ספורנו — עקב אשר שמע אברהם בקולי / וישמר משמרתי

Q — Why does Hashem bless Yitzchak בזכות אברהם — "עקב אשר שמע אברהם בקולי" — when Avraham was blessed for his own merit, and Yaakov is later blessed without invoking ancestral zechut?

A — For Sforno, the difference traces to a specific spiritual stage. At this point in his life Yitzchak had not yet "התעורר לקרוא בשם ה'" — to actively spread Hashem's name in the world. Avraham was paradigmatically איש חסד whose chesed included "להורות חטאים בדרך... כמופת לרבים," earning his own zechut. Yaakov, "מנעוריו יושב אהלים, ללמוד וללמד דעת את העם, בפרט באהלי שם ועבר," likewise. Yitzchak's bracha is tied to his father's only as long as he hasn't yet stepped into the teach-and-publicize role. The shift comes in 26:25 ("ויבן שם מזבח ויקרא בשם ה'"), after which "ולא מצאוהו עוד תלאות מקנאים ודברי ריבות כאשר בראשונה" — Avimelech sues for peace, the wells succeed. The avot are distinguished here not by their righteousness as such, but by whether they are מורי הוראה ברבים.