根因: _call_with_fallback() 回傳 5-tuple,但 call() 直接 return 導致呼叫端 (diagnostician/solver/critic agents) 3-var unpack → ValueError: too many values to unpack → 全部 降級 20% 修復: call() 明確解包再回傳 (response, provider, success) Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>