Flame Javelin can't be cast for RRRR

[strange obsidian]

Today I learned that I can't cast Flame Javelin for RRRR (or RRRRR, or RRRRRR). This isn't relevant in many scenario's, but it could allow you to dodge life loss from an Lightning Greaves-ed Yurlok of Scorch Thrash.

When casting Flame Javelin, you should be able to pick whether you are going to cast it for RRR, 2RR, 4R, or 6, and then pay that cost. You can pay for colorless mana with red mana, so you could, for example, choose the 4R mode, and then pay for the cost with five red mana.

Full disclosure: I actually discovered this when testing a custom {2/G}{2/U}{2/R} legendary with Marath, Will of the Wild's +1/+1 counters equal to the mana spent ability. Forge actually won't let me play my custom as a 6/6 if I'm spending Temur mana!

[strange obsidian]

No easy solution for this? You’ll have to do some dialog like with Spree cards, I assume, to first lock in the desired casting cost before you go to actually paying for the spell?

[obtuse orchid]

No there's no easy solution for this