Does the green-flame blade extra damage work on normal attacks?

Question

Can you do a green-flame blade cantrip (duration of 1 round), do an Action Surge and then a normal attack action with Extra Attack and have all attacks have the green-flame blade bonus damage since it have a duration of 1 round and should also work with opportunity attack in the same round?

Answer 1

You don't get extra damage on the later attacks

Green flame blade doesn't have a duration of 1 round. It is Instantaneous. The spell has you make one attack, which gets extra damage. No other attacks do so.

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails. On a hit, the target suffers the attack’s normal effects, and [extra damage is applied]

If other attacks would get damage from green flame blade it would have a longer duration, as would say something along the lines of

For the duration of this spell, when you hit with a melee weapon attack a creature within 5 feet of the target takes [...]

You can use Action Surge to get an another attack with extra damage by casting green flame blade using the extra action, but you would only get the attack described by the spell.