How does diehard work for summons?

Question

Im building a good cleric who will be taking the Summon good monster feat. I get that it gives me an expanded summoning list in which to choose from. What I do not understand is how the diehard feat will work for the summons.

Diehard

Benefit: When your hit point total is below 0, but you are not dead, you automatically stabilize. You do not need to make a Constitution check each round to avoid losing additional hit points. You may choose to act as if you were disabled, rather than dying. You must make this decision as soon as you are reduced to negative hit points (even if it isn't your turn). If you do not choose to act as if you were disabled, you immediately fall unconscious. When using this feat, you are staggered. You can take a move action without further injuring yourself, but if you perform any standard action (or any other action deemed as strenuous, including some swift actions, such as casting a quickened spell) you take 1 point of damage after completing the act. If your negative hit points are equal to or greater than your Constitution score, you immediately die.

My understanding of how summons work is that they keep fighting until either the duration expires, they are banished/dismissed somehow, or they are killed. By killed I mean their HP is reduced to 0, since at that point they are returned to wherever they came from since you cant easily kill a summon.

So does the feat simply mean they are able to continue in the fight until they are reduced to their con value?

Answer 1

When a similar question's posed on the Paizo messageboards (like in 2012 sort of twice and in 2013 twice and in 2016), they often link to or mention this Advanced Race Guide FAQ exchange:

How does this ability [the feat Ferocious Summons] work if a summoned creature disappears when it reaches 0 hp?
The summoned creature does not disappear at 0 hp, instead it disappears when killed (when its current hp get to a negative amount equal to its Constitution score).

Since the special ability ferocity that's granted to summoned creature by the feat Ferocious Summons really is a lot like the feat Diehard that's granted to summoned creatures by the feat Summon Good Monster, the implication is that rules similar to Ferocious Summons should apply to the feat Summon Good Monster.

Answer 2

The creatures affected by the Diehard feat granted from Summon Good Monster would remain summoned and conscious until their HP is reduced to a negative value equal to their Constitution score. Using any standard actions beyond 0 hit points would result in losing 1 hp per round until the creature disappears.

A normal summon would disappear at less than 0 hit points as normal, as soon as it falls unconscious.