diff --git a/tests/python/test_examples.py b/tests/python/test_examples.py
index 35c0c436..a66655ac 100644
--- a/tests/python/test_examples.py
+++ b/tests/python/test_examples.py
@@ -200,7 +200,15 @@ def check_saved_index_and_query(tmp_path, kind, metric, data):
     search_args = {"nprobe": 2} if kind == "ivf" else {"ef": len(data)}
     ids, distances = index.search(queries, k=5, num_threads=2, **search_args)
     assert index.metric == metric
-    np.testing.assert_array_equal(ids[:, 0], np.arange(len(queries)))
+    if kind == "ivf":
+        np.testing.assert_array_equal(ids[:, 0], np.arange(len(queries)))
+    else:
+        # The examples build graphs with two threads (SymphonyQG also without a fixed
+        # seed), and graph indexes do not guarantee that every point is reachable:
+        # check the self-recall of every point instead.
+        self_ids, _ = index.search(data, k=1, num_threads=2, **search_args)
+        found = np.count_nonzero(self_ids[:, 0] == np.arange(len(data)))
+        assert found >= 0.95 * len(data)
     assert np.isfinite(distances).all()
     restored_path = tmp_path / "restored.index"
     index.save(str(restored_path))
diff --git a/tests/python/test_hnsw.py b/tests/python/test_hnsw.py
index bf612c6d..c478fcb5 100644
--- a/tests/python/test_hnsw.py
+++ b/tests/python/test_hnsw.py
@@ -86,13 +86,14 @@ def test_seed_reproduces_serial_build_and_search(tmp_path):
 
 
 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.
+    # M equals the point count, so the base layer holds 2M >= N - 1 links and never
+    # prunes a reverse link: every point stays reachable under any thread
+    # interleaving. This checks parallel construction, not sparse-graph recall.
     idx = HnswIndex(DIM, N_VECTORS, M=N_VECTORS, ef_construction=N_VECTORS, nbits=4)
     centroids, cluster_ids = clusters
     idx.build(base_data, centroids, cluster_ids, num_threads=8)
-    ids, distances = idx.search(base_data[:10], k=1, ef=N_VECTORS)
-    np.testing.assert_array_equal(ids[:, 0], np.arange(10))
+    ids, distances = idx.search(base_data, k=1, ef=N_VECTORS)
+    np.testing.assert_array_equal(ids[:, 0], np.arange(N_VECTORS))
     assert np.isfinite(distances).all()
 
 
@@ -251,7 +252,10 @@ def test_add_returns_dense_labels_and_preserves_search(
     _, cluster_ids = clusters
     assert partial_hnsw.num_points == 400
     before, _ = partial_hnsw.search(base_data[:10], k=1, ef=N_VECTORS)
-    ids = partial_hnsw.add(base_data[400:], cluster_ids=cluster_ids[400:])
+    # One thread, as the build used, so the grown graph and `after` are reproducible.
+    ids = partial_hnsw.add(
+        base_data[400:], cluster_ids=cluster_ids[400:], num_threads=1
+    )
     np.testing.assert_array_equal(ids, np.arange(400, N_VECTORS))
     assert partial_hnsw.num_points == N_VECTORS
     # A sparse graph can isolate the odd point, as it can after build, so this
@@ -265,8 +269,8 @@ def test_add_returns_dense_labels_and_preserves_search(
 
 
 def test_add_into_a_dense_graph_retrieves_every_point(base_data, clusters):
-    # Keeping every neighbor removes the pruning that isolates points, so the
-    # added points must all be found.
+    # M equals the point count, so the base layer holds 2M >= N - 1 links and never
+    # prunes a reverse link: even a parallel add keeps every point reachable.
     centroids, cluster_ids = clusters
     idx = HnswIndex(DIM, N_VECTORS, M=N_VECTORS, ef_construction=N_VECTORS, nbits=4)
     idx.build(base_data[:400], centroids, cluster_ids[:400])
@@ -331,7 +335,7 @@ def test_resize_grows_capacity_and_allows_more(base_data, clusters):
     assert idx.max_elements == N_VECTORS
     assert idx.num_points == 400
 
-    idx.add(base_data[400:], cluster_ids=cluster_ids[400:])
+    idx.add(base_data[400:], cluster_ids=cluster_ids[400:], num_threads=1)
     assert idx.num_points == N_VECTORS
     ids, _ = idx.search(base_data[:10], k=1, ef=N_VECTORS)
     np.testing.assert_array_equal(ids[:, 0], np.arange(10))
@@ -457,7 +461,8 @@ def check(index):
     plain = HnswIndex.load(str(plain_path))
     for index in (idx, loaded, plain):
         index.resize(count + len(new))
-        index.add(new)
+        # One thread: graphs grown in parallel differ, and the searches compare them.
+        index.add(new, num_threads=1)
         assert index.num_stored_points == count + len(new)
         assert index.num_points == count + len(new) - (0 if index is plain else 4)
 
diff --git a/tests/python/test_ivf_initializers.py b/tests/python/test_ivf_initializers.py
index 57c08f9e..93393aba 100644
--- a/tests/python/test_ivf_initializers.py
+++ b/tests/python/test_ivf_initializers.py
@@ -17,10 +17,13 @@ def test_hnsw_parallel_build_routes_and_reloads(tmp_path, metric, threads):
     data /= np.linalg.norm(data, axis=1, keepdims=True)
     index = IvfIndex(dim, count, count, 4, metric, initializer="hnsw")
     index.build(data, data, np.arange(count, dtype=np.uint32), num_threads=threads)
+    # A parallel build may leave a centroid unreachable in the routing graph, so
+    # check the aggregate self-recall of every point (one point per cluster).
+    every, _ = index.search(data, 1, 8, num_threads=threads)
+    assert np.count_nonzero(every[:, 0] == np.arange(count)) >= 0.99 * count
     labels = np.array([0, 17, 511, 1023], dtype=np.uint32)
     queries = data[labels]
     expected = index.search(queries, 1, 8, num_threads=threads)
-    np.testing.assert_array_equal(expected[0][:, 0], labels)
     assert np.isfinite(expected[1]).all()
     path = tmp_path / "parallel-build.index"
     index.save(str(path))
diff --git a/tests/python/test_large_dimensions.py b/tests/python/test_large_dimensions.py
index 29765daf..708680b2 100644
--- a/tests/python/test_large_dimensions.py
+++ b/tests/python/test_large_dimensions.py
@@ -28,12 +28,13 @@ def test_large_dimension_round_trip(tmp_path, dim, metric, kind):
         search_args = {"ef": count}
     else:
         index = SymqgIndex(
-            dim, max_degree=32, ef_construction=count, nbits=4, metric=metric
+            dim, max_degree=32, ef_construction=count, nbits=4, metric=metric, seed=42
         )
         search_args = {"ef": count}
 
+    # A seeded single-thread build is reproducible, so every self-query must succeed.
     if kind == "qg":
-        index.add(data, num_threads=2)
+        index.add(data, num_threads=1)
     else:
         index.set_centroids(data.mean(axis=0, keepdims=True))
         index.add(data, cluster_ids=np.zeros(count, np.uint32), num_threads=1)
diff --git a/tests/python/test_portable_indexes.py b/tests/python/test_portable_indexes.py
index cf9e9db0..35476f1e 100644
--- a/tests/python/test_portable_indexes.py
+++ b/tests/python/test_portable_indexes.py
@@ -25,10 +25,18 @@ def check_index_round_trip(tmp_path, metric, dim, kind, bits):
         )
         search_args = {"ef": 96}
     else:
-        index = SymqgIndex(dim, max_degree=32, quantization_bits=bits, metric=metric)
+        # Seeded and single-threaded, so every self-query below must succeed (#216).
+        index = SymqgIndex(
+            dim,
+            max_degree=32,
+            nbits=bits or 32,
+            ef_construction=96,
+            metric=metric,
+            seed=42,
+        )
         search_args = {"ef": 96}
     if kind == "qg":
-        index.build(data, ef_construction=96)
+        index.add(data, num_threads=1)
     else:
         index.build(
             data, data.mean(axis=0, keepdims=True), np.zeros(len(data), np.uint32)
diff --git a/tests/python/test_symqg.py b/tests/python/test_symqg.py
index 220acd7b..4d2be889 100644
--- a/tests/python/test_symqg.py
+++ b/tests/python/test_symqg.py
@@ -21,8 +21,10 @@
 
 @pytest.fixture(scope="module")
 def built_symqg(base_data):
-    idx = SymqgIndex(DIM, max_degree=_MAX_DEGREE)
-    idx.build(base_data, ef_construction=_EF_BUILD)
+    # A seeded single-thread build is reproducible, so the self-retrieval test can
+    # require every probe (#216).
+    idx = SymqgIndex(DIM, max_degree=_MAX_DEGREE, ef_construction=_EF_BUILD, seed=42)
+    idx.add(base_data, num_threads=1)
     return idx
 
 
@@ -234,7 +236,8 @@ def test_failed_rebuild_preserves_index(
 
 @pytest.mark.parametrize("bits", [0, 4, 8])
 def test_successful_rebuild_replaces_index(base_data, tmp_path, bits):
-    index = SymqgIndex(DIM, max_degree=_MAX_DEGREE, quantization_bits=bits)
+    # Seeded, and build uses one thread, so every self-query below must succeed.
+    index = SymqgIndex(DIM, max_degree=_MAX_DEGREE, quantization_bits=bits, seed=42)
     index.build(base_data[:40], ef_construction=_EF_BUILD)
     replacement = base_data[40:88] + 10
     index.build(replacement, ef_construction=_EF_BUILD)
diff --git a/tests/unit/rabitqlib/index/hnsw_test.cpp b/tests/unit/rabitqlib/index/hnsw_test.cpp
index 8835a2c4..1d778ff1 100644
--- a/tests/unit/rabitqlib/index/hnsw_test.cpp
+++ b/tests/unit/rabitqlib/index/hnsw_test.cpp
@@ -716,8 +716,9 @@ TEST(HnswResizeTest, SurvivesSaveAndLoad) {
     }
 }
 
-// Labels must not depend on the order threads happen to finish in, and every
-// added point must still be reachable.
+// Labels must not depend on the order threads happen to finish in. M equals the point
+// count, so the base layer holds 2M >= N - 1 links and never prunes a reverse link:
+// every point stays reachable under any thread interleaving, and must be found.
 TEST(HnswAddTest, ParallelAddKeepsLabelsAndTheGraph) {
     constexpr size_t kBuilt = 256;
     constexpr size_t kAdded = 256;