Being asked to "leave in 3-6 months" is the new CTO's opening gambit in the negotiation you're about to start.
Your _worst_ negotiating option will be to say "OK, bye".
Work out what you want.
Work out your best idea of what the CTO/company wants (depending on what you want, the first step here is most likely to be asking them outright - if what you want most is to get rid of this CTO, this step will need to be approached much more subtly and delicately, and probably with someone much more experience helping you out).
Work out some "next best alternatives" to your optimal outcome, and what you'd accept as compensation for agreeing to those non-optimal alternatives.
Mostly though - work out what _you_ want - and why, and once you've done that, look at it with the most objective view you can and determine if it's actually plausible and realistic. (To be honest, a guy who wasn't even writing tests a few months ago _isn't_ going to be the technical lead or chief architect of an enterprise software startup - at least not a startup with a high chance of succeeding or getting serious funding. Be realistic here.)
If it's just the vesting(/money) - you'll negotiate one way. It's it's actually an ego/ownership/founderdhip issue for you, then acknowledge that (at least to yourself) and work out how to negotiate your desired outcome while keeping that perspective firmly in mind (and seriously, think about whether you need to rethink that - if it's mostly an ego thing).
Being asked to "leave in 3-6 months" is the new CTO's opening gambit in the negotiation you're about to start.
Your _worst_ negotiating option will be to say "OK, bye".
Work out what you want.
Work out your best idea of what the CTO/company wants (depending on what you want, the first step here is most likely to be asking them outright - if what you want most is to get rid of this CTO, this step will need to be approached much more subtly and delicately, and probably with someone much more experience helping you out).
Work out some "next best alternatives" to your optimal outcome, and what you'd accept as compensation for agreeing to those non-optimal alternatives.
Mostly though - work out what _you_ want - and why, and once you've done that, look at it with the most objective view you can and determine if it's actually plausible and realistic. (To be honest, a guy who wasn't even writing tests a few months ago _isn't_ going to be the technical lead or chief architect of an enterprise software startup - at least not a startup with a high chance of succeeding or getting serious funding. Be realistic here.)
If it's just the vesting(/money) - you'll negotiate one way. It's it's actually an ego/ownership/founderdhip issue for you, then acknowledge that (at least to yourself) and work out how to negotiate your desired outcome while keeping that perspective firmly in mind (and seriously, think about whether you need to rethink that - if it's mostly an ego thing).