I’m looking for an active voice word or phrase that means the reciprocal of “depends on”
For example if A depends on B, what does B do to A? I could say “B is depended on by A” but not only is that verbose, it’s also passive voice.
I thought of “B services A” but that’s not quite right, too specific.
Any ideas?
Precedes?
Enables.
What do you mean by “depends on”? I can think of at least two meanings:
(1) In statistics, that there is a positive correlation between A and B. So, you might say that for a typical population of humans, if A is height and B is weight, then A depends on B and B depends on A, because there is a correlation.
(2) You could mean that the value of A is fixed by the value of B. In mathematics, B would be an independent variable and A would be the dependent variable. So, in a transit system, if A is the fare and B is the distance travelled, then A might depend on B. Here you might say that B fixes or sets the value of A.
“…is a prerequisite for…?”
Can you narrow the context down a little bit? There are so many different types of dependencies.
I think what you’re describing determines the right word. For instance, for people, the reciprocal is probably ‘supports’. In other contexts, I could see ‘influences’, ‘controls’, or, oh I don’t know, maybe ‘determines’. 
I am documenting build dependencies. Some of the components of the system must be built before others can.
If I try to build component A, it will fail unless I first build component B. In that context A depends on B.
I have an elegant diagram which describes the dependencies, but there are so many arrows that it’s easy to miss a relationship when doing an impact analsys. So I’m distilling the diagram into two lists.
The first list was easy: A depends on B, C, Q; B depends on C, G etc…
In listing the reciprocals C(does something) to A, B. This is a useful list to have because if I change C I can see the scope of the impact: A, B.
To say, though, that C impacts A, B is a bit misleading. C is used by A and B, is depended on by A and B, is there for A and B when needed… will change A and B’s behaviour indirectly if modified… Arrrghh…
So far I’m leaning to either “enables” (thanks Gary T) or just “is depended on by”
So you’re saying the reciprocal of “depends on” depends on the dependency? 
In your context I guess I’d use the word “supports” rather than “enables”.
In abstract terms, the proper word is “governs.” If A depends on B, B governs A. You would avoid this in real-world situations where the other meanings of “govern” might be misundersood as what is intended, though.
Not a single word, but “is necessary for” may be the most accurate.
A depends on B, B is depended on by A
Trying to reduce a complex relationship to a simple term adds confusion, as noted in previous threads. Use prepositions for clarity in the direction of the relationship, and the same term for both sides of the relationship.
A depends on B, B is dependent on A.
To do any better, you’d have to be more specific about what kind of “depending” you’re talking about.
“is dependant on” it is. Thanks for the sounding board all!
What? No. Those mean the same thing. You’re saying that each depends on the other. “Enables,” “governs,” “controls” and “determines” are conceivable suggestions thus far that work, depending on context. “Supports” is similar, but doesn’t connote a need, which is what dependency is.
Use prepositions for clarity in the direction of the relationship
You asked for an active verb. “is” is not an active verb. You want “determines”. The hours worked determines the pay check. The atmospheric conditions determine the weather. The basket of goods determines the price.
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On unix systems using apt, isn’t it called a reverse-dependency? Not sure thats what you want but since this is a software project, that’s a known software term.
I was hoping for active voice, but I’m convinced now that in this case the passive “is dependent on” is the only way to minimize misunderstanding.
If A and B were “paycheque” and “hours worked”, A depends on B and B determines A is accurate. But what if A and B were “camper” and “weather report”? The camper depends on the weather report- that’s true. But “the weather report determines the camper” doesn’t scan. It could be the weather report determines what the camper will bring, but it could also be it determines whether the camper will postpone. Without context it is uncommunicative to simply use “determines”, or any other word.
English seems to be missing a word that simply means the reciprocal of depends. In fact, I’m struggling to think of any unidirectional-relationship-verb that has a pure reciprocal. Maybe TriPolar is right about using the same word with prepositions: if A binkles B, then the only thing you can say of the reverse is “B is binkled by A.”