Does Phase Bolt allow Spellstrike to reduce the target's circumstance bonus to AC?

Question

My Magus takes Phase Bolt as a cantrip. They use it with Spellstrike. My target has Raised a Shield for a +2 circumstance bonus to their AC. Do I Spellstrike against their AC with or without their circumstance bonus? Phase Bolt normally allows me to ignore it, but I'm not positive on how it interacts with Spellstrike. The key phrase in Phase Bolt:

Make a ranged spell attack roll against your target's AC; if the target has any circumstance bonuses to AC (such as from a shield or cover), reduce that bonus by 2 for this attack.

Answer 1

No, you don't gain that (or any other similar spells') benefit

You are not using the spell to attack. Spellstrike is pretty clear...

You channel a spell into a punch or sword thrust to deliver a combined attack. You Cast a Spell that takes 1 or 2 actions to cast and requires a spell attack roll. The effects of the spell don't occur immediately¹ but are imbued into your attack instead. Make a melee Strike with a weapon or unarmed attack. ² Your spell is coupled with your attack, using your attack roll result to determine the effects of both the Strike and the spell.³

1. The effects of the spell do not occur as they normally would.
2. You are making a melee Strike, which is a type of basic action. Nothing about "the effects" of the spell being "imbued into your attack" alters how you make the Strike.
3. Your (melee) attack roll is used to determine the effect of your spell.*

• Arguably, you could partially benefit from phase bolt and similar effect but not for the Strike; you could theoretically miss the target's shielded AC but have phase bolt function as it's success condition by hitting their reduced AC because you use the "roll", not the "result" of your Strike.

Answer 2

Strike and Spell as One Attack

You channel a spell into a punch or sword thrust to deliver a combined attack.

Spellstrike involves Casting the Spell and making single attack roll for the results of both the Strike and the spell. Normally this spell's effect is coupled with the Strike that is made, though there are some specific interactions with certain effects that work differently.

Splitting up the steps of making a Spellstrike for clarity, you get:

• You Cast a Spell that requires a spell attack roll. The effects of this spell don't occur immediately but are imbued into your attack instead.
• Make a melee Strike. Your spell is coupled with your attack, using your attack roll to determine the effects of both.

Casting a Spell normally requires immediate resolution by rolling a spell attack, but this ability delays the spells effects until you make an attack roll as a melee Strike. The spell is imbued into that Strike, and both are delivered as "a combined attack".

Phase Bolt

Make a ranged spell attack roll against your target's AC; if the target has any circumstance bonuses to AC (such as from a shield or cover), reduce that bonus by 2 for this attack.

With only one attack being made, Phase bolt's effect reducing AC would apply to determine the effects of both Strike and spell.

There are other spells with similar effects that would also apply for a Spellstrike with the same reasoning, such as shocking grasp that gives a +1 circumstance bonus on the attack roll if the target is made of metal or wearing metal armor.

Spellstrike Specifics

There are those specific interactions described for certain types of effects used as part of a Spellstrike that could change how this works, but none of these apply here.

The closest in Ancillary Effects only covers "non-targeted effects that might affect creatures other than the target, as well as any ongoing effects starting from the moment you hit with the Strike", but phase bolt's effect to reduce the bonus to a target's AC applies only for that single attack and only to the target.