Exclusivity clauses of the type eg mentions are common in Silicon Valley. From the agency's point of view, they're protecting the income stream they feel they're due after doing the upfront work of getting the gig set up. If you sign on full time with a client right after a, they lose.

You can try to whittle the agency down to 6 months, or have them add a buyout clause, but some might not go for it. You can hope that if a company likes you enough, they'll cut a deal with the agency to spring you loose, but that might not work, either. (I just saw that happen.)

Bottom line: This is one of the tradeoffs you'll have to factor into your decision to work through an agency.