How does the Celestial Warlock's Searing Radiance feature interact with the Sun Soul Monk's Radiant Sun Bolt?

Question

I am curious about how a Celestial warlock/Sun Soul Monk Multiclass would work.

From Xanathar's Guide to Everything's Celestial Warlock's feature Searing Radiance:

when you cast a spell that deals radiant or fire damage, you can add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets

And from Xanathar's Guide to Everything's Sun Soul Monk's feature Radiant Sun Bolt:

You gain a new attack option that you can use with the Attack action. This special attack is a ranged spell attack [...] Its damage is radiant [...].

I am curious if RAW or RAI you would be able to add the charisma modifier to the Radiant Sun Bolt, since you are not technically casting a spell, just using an attack that is considered a spell attack.

Answer 1

Warlock's Searing Radiance requires you to take Cast a spell action, so Radiant Sun Bolt won't receive bonus damage.

Radiant Sun Bolt is a ranged spell attack, but it is not a spell and it requires you to take Attack action, not Cast a spell.

Answer 2

First, the Warlock ability:

when you cast a spell

read: use the 'cast a spell' action

that deals radiant or fire damage, you can add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets

Now, the Monk ability:

You gain a new attack option that you can use with the Attack action.

read: use the 'attack' action

This special attack is a ranged spell attack [...] Its damage is radiant [...].

By this reading, the two don't interact with each other at all;

However, I wouldn't question a GM who allows these two to work together as how thematically similar the two are, and how Monk some abilities are indeed very magic-like (yet still not technically 'magic').