diff --git a/docs/docs/index/hnsw.md b/docs/docs/index/hnsw.md
index cbf7e36e..42cababb 100644
--- a/docs/docs/index/hnsw.md
+++ b/docs/docs/index/hnsw.md
@@ -63,6 +63,11 @@ see [C++ integration](../cpp_integration.md).
 | `metric`, `random_seed` | Python defaults are `"l2"` and 100; `"ip"` is also supported. |
 | `k`, `ef` | Positive result count and search window. Python requires `k <= max_elements`; omitted/zero `ef` uses `max(k, 10)`, and positive `ef` is raised to at least `k`. |
 
+`random_seed` controls graph levels and the FHT/Kac rotation (using its low 32 bits).
+With identical inputs and parameters, single-threaded construction is reproducible
+within the same build environment. Parallel insertion order and different toolchains
+or SIMD backends can still change results. Loading restores the saved rotation.
+
 Results use squared L2 or `1 - dot` estimates, ordered from smallest to largest.
 Python fills missing results with ID `2**32 - 1` and distance `+inf`; C++ returns
 only the available pairs. See [result handling](../quick_start.md#read-the-results)
diff --git a/include/rabitqlib/index/hnsw/hnsw.hpp b/include/rabitqlib/index/hnsw/hnsw.hpp
index 55f7f8b8..22e97c27 100644
--- a/include/rabitqlib/index/hnsw/hnsw.hpp
+++ b/include/rabitqlib/index/hnsw/hnsw.hpp
@@ -46,6 +46,8 @@ class HierarchicalNSW {
    public:
     explicit HierarchicalNSW();
     // dim is the original input dimension; total_bits includes the sign bit.
+    // random_seed controls graph levels and (through its low 32 bits) rotation.
+    // Parallel insertion order can still vary with thread scheduling.
     explicit HierarchicalNSW(
         size_t max_elements,
         size_t dim,
diff --git a/src/index/hnsw.cpp b/src/index/hnsw.cpp
index da678eba..7c66e881 100644
--- a/src/index/hnsw.cpp
+++ b/src/index/hnsw.cpp
@@ -82,7 +82,10 @@ HierarchicalNSW::HierarchicalNSW(
     dim_ = dim;
     rotator_.reset(
         choose_rotator<float>(
-            dim, RotatorType::FhtKacRotator, round_up_to_multiple(dim_, 32)
+            dim,
+            RotatorType::FhtKacRotator,
+            round_up_to_multiple(dim_, 32),
+            static_cast<uint32_t>(random_seed)
         )
     );
     padded_dim_ = rotator_->size();
diff --git a/tests/python/test_hnsw.py b/tests/python/test_hnsw.py
index 27be3e40..a0c509f8 100644
--- a/tests/python/test_hnsw.py
+++ b/tests/python/test_hnsw.py
@@ -68,6 +68,29 @@ def test_fast_quantization_builds(base_data, clusters):
     assert idx.is_built
 
 
+def test_seed_reproduces_serial_build_and_search(tmp_path):
+    rng = np.random.default_rng(47)
+    data = rng.standard_normal((65, 65), dtype=np.float32)
+    queries = rng.standard_normal((5, 65), dtype=np.float32)
+    centroids = data[:2].copy()
+    clusters = (np.arange(len(data)) % 2).astype(np.uint32)
+    snapshots = []
+    results = []
+    for run, seed in enumerate((42, 42, 43)):
+        index = HnswIndex(
+            65, len(data), M=8, ef_construction=32, nbits=4, random_seed=seed
+        )
+        index.build(data, centroids, clusters, num_threads=1)
+        path = tmp_path / f"seed-{run}.index"
+        index.save(str(path))
+        snapshots.append(path.read_bytes())
+        results.append(index.search(queries, k=5, ef=65, num_threads=1))
+    assert snapshots[0] == snapshots[1]
+    assert snapshots[0] != snapshots[2]
+    for actual, expected in zip(results[1], results[0], strict=True):
+        np.testing.assert_array_equal(actual, expected)
+
+
 def test_parallel_build_preserves_self_retrieval(base_data, clusters):
     # Keep every neighbor so insertion-order-dependent pruning cannot isolate a
     # point. This checks parallel construction, not sparse-graph recall.
diff --git a/tests/unit/rabitqlib/index/hnsw_test.cpp b/tests/unit/rabitqlib/index/hnsw_test.cpp
index 04b97f38..4e59a3f4 100644
--- a/tests/unit/rabitqlib/index/hnsw_test.cpp
+++ b/tests/unit/rabitqlib/index/hnsw_test.cpp
@@ -987,6 +987,39 @@ std::vector<char> saved_bytes(const HierarchicalNSW& index, const std::string& p
     return {std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>()};
 }
 
+TEST(HnswConstructionTest, SeedReproducesRotationCodesAndSerialSearch) {
+    constexpr size_t kCount = 65;
+    AddFixture fixture(kCount, 47);
+    const std::string path = ::testing::TempDir() + "rabitq_hnsw_seed.index";
+    for (auto metric : {METRIC_L2, METRIC_IP}) {
+        std::vector<char> expected_bytes;
+        Results expected_results;
+        for (size_t seed : {42U, 42U, 43U}) {
+            HierarchicalNSW index(kCount, AddFixture::kDim, 4, 8, 32, seed, metric);
+            index.construct(
+                1,
+                fixture.centroid.data(),
+                kCount,
+                fixture.data.data(),
+                fixture.cluster_ids.data(),
+                1
+            );
+            const auto bytes = saved_bytes(index, path);
+            const auto results = index.search(fixture.data.data(), 5, 10, kCount, 1);
+            if (expected_bytes.empty()) {
+                expected_bytes = bytes;
+                expected_results = results;
+            } else if (seed == 42) {
+                EXPECT_EQ(bytes, expected_bytes);
+                EXPECT_EQ(results, expected_results);
+            } else {
+                EXPECT_NE(bytes, expected_bytes);
+            }
+        }
+    }
+    std::remove(path.c_str());
+}
+
 TEST(HnswRemoveTest, ExcludesRemovedPointsAndKeepsSurvivors) {
     constexpr size_t kCount = 120;
     constexpr size_t kQueries = 8;