Can the Ultegra Di2 Climbers switch be used without the STI Levers? I know that with DA it was reliant on the STI Levers.

I was hoping to build up a super light flat bar road bike with Ultegra di2.

If there is simple tuning that needs to be done, I will do it. However, I am not very experienced with wiring and I wont be able to do anything too advanced. Also definitely not gonna get DA, even if that is easier to set up in this fashion. \

I have read on a few sites, and many have said that the ultegra climbers switch cannot be used on its own, these sites also listed the switch as not compatible with Ultegra. I have also read that the brain of the ultegra di2 is in the front derailleur, which makes me think that it should be able to work.

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each part of the ultegra di2 system has some part of a "brain". shifters send coded signals, not just a "click" like 7970 di2. in this respect, it's less hackable than dura ace di2.
some places probably list the climbers switch as not compatible with ultegra di2 because there is a specific ultegra version. i have not investigated it to see if the climber switch has its own brain. i know that the upcoming sprinter switch does NOT have its own brain and is totally reliant on the sti shifter, and i think it has its own dedicated port.