feat(titles): language-aware session titles with optional pinned language

Inspired by Claude Code v2.1.176 (June 2026): session titles are
generated in the conversation's language by default, and
auxiliary.title_generation.language pins a specific one.

- title_generator.py: default prompt now instructs the model to match
  the user's language (was unspecified, biasing titles to English);
  a pinned language from config swaps in an override prompt
- config.py: language key added to title_generation defaults
- docs: title_generation block added to the auxiliary config reference

agent/title_generator.py

@@ -22,9 +22,34 @@ 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."
 )
 
+# When the user pins a title language in config.yaml
+# (auxiliary.title_generation.language), this replaces the
+# match-the-conversation instruction. Inspired by Claude Code v2.1.176
+# (June 2026): "Session titles now generated in the conversation's
+# language (`language` setting pins a specific one)."
+_TITLE_PROMPT_PINNED = (
+    "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}, regardless of the conversation's language. "
+    "Return ONLY the title text, nothing else. No quotes, no punctuation at the end, no prefixes."
+)
+
+
+def _pinned_title_language() -> str:
+    """Return the configured title language, or '' to match the conversation."""
+    try:
+        from hermes_cli.config import load_config
+
+        aux = (load_config() or {}).get("auxiliary") or {}
+        task_cfg = aux.get("title_generation") or {}
+        return str(task_cfg.get("language") or "").strip()
+    except Exception:
+        return ""
+
 
 def generate_title(
     user_message: str,
@@ -48,8 +73,13 @@ def generate_title(
     user_snippet = user_message[:500] if user_message else ""
     assistant_snippet = assistant_response[:500] if assistant_response else ""
 
+    pinned = _pinned_title_language()
+    system_prompt = (
+        _TITLE_PROMPT_PINNED.format(language=pinned) if pinned else _TITLE_PROMPT
+    )
+
     messages = [
-        {"role": "system", "content": _TITLE_PROMPT},
+        {"role": "system", "content": system_prompt},
         {"role": "user", "content": f"User: {user_snippet}\n\nAssistant: {assistant_snippet}"},
     ]
 

hermes_cli/config.py

@@ -1309,6 +1309,10 @@ DEFAULT_CONFIG = {
             "api_key": "",
             "timeout": 30,
             "extra_body": {},
+            # Pin session titles to a specific language (e.g. "English",
+            # "Japanese"). Empty = match the conversation's language.
+            # Inspired by Claude Code v2.1.176's `language` setting.
+            "language": "",
         },
         "tts_audio_tags": {
             "provider": "auto",

tests/agent/test_title_generator.py

@@ -22,6 +22,43 @@ 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_conversation_language(self):
+        """Without a pinned language, the prompt asks to match the user's language."""
+        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._pinned_title_language", return_value=""):
+            generate_title("質問です", "回答です")
+            system_msg = llm.call_args.kwargs["messages"][0]["content"]
+            assert "same language the user is writing in" in system_msg
+
+    def test_pinned_language_overrides_prompt(self):
+        """auxiliary.title_generation.language pins the title language."""
+        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._pinned_title_language", return_value="English"):
+            generate_title("質問です", "回答です")
+            system_msg = llm.call_args.kwargs["messages"][0]["content"]
+            assert "Write the title in English" in system_msg
+            assert "regardless of the conversation's language" in system_msg
+
+    def test_pinned_language_config_read(self):
+        """_pinned_title_language reads auxiliary.title_generation.language."""
+        from agent.title_generator import _pinned_title_language
+
+        cfg = {"auxiliary": {"title_generation": {"language": "  French "}}}
+        with patch("hermes_cli.config.load_config", return_value=cfg):
+            assert _pinned_title_language() == "French"
+        with patch("hermes_cli.config.load_config", return_value={}):
+            assert _pinned_title_language() == ""
+        with patch("hermes_cli.config.load_config", side_effect=RuntimeError):
+            assert _pinned_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 (first exchange of each session)
+  title_generation:
+    provider: "auto"
+    model: ""                  # cheap/fast model recommended (e.g. gemini-flash, haiku)
+    base_url: ""
+    api_key: ""
+    timeout: 30
+    language: ""               # pin titles to a language, e.g. "English" or "Japanese";
+                               # empty = title follows the conversation's language
+
   # Skills hub — skill matching and search
   skills_hub:
     provider: "auto"