system
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human
[Halo] How did the Forerunners test the Halo array.

I haven't read the forerunner trilogy so it may have been explained in there, but how did the forerunner know that the halos would kill all sentient life? a WMD on that scale would have to be tested on some scale to guarantee its success, which seems backward since the forerunner built it to save biodiversity in the galaxy. Any ideas?
gpt
It's explained in the Forerunner Trilogy. The original Halo's were bigger (considerably, like 30,000km in diameter instead of 10,000) and were more... Directed. You could specificity target things with them instead of just generating a spherical pulse. 

The first known test of a ring was at Charrum Hakkor. It was used again on the San'Shyum (Prophet) homeworld by the Master Builder, then again in the destruction of the Forerunner Capital World. The final confirmed use was during the battle of the Greater Ark, where in a show of defiance the Master Builder obliterated whatever life may remain in the large Magellanic Cloud. 

The rings which were later used to defeat the flood were designed with a limited, omnidirectional 'blast radius' before being deployed with overlapping fields of effect. They actual technology had already been tested, it was just being used in a less targeted manner.