While the usual HTCondor command line tools are often not thought of
as an API, they are frequently the best choice for a programmatic
interface to the system.
They are the most complete, tested and debugged way to work with the system.
The major down side to running the tools
is that spawning an executable may be relatively slow;
many applications do not need an extreme level of performance,
making use of the command line tools acceptable.
Even some of the HTCondor tools themselves work this way.
For example, when condor_dagman needs to submit a job,
it invokes the condor_submit program,
just as an interactive user would.