float total = 0.0f;
+        float probs[128]; float total = 0.0f;
         for (int i = 0; i < gn; i++) {
             probs[i] = expf(gsc[i] - mx);
             total += probs[i];