feat(titles): support language-aware title generation

Make auxiliary title prompts match the user language by default, with an optional pinned `auxiliary.title_generation.language` config.

agent/title_generator.py

@@ -22,9 +22,31 @@ TitleCallback = Callable[[str], None]
 _TITLE_PROMPT = (
     "Generate a short, descriptive title (3-7 words) for a conversation that starts with the "
     "following exchange. The title should capture the main topic or intent. "
+    "Write the title in the same language the user is writing in. "
     "Return ONLY the title text, nothing else. No quotes, no punctuation at the end, no prefixes."
 )
 
+_TITLE_PROMPT_PINNED_LANGUAGE = (
+    "Generate a short, descriptive title (3-7 words) for a conversation that starts with the "
+    "following exchange. The title should capture the main topic or intent. "
+    "Write the title in {language}. "
+    "Return ONLY the title text, nothing else. No quotes, no punctuation at the end, no prefixes."
+)
+
+
+def _title_language() -> str:
+    """Return configured title language, or empty string to match the user."""
+    try:
+        from hermes_cli.config import load_config
+
+        return str(
+            ((load_config() or {}).get("auxiliary") or {})
+            .get("title_generation", {})
+            .get("language", "")
+        ).strip()
+    except Exception:
+        return ""
+
 
 def generate_title(
     user_message: str,
@@ -48,8 +70,11 @@ def generate_title(
     user_snippet = user_message[:500] if user_message else ""
     assistant_snippet = assistant_response[:500] if assistant_response else ""
 
+    language = _title_language()
+    prompt = _TITLE_PROMPT_PINNED_LANGUAGE.format(language=language) if language else _TITLE_PROMPT
+
     messages = [
-        {"role": "system", "content": _TITLE_PROMPT},
+        {"role": "system", "content": prompt},
         {"role": "user", "content": f"User: {user_snippet}\n\nAssistant: {assistant_snippet}"},
     ]
 

hermes_cli/config.py

@@ -1309,6 +1309,7 @@ DEFAULT_CONFIG = {
             "api_key": "",
             "timeout": 30,
             "extra_body": {},
+            "language": "",
         },
         "tts_audio_tags": {
             "provider": "auto",

tests/agent/test_title_generator.py

@@ -7,6 +7,7 @@ from agent.title_generator import (
     generate_title,
     auto_title_session,
     maybe_auto_title,
+    _title_language,
 )
 
 
@@ -22,6 +23,42 @@ class TestGenerateTitle:
             title = generate_title("help me fix this import", "Sure, let me check...")
             assert title == "Debugging Python Import Errors"
 
+    def test_default_prompt_matches_user_language(self):
+        mock_response = MagicMock()
+        mock_response.choices = [MagicMock()]
+        mock_response.choices[0].message.content = "Some Title"
+
+        with patch("agent.title_generator.call_llm", return_value=mock_response) as llm:
+            generate_title("質問です", "回答です")
+
+        system_prompt = llm.call_args.kwargs["messages"][0]["content"]
+        assert "same language the user is writing in" in system_prompt
+
+    def test_configured_language_pins_prompt(self):
+        mock_response = MagicMock()
+        mock_response.choices = [MagicMock()]
+        mock_response.choices[0].message.content = "Some Title"
+
+        with (
+            patch("agent.title_generator.call_llm", return_value=mock_response) as llm,
+            patch("agent.title_generator._title_language", return_value="Japanese"),
+        ):
+            generate_title("hello", "hi")
+
+        system_prompt = llm.call_args.kwargs["messages"][0]["content"]
+        assert "Write the title in Japanese" in system_prompt
+        assert "same language the user" not in system_prompt
+
+    def test_title_language_reads_config(self):
+        cfg = {"auxiliary": {"title_generation": {"language": "  French "}}}
+
+        with patch("hermes_cli.config.load_config", return_value=cfg):
+            assert _title_language() == "French"
+        with patch("hermes_cli.config.load_config", return_value={}):
+            assert _title_language() == ""
+        with patch("hermes_cli.config.load_config", side_effect=RuntimeError("bad config")):
+            assert _title_language() == ""
+
     def test_strips_quotes(self):
         mock_response = MagicMock()
         mock_response.choices = [MagicMock()]

website/docs/user-guide/configuration.md

@@ -938,6 +938,16 @@ auxiliary:
   compression:
     timeout: 120               # seconds — compression summarizes long conversations, needs more time
 
+  # Auto-generated session titles. Empty language follows the conversation;
+  # set e.g. "English" or "Japanese" to pin titles to one language.
+  title_generation:
+    provider: "auto"
+    model: ""
+    base_url: ""
+    api_key: ""
+    timeout: 30
+    language: ""
+
   # Skills hub — skill matching and search
   skills_hub:
     provider: "auto"