Use 87W MacBook Pro adapter with 2026 M5 MacBook Air?

If the original Power Adapter presents a USB-C connection, then yes, it is perfectly suitable you to charge your new MacBook Air.

All recent models of Mac and iPad that have a USB-C power connection support USB PD (Power Delivery). All Apple USB-C Power Adapters also support USB PD. The Power Delivery protocol actively negotiates the charging voltage and current between the Power Adapter and connected device, taking into account the rating and capabilities of the associated charging cable.

The suggested Power Adapter for your MacBook Air M5 is 30-70W. As such, your existing USB-C Power Adapter is more than adequate to charge the MacBook at its maximum supported charging rate.

It will charge slightly faster then when using 70w, but much faster then 30w