diff --git a/barcode/codex.py b/barcode/codex.py
index 9a60c31..987e065 100755
--- a/barcode/codex.py
+++ b/barcode/codex.py
@@ -278,6 +278,10 @@ def _calculate_checksum(self, encoded: list[int]) -> int:
         return sum(cs) % 103
 
     def _build(self) -> list[int]:
+        # Reset the state that gets mutated while building so that repeated
+        # calls on the same instance are deterministic (see #143).
+        self._charset = "C"
+        self._digit_buffer = ""
         encoded: list[int] = [code128.START_CODES[self._charset]]
         for i, char in enumerate(self.code):
             encoded.extend(self._maybe_switch_charset(i))
diff --git a/tests/test_code128.py b/tests/test_code128.py
new file mode 100644
index 0000000..7f4765c
--- /dev/null
+++ b/tests/test_code128.py
@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+from barcode.codex import Code128
+
+
+def test_build_is_stable_across_repeated_calls() -> None:
+    """Calling build() repeatedly on the same instance must be deterministic.
+
+    Regression test for #143: ``_build`` mutated ``_charset`` and
+    ``_digit_buffer`` without resetting them, so a second call started from
+    stale state and produced a different bit string.
+    """
+    code = Code128("12A")
+    first = code.build()
+    assert code.build() == first
+    assert code.build() == first
+
+
+def test_encoded_is_stable_across_repeated_calls() -> None:
+    """The ``encoded`` property must also be deterministic across calls."""
+    code = Code128("12A")
+    first = code.encoded
+    assert code.encoded == first
+    assert code.encoded == first
+
+
+def test_reused_instance_matches_fresh_instance() -> None:
+    """A reused instance must encode identically to a freshly built one."""
+    reused = Code128("123ABC456")
+    reused.build()  # dirties the internal charset/buffer state
+    assert reused.build() == Code128("123ABC456").build()