Can a genie warlock inside its vessel communicate with a familiar target (like another player character) by using message cantrip?

You can cast Message through solid objects if you are familiar with the target and know it is beyond the barrier.

But some DM might rule it out since when inside the vessel you're in an extradimensional space.

Heh. Message in a bottle.

But actually, I'd say no. I was on the fence until I read one line.

While inside, you can hear the area around your vessel as if you were in its space.

Saying "as if you were in its space" implies that you aren't in the vessel's space, which means you're fully extradimensional. The 120 foot range of message probably isn't going to get through dimensions.

Here's the official ruling in Sage Advice Compendium on the subject;

Are extradimensional spaces, such as a demiplane or the space created by rope trick, considered to be on a different plane of existence? An extradimensional space (aka an extraplanar space) is outside other planes. Thereore, if you’re on the Material Plane and your foe is in an extradimensional space, the two of you aren’t on the same plane of existence.

Quote from ArntItheBest >>

But message doesn't actually say you have to be on the same plane of existence. Your extradimensional space is contained within the bottle, so I would say anyone/thing within 120 ft. of the bottle is fair game (so long as you are familiar with it).

 To me it's the contrary spells don't affect creatures on different plane of existence unless noted otherwise.

Thanks, I think that settles it. 

With this ruling in mind, can anyone think about a cantrip or a 1st level spell that would work with bottled respite? And but "work" I mean any cool interaction, it doesn't have to be some kind of telepathy or anything.

A spell like Contact Other Plane comes to mind.

I don't think Contact Other Plane is a cantrip or a first level spell. But anyways...

• You could go into the vessel and have somebody else cast catapult on it. Since the vessel has hit points, it would most likely break, which sets you outside of its space (the target of the spell). Being able to send somebody 60 feet away (and even longer if they have Metamagic) is a solid rider to a solid spell.
• Although you can't communicate with it while in the vessel, find familiar would let you give your familiar a command to carry your vessel somewhere and then enter the vessel. Seems like a pretty effective method of travel.
• You might be able to do a similar carrying thing with unseen servant, but the wording on that spell could make it harder.

The spell Contact Other Plane is not a cantrip or first level spell, but most spell's effect don't cross planes so i gave one that would anyways.

Since concentrating doesn't require to remain within range of the spell, you could have cool interactions where you cast a spell and go in the vessel and keep concentrating on it, such as Tasha's Caustic Brew, Hideous Laughter for exemple, or Wtich Bolt if you're able to take multiple actions with Action Surge in exemple.  

Quote from ArntItheBest >>

Quote from Plaguescarred >>

The spell Contact Other Plane is not a cantrip or first level spell, but most spell's effect don't cross planes so i gave one that would anyways.

Since concentrating doesn't require to remain within range of the spell, you could have cool interactions where you cast a spell and go in the vessel and keep concentrating on it, such as Tasha's Caustic Brew, Hideous Laughter for exemple, or Wtich Bolt if you're able to take multiple actions with Action Surge in exemple.  

 If you think that Message won't work due to range, then Witch Bolt absolutely would break as soon as you entered the spell. I think that it would break even if you didn't rule against Message, as WB requires line of sight.

 I went by memory but after rereading Witch Bolt would not work

Thanks for all the ideas guys!