If a wizard is under the invisibilty spell, does using his familiar to channel a spell break his invisibility?

Yes, because whether channeled through familiar or not it is still the wizard that must cast the spell.

… but he could do this from complete cover.

Quote from OldElf86 >>

… but he could do this from complete cover.

 
Depends on the spell. If the spell requires you to see the target then you cannot cast it at the target. While the familiar delivers the spell, you still must adhere to all the same rulings of that casting, including seeing the target. If you have total cover from the target it means you are entirely and completely blocked/obscured by some physical object from the target meaning you would not be able to see them. Thankfully most Touch spells (the only spells that can be delivered by a familiar) don't require sight, but the distinction is important.

TL;DR: having a familiar does not alter the spell casting or use in any way at all. It just means instead of you touching the target to deliver the spell then your familiar does it for you, allowing you to use it indirectly at range.

An invisible wizard could be behind total cover and use his action to perceive through his familiar who is out of sight. Then the wizard could use his bonus action to cast Misty Step to a location within range that his familiar can see even if he cannot. Since the wizard cast a spell, it would end his invisibility.

It's probably not what you were thinking of, but this is an example of a wizard using his familiar to channel a spell to a location out of his sight while he is behind total cover.

Quote from TexasDevin >>

An invisible wizard could be behind total cover and use his action to perceive through his familiar who is out of sight. Then the wizard could use his bonus action to cast Misty Step to a location within range that his familiar can see even if he cannot. Since the wizard cast a spell, it would end his invisibility.

It's probably not what you were thinking of, but this is an example of a wizard using his familiar to channel a spell to a location out of his sight while he is behind total cover.

 
Unless we want to get super-semantic because that is channelling only senses and is not channelling the spell (you can also do the above trick using items/invocations to see through objects or spells like Arcane Eye or Clairvoyance, etc). Really useful, but not spell channelling.

Regardless, in terms of answering the question posed in the original post, if the wizard was under the effect of the invisibility spell, then casting a spell would end the invisible condition whether a familiar was involved or not.

Quote from Cyb3rM1nd >>

Yes, because whether channeled through familiar or not it is still the wizard that must cast the spell.

 As you pointed out :)

I'm an old core player and we could see through the eyes of our familiars under the old rules.  Does that not hold true today?  Or have they fleshed out the rules more deeply to say that although you can see through the eyes of your familiar that ability does not extend to include 'for the use of casting spells?'

Quote from OldElf86 >>

I'm an old core player and we could see through the eyes of our familiars under the old rules.  Does that not hold true today?  Or have they fleshed out the rules more deeply to say that although you can see through the eyes of your familiar that ability does not extend to include 'for the use of casting spells?'

 
It takes an action and only lasts until end of current turn. So, you cannot use spells that cost an action to cast on the same turn you see through a familiar's eyes, hence the example of Misty Step which is a bonus action, thus letting you use it while using familiar's eyes, but the 30 ft range for the unoccupied space is from you not from the familiar.

Invisibility is a great exploration spell but it’s kind of weak for use in combat because most actions other than moving, dodging, and disengaging end the spell, even casting a healing spell. Greater Invisibility is a great spell for combat though because it overcomes that weakness, but it only lasts for 1 minute instead of 1 hour. 

Both spells require concentration though.

Creatures being affected by the invisibility spell can use the following actions without ending the spell.

Dash- great since you’re probably not provoking attacks of opportunity.

Disengage- if creatures can see you with special senses invisibility isn’t doing much for you anyways..

Dodge- mostly ineffective if creatures already have disadvantage from not being able to see you, but perhaps taking this action for advantage on dex saves you can see will help you mitigate an AOE if a caster tries to blow you up instead of find you.

Help- also really good. Giving an ally advantage on an attack and being able to move out of reach can be very effective if your low level of r out of high slots.

Hide- be able to make a successful get away or have a caster actually have to guess where to put that AOE if they’ve truly don’t know where you are.

Ready- can be quite interesting to immediately close a door again after a creature opened it during their turn with their one interaction. Potential cost them an action to try to open it again. Why initiate a strength contest when you can take someone’s action.

Search- while seldom used in games in my experience, being able to search for hidden creatures or objects in combat while creatures can’t actually see you may come in handy.

Use an object- interestingly enough, there are a couple of items like ball bearings and caltrops that can be used to have direct and lasting impact on combat without breaking invisibility. The portable ram can be used to break down doors too, Incase a creatures readied action may be “I attack the first creature I see through the doorway!”.

another interesting point about the invisibility spell is that casting a spell and making an attack end the spell, but not actually causing damage. Spells that give a creature an reusable action/bonus action to force saves don’t break invisibility. For example, if a cleric casts spirit guardians on itself and then another party member casts invisibility on that cleric, there spirit guardians still decimate foes while imposing disadvantage on their attacks against the cleric. A sorcerer or wizard casting the dragons breath spell on themselves while receiving the invisibility spell from another ally would work incredibly well too, as they would be better able to maintain concentration and position themselves appropriately to make better use of its AOE without provoking attacks of opportunity.

artificers spell storing item could be used to decent effect if the spell stored used a saving throw. Since you’re not actually casting and the spells effect is produced with the “use an object” action.

channel divinities and other class features that impose saves, but aren’t spells, also don’t break invisibility.

breath weapons, like those possessed by dragons and dragonborn, don’t break the invisibility spell either. 

It's not a spell and it's not an attack.

I don't care for it myself. I have always felt like dragon breath was a loophole. It feels to me like it violates the spirit of the restriction on invisibility the way it violates the spirit of the restriction of find familiar. But this is the rules forum and I digress...

Quote from TexasDevin >>

It's not a spell and it's not an attack.

I think you are being too literal. Invisibility breaks when you attack(meaning the general sense of the word, intending to inflict harm. It does not mean game mechanic, take the attack action) or cast a spell. This is MY opinion of RAI. In this case, I assume that "attack" means what I stated but, the rules do not specifically state that. If you want to say Breath Weapons aren't a kind of attack, I guess that would be why some DM have so many house rules.

I agree that attack means any form of attack, other it would be worded 'attack action'.

The invisibility spell states it is broken specifically by two things. Making an attack or casting a spell. It does not state that it is broke by “harmful effects” or “inflicting damage” which are what I believe you may be attempting to erroneously change the meaning of “attack” to. 

if you can find a way to inflict harm or damage without making an attack roll or casting a spell, then invisibility continues. This fits perfectly with the spells stated pentameters.

as far as what you believe to be rai or raw for what defines an attack, the definition is luckily spelled out in the rule book itself.

PHB pg 193. MAKING AN ATTACK
”Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

1 . Choose a target. Pick a target within your attack's range: a creature, an object, or a location.
2. Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.
3. Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.”

the last sentence is the part that seems to matter for this discussion. The very next paragraph on that page further defines attack identification. Seeing as an attack is one of the 3 major rolls (saving throws and ability checks being the other 2) that make up the entire game, I’d spend time researching their differences. Since they make up the foundation of almost every in game feature or interaction, a different implementation of their rules will have a huge impact on the majority of the rest of the game.

All that being said, everyone is free to play as they please. If for some reason you personally can’t accept clever play as defined by the rules, which is also probably being applied in a way to promote teamwork since the spell requires concentration, homebrew anything you want. The system is modular and can probably handle whatever you throw at it.

I don't personally feel that finding loopholes or exploiting poor wording for the sake of pointing to it as RAW constitutes clever play, apparently you do so, agree to disagree.

How is any of that a loophole? Everything I posted, with the exception of the dragons breath spell and artificer references, is within the PHB. It’s not “hidden” and from everyone by being in other source books. It wasn’t out of sight or mind during development of the game. The PHB has been errata’ed quite a few times. I’d say there’s a decent chance there would be changes to the spell description if “harmful effect” or “cause damage” were supposed to break the spell. Especially taking into account how popular the spell is.

I opened up the spell description and went down the list of general rules for “Actions in Combat” starting on PHB 192. There’s no stretching of wording or miscommunication about what the text says in any of those pages.

you guys even try it out? Or are you just taking the position of that’s how I first understood it so I’m not changing it?

It seems to me that it’s being called poor wording because the rules seem to contradict some individuals previous assumptions on these particular game feature interactions. It’s fine to alter the game in any way you want to play, especially if you and your table have set a standard of understanding various rules. The “juice” may not be worth the “squeeze” to try to change or alter game play based on different rules interpretations.

Just because the RAW doesn’t line up with how you’ve personally been playing doesn’t mean that the RAW is somehow negative or that it needs to be ignored by the rest of the community. 

I don’t  understand.

You want proof, rules and references supplied.

The rules as proof don’t  matter because the people who designed couldn’t have thought of everything? Well I suppose thinking of “everything” would be a tall order, but this isn’t thinking of everything.

its pointing out not only what was thought about, but what was theory crafted, play tested, printed, released for play, and made it though at least 10 erratas and updates over 6 years. Including an errata that was directly focused on PHB spell wording And feature interaction.

As for my arguement, there really isn’t one at this point. Just me explaining the features as both written and intended, and you stating you don’t play in that way with regard to one spell.

I think at this point I’m trying to understand what exactly the big deal is with playing the spell the way it’s written?

https://www.sageadvice.eu/2018/11/30/a-dragon-with-invisibility-may-use-its-breath-weapon-without-becoming-visible-is-it-correct/

Well all you really are doing is making a broad generalization about how you believe people would incorrectly interpret the rules the way you have. And that this is based on your having played prior editions and are carrying over old rules to a new edition.

there is no more or less general interpretation of attacks, saves, and checks. They are distinctly different and foundational parts of the game.

If a feature says attack, it doesn’t also mean all ability checks or saving throws that cause damage.

The spell gives more than the ability to stealth. It gives defensive buffs, and a niche ability to be offensive so long as the spell descriptions specifications are met. Also invisibility doesn’t make you undetectable.