system
Given the following post, respond as if you're commenting on Reddit:
human
[D&D] How much heat do items in a bag of holding lose?

Alright, so items in a bag of holding get shunted to a pocket dimension where they have no air and no contact with other objects.  That means no conduction or convection for heat exchange.  However, they will still radiate heat causing them to slowly cool.  The question is how quickly?

To do that we need to know the background temperature of the body they are radiating to.  So, are they radiating heat to an absolute zero cosmic background?  Or is it closer to background radiation of the universe (about 3 K)?  Is there one fixed temperature for the bag, or is it unique to each item?  Does that background source have enough thermal mass to treat it as static or does it come to steady state quickly?

I realize that I could conduct this research myself by procuring a bag, then precisely measuring the temperatures before and after a trip into the bag, but the first step in proper science is always to check existing literature.
gpt
No mention is ever made of the temperatures inside being any kind of extreme.  No checks are necessary against it, and no reports of items coming out being particularly hot or frozen, regardless of time spent in the bag.  
This leads me to believe that the boundaries of the pocket dimension are, in fact, the cloth of the bag itself.  Therefor ambient temperature inside the bag is whatever the current environment outside the bag is.