What usually fails first on a MagSafe charger — the cable or the port?

The cable frays at the connector end, not the port. The rubber strain relief where it meets the MagSafe head cracks after a year or two. For MagSafe 2, dust in the port can cause issues, but you can clean it gently. MagSafe 3 is more durable, but third-party chargers may trigger a “not charging” warning. Check the cable first — replace it, not the laptop.