Contractor through agent

I got an offer from an agency to work in a big company. The agent promised to sponsr me H1b. However, both the hiring manager from the company and the agent promised 1 year contract only. They say there might be chance to move on to be full time in the company after the first year but no garantee from either of them.

Is thia an takable offer? The company is really famous. I am just worried what will happen to me after the one year contract. Any advices? Thanks.