fix: resolve #7693 — Cross-script title guard rejects valid CJK+English titles

When the user writes in CJK, the title language mismatch guard now allows
Latin characters in the title as long as the title also contains CJK
characters (cjk >= 2). This permits CJK titles with borrowed English
product/technical terms (e.g. 'WeChat Pay 回调失败排查', 'Python 代码修复')
while still rejecting pure-Latin titles for CJK conversations and
unrelated scripts (Cyrillic, Arabic, etc.).

Added regression tests: test_cjk_start_mixed_cjk_latin_title_is_allowed
and test_cjk_start_pure_latin_title_still_rejected.

api/streaming.py

@@ -4040,13 +4040,26 @@ def _title_language_mismatch(user_text: str, title: str) -> bool:
         return False
 
     # (1) Cross-script mismatch — language-agnostic.
+    # CJK text routinely borrows Latin product/technical terms (e.g. "WeChat
+    # Pay", "Python", "ProRes RAW"), so when the user writes in CJK, Latin
+    # characters in the title are acceptable as long as the title also
+    # contains CJK — i.e. the title is genuinely mixed, not pure drift.
+    # A pure-Latin title for a CJK conversation is still rejected.
+    # Unrelated scripts (Cyrillic, Arabic, Greek …) are always flagged.
     user_script = _dominant_script(user_text)
     if user_script:
         title_counts = _script_counts(candidate)
         title_total = sum(title_counts.values())
         if title_total >= 2:
             for script, n in title_counts.items():
-                if script != user_script and n >= 2 and (n / title_total) >= 0.35:
+                if script == user_script:
+                    continue
+                # When user writes in CJK, Latin in the title is a borrowed
+                # term as long as the title also contains CJK characters.
+                if user_script == 'cjk' and script == 'latin':
+                    if title_counts.get('cjk', 0) >= 2:
+                        continue
+                if n >= 2 and (n / title_total) >= 0.35:
                     return True
 
     # (2) Legacy same-script German→English heuristic.

tests/test_issue3293_title_language_drift.py

@@ -71,6 +71,35 @@ def test_cjk_start_english_title_is_rejected():
     assert _title_language_mismatch("如何修复这个错误问题", "Fixing the Bug") is True
 
 
+def test_cjk_start_mixed_cjk_latin_title_is_allowed():
+    """CJK conversations frequently embed English product/technical terms in
+    titles (e.g. 'WeChat Pay 回调失败排查').  As long as the title also
+    contains CJK characters, Latin borrowed terms should not trigger rejection.
+    Regression test for #7693."""
+    from api.streaming import _title_language_mismatch
+
+    # Pure CJK user, CJK title with English product name
+    assert _title_language_mismatch(
+        "微信支付回调一直失败怎么办", "WeChat Pay 回调失败排查"
+    ) is False
+    # CJK user, CJK title with English tech term
+    assert _title_language_mismatch(
+        "如何修复这个错误问题", "Python 代码修复"
+    ) is False
+    # Mixed CJK+Latin user, mixed title
+    assert _title_language_mismatch(
+        "prores raw是否能选择压缩？", "ProRes RAW 压缩选项与 BRAW 对比"
+    ) is False
+
+
+def test_cjk_start_pure_latin_title_still_rejected():
+    """A purely Latin title for a CJK conversation is still a genuine drift."""
+    from api.streaming import _title_language_mismatch
+
+    assert _title_language_mismatch("如何修复这个错误问题", "Fixing the Bug") is True
+    assert _title_language_mismatch("如何修复这个错误问题", "Python Error Guide") is True
+
+
 # ── regression guards: legitimate same-script titles must NOT be rejected ───
 
 def test_english_start_english_title_allowed():