12" MacBook battery is dead after 386 cycles

Solution 1:

An inflated/bulging battery is a safety risk and you should get it repaired ASAP. The reason for this could be anything from manufacturing defects to improper care.

If you got AppleCare when you bought your Mac, you may still be able to replace the battery for free. Otherwise, Apple won't change the battery for free, as warranties only last one year.(see here)

If you schedule a repair without AppleCare or warranty, it can cost anywhere from $129-199, depending on your Mac's model. You can schedule a repair here.

Or, if you are willing to take the risk, you can replace the battery on your own using an iFixit guide.

Solution 2:

Bulging batteries mean only one thing- buildup of gas inside. The gases are produced due to electrochemical oxidation of the electrolyte.

There's also no single cause for a swollen battery. Some are due to manufacturing defects, others are caused by the age of the battery, and still other cases can be caused by misuse, such as not properly exercising the battery over time, or by using the wrong power charger.

If Apple will care, I do not know, they must not when it is outside the warranty but it depends, in your case it is clearly a manufacturing defect and a Safety risk that should not have happened.

Do not wait to have it fixed.