diff --git a/python/fasttext_module/fasttext/tests/test_vector_bounds.py b/python/fasttext_module/fasttext/tests/test_vector_bounds.py
new file mode 100644
index 0000000..7b085dd
--- /dev/null
+++ b/python/fasttext_module/fasttext/tests/test_vector_bounds.py
@@ -0,0 +1,54 @@
+# SPDX-FileContributor: Arthit Suriyawongkul
+# SPDX-FileCopyrightText: 2026-present, fasttext-community
+# SPDX-FileType: SOURCE
+# SPDX-License-Identifier: MIT
+
+"""Vector index and size errors must raise, not access out of bounds."""
+
+import pytest
+
+import fasttext_pybind
+
+from .helpers import build_supervised_model, get_random_data
+
+
+def _model(data=None):
+    # thread=12: thread <= 10 leaves the input matrix partly uninitialized.
+    data = data or get_random_data(300, max_vocab_size=100)
+    return build_supervised_model(data, {"thread": 12, "dim": 16, "verbose": 0})
+
+
+def _shrink_output_dim(model):
+    output = model.get_output_matrix()[:, :8].copy()  # input dim is 16
+    model.set_matrices(model.get_input_matrix(), output)
+
+
+@pytest.mark.parametrize("ind", [-1, 10**9])
+def test_get_input_vector_out_of_range_raises(ind):
+    with pytest.raises(ValueError, match="out of range"):
+        _model().get_input_vector(ind)
+
+
+def test_get_word_vector_size_mismatch_raises():
+    model = _model()
+    vec = fasttext_pybind.Vector(model.get_dimension() - 1)
+    with pytest.raises(ValueError, match="size mismatch"):
+        model.f.getWordVector(vec, model.words[1])
+
+
+def test_predict_dim_mismatch_raises():
+    model = _model()
+    _shrink_output_dim(model)
+    with pytest.raises(ValueError, match="size mismatch"):
+        model.predict(model.words[1])
+
+
+def test_training_thread_error_raises(tmp_path):
+    """Used to call std::terminate: exception escaped a training thread."""
+    data = get_random_data(3000, max_vocab_size=600)  # cutoff needs >= 256 rows
+    model = _model(data)
+    _shrink_output_dim(model)
+    train_txt = tmp_path / "train.txt"
+    train_txt.write_text("".join(f"__label__{line}\n" for line in data))
+    with pytest.raises(ValueError, match="size mismatch"):
+        model.quantize(input=str(train_txt), cutoff=300, retrain=True)
diff --git a/src/fasttext.cc b/src/fasttext.cc
index a659c55..66977d8 100644
--- a/src/fasttext.cc
+++ b/src/fasttext.cc
@@ -806,6 +806,12 @@ namespace fasttext
     {
       trainException_ = std::current_exception();
     }
+    catch (const std::exception &)
+    {
+      // E.g. a Vector size check. An exception escaping a std::thread
+      // calls std::terminate(); store it so startThreads() rethrows it.
+      trainException_ = std::current_exception();
+    }
     if (threadId == 0)
       loss_ = state.getLoss();
     ifs.close();
diff --git a/src/vector.cc b/src/vector.cc
index e5c288a..e877ebd 100644
--- a/src/vector.cc
+++ b/src/vector.cc
@@ -8,10 +8,9 @@
 
 #include "vector.h"
 
-#include <assert.h>
-
 #include <cmath>
 #include <iomanip>
+#include <stdexcept>
 
 #include "matrix.h"
 
@@ -38,36 +37,47 @@ void Vector::mul(real a) {
 }
 
 void Vector::addVector(const Vector& source) {
-  assert(size() == source.size());
+  if (size() != source.size()) {
+    throw std::invalid_argument("Vector::addVector: size mismatch");
+  }
   for (int64_t i = 0; i < size(); i++) {
     data_[i] += source.data_[i];
   }
 }
 
 void Vector::addVector(const Vector& source, real s) {
-  assert(size() == source.size());
+  if (size() != source.size()) {
+    throw std::invalid_argument("Vector::addVector: size mismatch");
+  }
   for (int64_t i = 0; i < size(); i++) {
     data_[i] += s * source.data_[i];
   }
 }
 
 void Vector::addRow(const Matrix& A, int64_t i, real a) {
-  assert(i >= 0);
-  assert(i < A.size(0));
-  assert(size() == A.size(1));
+  if (i < 0 || i >= A.size(0)) {
+    throw std::invalid_argument("Vector::addRow: row index out of range");
+  }
+  if (size() != A.size(1)) {
+    throw std::invalid_argument("Vector::addRow: size mismatch");
+  }
   A.addRowToVector(*this, i, a);
 }
 
 void Vector::addRow(const Matrix& A, int64_t i) {
-  assert(i >= 0);
-  assert(i < A.size(0));
-  assert(size() == A.size(1));
+  if (i < 0 || i >= A.size(0)) {
+    throw std::invalid_argument("Vector::addRow: row index out of range");
+  }
+  if (size() != A.size(1)) {
+    throw std::invalid_argument("Vector::addRow: size mismatch");
+  }
   A.addRowToVector(*this, i);
 }
 
 void Vector::mul(const Matrix& A, const Vector& vec) {
-  assert(A.size(0) == size());
-  assert(A.size(1) == vec.size());
+  if (A.size(0) != size() || A.size(1) != vec.size()) {
+    throw std::invalid_argument("Vector::mul: size mismatch");
+  }
   for (int64_t i = 0; i < size(); i++) {
     data_[i] = A.dotRow(vec, i);
   }