diff --git a/packages/services/voice-agent/src/plugins/speechmatics_stt.py b/packages/services/voice-agent/src/plugins/speechmatics_stt.py
index edcf9cb..6219073 100644
--- a/packages/services/voice-agent/src/plugins/speechmatics_stt.py
+++ b/packages/services/voice-agent/src/plugins/speechmatics_stt.py
@@ -40,5 +40,9 @@ def create_speechmatics_stt(language: str = "cmn") -> speechmatics.STT:
         language=sm_lang,
         include_partials=True,
     )
+    # Workaround: LiveKit's LanguageCode normalizes ISO 639-3 "cmn" back to
+    # ISO 639-1 "zh", but Speechmatics expects "cmn".  Override the internal
+    # option after construction so the raw Speechmatics code is sent.
+    stt._stt_options.language = sm_lang  # type: ignore[assignment]
     logger.info("Speechmatics STT created: language=%s (input=%s)", sm_lang, language)
     return stt