fix(chat): harden session auto-title and keep a single no-tool LLM call

- Add session_auto_title prompts, finalize_auto_title (reject prose, truncate, fallback).
- Third-round naming: one model.chat(system, user) with empty tools; reuse executor.model.
- Extend worker title tests for garbage model output.

Co-authored-by: Cursor <cursoragent@cursor.com>

runtime/session_auto_title.py

@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+AUTO_TITLE_CHAR_MAX = 18
+
+# Minimal system prompts: naming-only role (full rules are enforced in finalize_auto_title).
+AUTO_TITLE_SYSTEM_PROMPT_EN = (
+    "Dedicated session naming agent: output one short title only, "
+    f"≤{AUTO_TITLE_CHAR_MAX} characters, same language as the user lines."
+)
+AUTO_TITLE_SYSTEM_PROMPT_ZH = (
+    "你是专职会话命名助手：只输出一条短标题，"
+    f"至多{AUTO_TITLE_CHAR_MAX}个字，语种与用户发言一致。"
+)
+# One-line titles longer than this are almost never valid (model wrote prose).
+_AUTO_TITLE_RAW_SOFT_MAX = 42
+
+
+def _collapse_ws(raw: str) -> str:
+    return " ".join(str(raw or "").strip().split())
+
+
+def should_reject_auto_title(raw: str) -> bool:
+    s = _collapse_ws(raw)
+    if not s:
+        return True
+    if len(s) > _AUTO_TITLE_RAW_SOFT_MAX:
+        return True
+    low = s.lower()
+    if "claude code" in low:
+        return True
+    if "软件工程助手" in s:
+        return True
+    if "您好" in s and "我是" in s and len(s) > 16:
+        return True
+    if low.startswith("hello! i am") or low.startswith("hi! i am"):
+        return True
+    return False
+
+
+def finalize_auto_title(*, raw: str, fallback: str, max_chars: int = AUTO_TITLE_CHAR_MAX) -> str:
+    """Turn model output into a short session title; reject prose and use fallback."""
+    fb = _collapse_ws(fallback)
+    pick = _collapse_ws(raw)
+    if not pick or should_reject_auto_title(raw):
+        base = fb
+    else:
+        base = pick
+    if not base:
+        return "会话"
+    if len(base) <= max_chars:
+        return base
+    return base[:max_chars]
+
+
+__all__ = [
+    "AUTO_TITLE_CHAR_MAX",
+    "AUTO_TITLE_SYSTEM_PROMPT_EN",
+    "AUTO_TITLE_SYSTEM_PROMPT_ZH",
+    "finalize_auto_title",
+    "should_reject_auto_title",
+]