fix(avatar): follow language changes each turn

digital-avatar-app/backend/routers/chat.py

@@ -478,6 +478,24 @@ def _qa_requires_language_adaptation(question: str, answer: str) -> bool:
     )
 
 
+def _qa_requires_per_turn_rendering(
+    question: str,
+    answer: str,
+    history: list[Any],
+) -> bool:
+    """Keep the direct QA fast path only when no conversation can bias language."""
+    return bool(history) or _qa_requires_language_adaptation(question, answer)
+
+
+def _per_turn_language_instruction() -> str:
+    return (
+        "本轮语言覆盖指令：只根据紧随其后的最新用户消息判断本轮回答语言。"
+        "即使此前整段对话一直使用另一种语言，只要最新消息切换了语言，本轮就必须立即切换到相同语言；"
+        "不要沿用上一轮语言。若最新消息明确指定回答语言，以该指定为准；若混用多种语言，使用其中占主导的"
+        "自然语言。不要说明你检测、切换或翻译了语言。"
+    )
+
+
 def _canonicalize_question(value: str) -> str:
     value = _normalize_question(value)
     replacements = (
@@ -709,6 +727,8 @@ def _build_prompt(
     messages = [{"role": "system", "content": system}]
     for item in history[-MAX_HISTORY_MESSAGES:]:
         messages.append({"role": item.role, "content": item.content} if hasattr(item, "role") else item)
+    # Keep the language instruction adjacent to the current turn so long histories cannot override it.
+    messages.append({"role": "system", "content": _per_turn_language_instruction()})
     messages.append({"role": "user", "content": question.strip()})
     return messages
 
@@ -845,7 +865,7 @@ def _resolve_reply(
         qa_pairs = db.query(QAPair).filter(QAPair.avatar_id == avatar.id).all()
     matched = _match_standard_qa(question, qa_pairs)
     adapt_qa_language = bool(
-        matched and _qa_requires_language_adaptation(question, matched.answer)
+        matched and _qa_requires_per_turn_rendering(question, matched.answer, history)
     )
     if matched and not adapt_qa_language and not image_contexts:
         return {"answer": matched.answer, "source": "qa", "references": []}
@@ -940,7 +960,7 @@ def _stream_reply(
     qa_pairs = db.query(QAPair).filter(QAPair.avatar_id == avatar.id).all()
     matched = _match_standard_qa(question, qa_pairs)
     adapt_qa_language = bool(
-        matched and _qa_requires_language_adaptation(question, matched.answer)
+        matched and _qa_requires_per_turn_rendering(question, matched.answer, history)
     )
     messages, reservation = [], None
     if matched and not adapt_qa_language and not image_contexts:

digital-avatar-app/backend/tests/test_chat_orchestration.py

@@ -11,6 +11,7 @@ from routers.chat import (
     _match_standard_qa,
     _public_avatar_payload,
     _qa_requires_language_adaptation,
+    _qa_requires_per_turn_rendering,
     _require_owned_avatar,
     _resolve_reply,
 )
@@ -86,6 +87,41 @@ class ChatOrchestrationTests(unittest.TestCase):
         self.assertTrue(_qa_requires_language_adaptation("안녕하세요", "你好"))
         self.assertFalse(_qa_requires_language_adaptation("你好", "您好"))
 
+    def test_conversation_qa_is_rendered_for_the_current_turn_language(self):
+        history = [SimpleNamespace(role="user", content="Please answer in English.")]
+        self.assertTrue(_qa_requires_per_turn_rendering("Quelle est votre adresse ?", "Our address is Test Road 1.", history))
+
+        fake_model = Mock(return_value="Notre adresse est Test Road 1.")
+        result = _resolve_reply(
+            None,
+            self.avatar,
+            "Quelle est votre adresse ?",
+            history,
+            qa_pairs=[SimpleNamespace(question="Quelle est votre adresse ?", answer="Our address is Test Road 1.", enabled=True)],
+            search_fn=Mock(),
+            model_client=fake_model,
+        )
+
+        self.assertEqual(result["source"], "qa")
+        self.assertEqual(result["answer"], "Notre adresse est Test Road 1.")
+        messages = fake_model.call_args.kwargs["messages"]
+        self.assertEqual(messages[-1], {"role": "user", "content": "Quelle est votre adresse ?"})
+        self.assertEqual(messages[-2]["role"], "system")
+        self.assertIn("本轮语言覆盖指令", messages[-2]["content"])
+        self.assertIn("不要沿用上一轮语言", messages[-2]["content"])
+
+    def test_latest_user_message_has_an_adjacent_language_override(self):
+        history = [
+            SimpleNamespace(role="user", content="请用中文回答"),
+            SimpleNamespace(role="assistant", content="好的，请问有什么可以帮你？"),
+        ]
+        messages = _build_prompt(self.avatar, history, "What can you help me with?", [])
+
+        self.assertEqual(messages[-1], {"role": "user", "content": "What can you help me with?"})
+        self.assertEqual(messages[-2]["role"], "system")
+        self.assertIn("最新用户消息", messages[-2]["content"])
+        self.assertIn("立即切换到相同语言", messages[-2]["content"])
+
     def test_conversational_paraphrase_matches_standard_qa(self):
         for question in ("请问一下，你们公司在哪里呀？", "请问去你们那边怎么走"):
             with self.subTest(question=question):