diff --git a/src/docgen/ai_client.py b/src/docgen/ai_client.py
index b17b2ff..65a9f62 100644
--- a/src/docgen/ai_client.py
+++ b/src/docgen/ai_client.py
@@ -822,18 +822,32 @@ def _grok_stt_segment(s: Any, index: int) -> dict[str, Any]:
     return {"start": start, "end": end, "text": str(s.get("text") or "")}
 
 
+def _require_openai_word_token(word: Any, start: float, end: float) -> str:
+    """OpenAI words use the same non-empty, ordered-interval contract as Grok."""
+    token = str(word or "").strip()
+    if not token:
+        raise AIError("STT words[].word must be a non-empty token")
+    _stt_ordered_interval(start, end, label="words[]")
+    return token
+
+
 def _openai_stt_word(w: Any) -> dict[str, Any]:
+    start = _stt_json_number(_stt_field(w, "start"), label="words[].start")
+    end = _stt_json_number(_stt_field(w, "end"), label="words[].end")
     return {
-        "start": _stt_json_number(_stt_field(w, "start"), label="words[].start"),
-        "end": _stt_json_number(_stt_field(w, "end"), label="words[].end"),
-        "word": _stt_field(w, "word"),
+        "start": start,
+        "end": end,
+        "word": _require_openai_word_token(_stt_field(w, "word"), start, end),
     }
 
 
 def _openai_stt_segment(s: Any) -> dict[str, Any]:
+    start = _stt_json_number(_stt_field(s, "start"), label="segments[].start")
+    end = _stt_json_number(_stt_field(s, "end"), label="segments[].end")
+    _stt_ordered_interval(start, end, label="segments[]")
     return {
-        "start": _stt_json_number(_stt_field(s, "start"), label="segments[].start"),
-        "end": _stt_json_number(_stt_field(s, "end"), label="segments[].end"),
+        "start": start,
+        "end": end,
         "text": _stt_field(s, "text"),
     }
 
diff --git a/tests/test_ai_client.py b/tests/test_ai_client.py
index 1e8c930..6049ec2 100644
--- a/tests/test_ai_client.py
+++ b/tests/test_ai_client.py
@@ -479,6 +479,61 @@ def test_openai_whisper_bool_word_start_raises(
             transcribe_audio(mp3, cfg=cfg)
 
 
+@pytest.mark.usefixtures("clear_ai_env")
+def test_openai_whisper_inverted_word_is_refused(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setenv("OPENAI_API_KEY", "sk-openai")
+    cfg = _cfg(tmp_path, {})
+    mp3 = tmp_path / "n.mp3"
+    mp3.write_bytes(b"fake-mp3")
+    fake = MagicMock()
+    fake.audio.transcriptions.create.return_value = _openai_whisper_result(
+        text="Hi",
+        words=[{"start": 0.4, "end": 0.1, "word": "Hi"}],
+    )
+    with patch("docgen.ai_client.openai_client", return_value=fake):
+        with pytest.raises(AIError, match=r"words\[\] end must be >= start"):
+            transcribe_audio(mp3, cfg=cfg)
+
+
+@pytest.mark.usefixtures("clear_ai_env")
+def test_openai_whisper_empty_word_is_refused(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setenv("OPENAI_API_KEY", "sk-openai")
+    cfg = _cfg(tmp_path, {})
+    mp3 = tmp_path / "n.mp3"
+    mp3.write_bytes(b"fake-mp3")
+    fake = MagicMock()
+    fake.audio.transcriptions.create.return_value = _openai_whisper_result(
+        text="Hi",
+        words=[{"start": 0.0, "end": 0.2, "word": "  "}],
+    )
+    with patch("docgen.ai_client.openai_client", return_value=fake):
+        with pytest.raises(AIError, match="non-empty token"):
+            transcribe_audio(mp3, cfg=cfg)
+
+
+@pytest.mark.usefixtures("clear_ai_env")
+def test_openai_whisper_inverted_segment_is_refused(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setenv("OPENAI_API_KEY", "sk-openai")
+    cfg = _cfg(tmp_path, {})
+    mp3 = tmp_path / "n.mp3"
+    mp3.write_bytes(b"fake-mp3")
+    fake = MagicMock()
+    fake.audio.transcriptions.create.return_value = _openai_whisper_result(
+        text="Hi",
+        words=[{"start": 0.0, "end": 0.2, "word": "Hi"}],
+        segments=[{"start": 0.5, "end": 0.1, "text": "Hi"}],
+    )
+    with patch("docgen.ai_client.openai_client", return_value=fake):
+        with pytest.raises(AIError, match=r"segments\[\] end must be >= start"):
+            transcribe_audio(mp3, cfg=cfg)
+
+
 def test_openai_whisper_maps_typed_words(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch, clear_ai_env: None
 ) -> None: