What's the rule when a portable hole is opened in a bag of holding?

You can't open a Portable Hole inside aBag of Holding (or similar) due to the act of placing the hole in the bag being catastrophic:

Placing a portable hole inside an extradimensional space created by a bag of holding, handy haversack, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it and deposited in a random location on the Astral Plane. The gate then closes. The gate is one-way only and can’t be reopened.

Quote from Davyd >>

You can't open a Portable Hole inside aBag of Holding (or similar) due to the act of placing the hole in the bag being catastrophic:

Placing a portable hole inside an extradimensional space created by a bag of holding, handy haversack, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it and deposited in a random location on the Astral Plane. The gate then closes. The gate is one-way only and can’t be reopened.

 

 Though the "catastrophic" results may be what is desired. Move to within 10ft of the BBEG, place a Portable hole into your bag of holding and Plane shift / Banish / word of reca/l  back to the material Plane leaving the bad guy stranded.

Its how you defeat the Tarasque as a Level 2 Artificer.

My character is a lv10 Artificer.

Going to use my construct to open it inside the bag of holding. Command,  toss, then evacuate.

Just need to know what happens officially.

The moment your construct puts the portable hole inside the bag of holding (not when the hole is opened...when the closed hole goes through the opening of the bag) a portal to the astral sea is opened and the hole, the bag, your construct, and anything else within 10' is immediately sucked through to the astral sea.  There is no save and there is no way back without a Plane Shift spell or something similar.

Also, please don't do this in your game...it is potentially game breaking, and it is one of those things that if you start doing it to bad guys then the bad guys will start doing it to you.  

Quote from pathofdaggers >>

My character is a lv10 Artificer.

Going to use my construct to open it inside the bag of holding. Command,  toss, then evacuate.

Just need to know what happens officially.

Officially you don't get passed the 'toss' step. The moment the bag of holding crosses the boundary into the portable hole, the following rule kicks in:

Placing a portable hole inside an extradimensional space created by a bag of holding, handy haversack, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it and deposited in a random location on the Astral Plane. The gate then closes. The gate is one-way only and can’t be reopened.

You cannot open a bag of holding inside a portal hole because you cannot get a bag of holding into a portable hole.