I've decided to leave my current job and I've made my intentions clear. My boss tells me leaving now will make me unhirable because I will appear to be a job hopper on my CV.
Is this job hopping and will it hurt my career?
Context is king here.
Here's a bunch of examples to illustrate the point:
Not a big deal - career contractors often take gigs of 6 months, 12 months, 2 years and rarely more than 3. They don't plan to stay, and the pattern to notice is that the person tends to go straight from gig to gig, without gaps of any size (or with the gaps showing a planned reason)
Not a big deal - a career of long term commitments with a single short term case because of an untenable situation.
Big deal - big gaps in the resume with a pattern of leaving because of frustration that could have been resolved with perserverance or better negotiation skills
Big deal - abrupt gaps due to being fired/laid off. Repeatedly.
Will raise questions - first job in the industry a very short term - when the first job is the only career history available, and it's very short, one wonders why. College grads are usually very cost effective, and eager to learn... so what failed?
Single cases - see the boss's perspective.
The big question is "will he do the same to me?" - the clearer the case for the answer to be "no" the better.
So - "I left the company because we'd had 2 layoffs, no customers, and I was asked to work on a project that didn't let me grow any in-demand skills" - is a pretty clear "no" - unless my company is similarly messed up, the risk of someone leaving is low. By comparison - "The work was fine, but management didn't support my near term hopes and dreams" - makes any outside observer wonder what happened.
You've been with the company 6 months. You committed (sight unseen) to a contract of 1 year. At the time of this arrangement, you didn't assert the requirement that you be a manager, and I'm going to bet you didn't come to the table with a background in management. I can see why they aren't eager to put you in a management role when you've got 6 months of experience. Management positions are tricky and rare, because the manager is an organizational representative for the company - the work of hiring personnel, and choosing which projects get resources defines how the organization invests in growth, and while technology skills are important, so is being aware of organizational requirements and culture. It sounds like the organization opted for someone who they trust as an organizational expert, vs. someone with the technology skills.
If you explained this to me in a job interview as your reason for leaving, I'd have a hard time buying it. 6 months ago, working as an individual contributor was a fine idea. My take is that your current frustration right now is in two parts - (1) you were denied an opportunity that opened up (management), (2) you will have to work a suboptimal manager who is much less of a mentor/teacher/help than your original boss. You still do the same work you were orginally hired to do, you get recognized for doing good work, so the fundamental basis of the negotiation you arranged 6 months ago has not changed. Given that in most organization, it's not unusual for the reporting structure to change over time, and I've seen plenty of cases where the bosses changed in the first year of someone's employement, I'd say it's a common enough case that most managers would say "I could see that happening here, too - so if this guy will heave if that happens, and he doesn't like the new boss, I'm at risk for loosing him here, too". Not a conclusion that will work in your favor.
Other things I'd try before up and quitting:
Talk to the new potential manager. Explain what you would like to see as a pattern for you and him working with the client - figure out some ideas for communication that alleviate the concern that he'll be a hindrance, not a help.
If you have serious concerns about whether you'll be trusted or listened to - bring them to your departing original boss, and/or upper management - if they've hired someone with expertise (you) and you're not trusted, they are getting less value for their dollar and should be made aware.
Stick it out and try to find a way for at least 3 months - they hypothetical and the actual of what it will be like to work with this person could be very different. Also, time to settle into a new role will improve just about any boss. They really can learn, over time, what works and what doesn't.
Keep in mind that the new boss has something to prove, too. You've been a great asset so far. If the productivity goes down because your expertise isn't considered valid, it'll show in the work, and your boss will have to explain why things aren't going well.
Is this job hopping and will it hurt my career?
Leaving after 6 months because your boss left and you aren't comfortable with the new hierarchy isn't job hopping if this is just a single instance. Doing so repeatedly is job hopping.
When I review a resume and see multiple, short-duration jobs, I do worry that there is a pattern here. Some folks get bored, or restless after a short period of time and want to move on. Others can't get along with people and feel the need to leave often. Some folks just quickly go where they can make a few dollars more. I usually want to hire people that I think will be around for a long time, so that my investment in their career growth will pay back to the company.
But if this is the only time when you left so soon, I wouldn't worry that you are exhibiting a pattern. You just want to be able to explain to an interviewer your good reasons for leaving so soon.
You indicate that you will be leaving 6 months into a 1 year contract. I might worry that you are abandoning a commitment, but that depends on the specifics of your contract.