diff --git a/src/store/hnsw.c b/src/store/hnsw.c
index 16a5f581..fb53b4b4 100644
--- a/src/store/hnsw.c
+++ b/src/store/hnsw.c
@@ -464,6 +464,13 @@ ray_hnsw_t* ray_hnsw_build(const float* vectors, int64_t n_nodes, int32_t dim,
                            ray_hnsw_metric_t metric,
                            int32_t M, int32_t ef_construction) {
     if (!vectors || n_nodes <= 0 || dim <= 0) return NULL;
+    /* Overflow guard: n_nodes * dim * sizeof(float) sizes the copied vector
+     * block and also indexes every distance computation (vectors + id*dim).
+     * If it wraps size_t the copy under-allocates and the memcpy below — and
+     * later reads during construction — run past the buffer.  Reject before
+     * any allocation.  Mirrors the per-layer neighbor guard in the loader.
+     * n_nodes and dim are > 0 here, so the divide is safe. */
+    if ((uint64_t)n_nodes > SIZE_MAX / sizeof(float) / (uint64_t)dim) return NULL;
     if (M <= 0) M = HNSW_DEFAULT_M;
     if (ef_construction <= 0) ef_construction = HNSW_DEFAULT_EF_C;
     if (metric < RAY_HNSW_COSINE || metric > RAY_HNSW_IP) metric = RAY_HNSW_COSINE;
diff --git a/test/test_embedding.c b/test/test_embedding.c
index ef8aca58..6dea9b10 100644
--- a/test/test_embedding.c
+++ b/test/test_embedding.c
@@ -969,6 +969,20 @@ static test_result_t test_hnsw_mmap_load(void) {
     PASS();
 }
 
+/* Regression: ray_hnsw_build must reject dimensions whose
+ * n_nodes * dim * sizeof(float) overflows size_t, before it copies the source
+ * vectors.  Values are chosen so the byte count wraps to 16: without the guard
+ * the copy under-allocates and memcpy over-reads the 1-element source (ASan
+ * traps it); with the guard the call returns NULL and the source is never
+ * touched.  (2^62 + 2) * 2 * sizeof(float) == 2^65 + 16 == 16 (mod 2^64). */
+static test_result_t test_hnsw_build_overflow_rejected(void) {
+    float dummy[1] = { 0.0f };
+    ray_hnsw_t* idx = ray_hnsw_build(dummy, ((int64_t)1 << 62) + 2, 2,
+                                     RAY_HNSW_L2, 4, 50);
+    TEST_ASSERT_NULL(idx);
+    PASS();
+}
+
 /* Trigger the maxheap_sift_down / results-replacement path in hnsw_search_layer.
  *
  * The replacement branch (lines 342-344) fires when:
@@ -1964,6 +1978,7 @@ const test_entry_t embedding_entries[] = {
     { "embedding/hnsw_dim_accessor", test_hnsw_dim_accessor, emb_setup, emb_teardown },
     { "embedding/hnsw_search_filter_null_accept", test_hnsw_search_filter_null_accept, emb_setup, emb_teardown },
     { "embedding/hnsw_mmap_load", test_hnsw_mmap_load, emb_setup, emb_teardown },
+    { "embedding/hnsw_build_overflow_rejected", test_hnsw_build_overflow_rejected, emb_setup, emb_teardown },
     { "embedding/hnsw_search_sift_down", test_hnsw_search_sift_down, emb_setup, emb_teardown },
 
     /* rerank coverage (S7) */