How these two meet
What Capricorn suspects is that this will turn out to be talk. What Sagittarius suspects is that nothing here will ever be allowed to start.
Jupiter says yes before it has checked. Saturn says not yet before it has finished listening. You are the oldest argument in the sky.
The word you use differently is promise. Jupiter commits to the direction and expects the details to arrange themselves, which they often do. Saturn commits to the structure, and a direction with no structure is an intention, not a promise.
So Sagittarius means it entirely and Capricorn hears something that has not been paid for yet. And Sagittarius cannot understand why sincerity is not accepted as sufficient, because for fire it always has been.
Which makes what you actually do together easy to miss. Sagittarius keeps Capricorn from building something joyless, and Capricorn turns one of a hundred possibilities into a thing that exists in the world. Almost nothing else does that.
For Sagittarius, the work is turning one intention into a structure: a date, a payment, something that costs you if you do not do it. That is what a promise sounds like to the other one.
For Capricorn, the work is believing the intention while the structure is still missing. Once. Out loud. So that sincerity is not treated as worthless.
The question
When one of you says yes, what has been promised?
One conscious action
one intention given a structure
Sagittarius takes one thing they have meant for months and gives it a date and a first step. Capricorn says out loud that they believed it before the date existed.
It is small on purpose. It is also the exact place these two stop misunderstanding each other.
Jupiter sees where it should go and Saturn makes the road. Neither of you has ever been able to do it alone.
That is not a compromise between you. It is the only way either of you gets there.