What would you do?

[Paralytic]

Stop thinking like an employee and/or taking it personally. You're there as a business delivering a service. If there are impediments on the client side that mean you cannot deliver that service, you have a right/responsibility to highlight those to your client and work with them to come up with a plan to make the work successful.

However, from what you're saying, I suspect the client sees you as an employee.

[GitMaster69]

... if you'd started a new contract and its become clear that the scope of work you've been asked to deliver is very much a poisoned chalice and you're on a hiding to nothing in terms of expectations with senior stakeholders, to the point where you're beginning to think you're going to be the scapegoat?

Very honest chat with your client to say its not what was represented in the interview and as such you think its best for both to look for someone else?

Or just tough it out because its all billable

Just milk the contract if you need dosh. Move on if you don’t

[jayn200]

If you're brought in mid contract then something went wrong, either they haven't resourced enough, they figured out they don't have the appropriate resource, you need to come in to fix someone's mess, or someone left (probably because it's a tulip show).

My current contract I was 3rd contractor, first 2 just couldn't figure it out. Part of this was due to unrealistic expectations of client who think they know everything due to one of their senior managers very minor involvement in a similar project at a previous employer (different industry and resourced differently). Project is/was a mess but I've been billing for over a year now, sure we were 6 months late (partially due to covid, partially due to what the previous 2 messed up) and delivered half the scope but being honest about everything with the client from the start has helped a lot. I told them from the beginning one key part of the project they wanted done wasn't going to work, and they still wanted to go ahead, that failed and didn't end up going live with it. Sure they weren't happy about that, but I didn't get the blame for it because I told them from the start it wasn't going to work and it was their decision to proceed not mine.

[GJABS]

If they've hired you to be the fall guy, then maybe accept that is what you're being paid to be.
You're an actor on a stage, nothing is really real as a contractor.

[lecyclist]

This is a situation where a face-2-face with the manager would be a plus, preferably over a beer. It would enable you to pivot the conversation to how it affects him / her, and how you want to make sure a solution is provided that makes them look good. Especially so if the manager is a political animal. As GJABS commented, we are just highly paid NPCs.

[LondonManc]

... if you'd started a new contract and its become clear that the scope of work you've been asked to deliver is very much a poisoned chalice and you're on a hiding to nothing in terms of expectations with senior stakeholders, to the point where you're beginning to think you're going to be the scapegoat?

Very honest chat with your client to say its not what was represented in the interview and as such you think its best for both to look for someone else?

Or just tough it out because its all billable

Two key things for me if I was in your situation:

1/ I am in charge of delivery and have a remit to deliver.

If so, then it's up to me to review the deliverable, the cast/crew, the kit, etc. If there's no project plan, there's your chance to pull one together to manage expectations. If it's going to be longer than they expected, tell them why. Don't be negative, be assertive about it. Behave as a firefighter, not a scapegoat. You're being pitched in as an expert to save them paying OTT for big four consultancy to charge who knows how many times as much. Think about the project triangle and explain that something has to give. It's only a walk-away moment if all three points are fixed. See it as a chance to shine and you become their go-to contractor and can charge accordingly in future.

2/ I am not in charge but I know it's likely to go south.

Articulate your concerns positively to your hiring manager, basing them on previous experience or tech articles as appropriate. They've not brought you in as a bum on a seat, they've brought you in to deliver and apply your contracting experience. If you don't feel you are being listened to, at least you have your concerns documented - keep one eye on the market though.