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You can add in some assumptions in the SoW / MSA / contract. Like, "Will only be responsible for work in XYZ Repo, not other Repos or 3rd Party Services." It's really whatever you want to put in there, I start with a pretty long standard list (based on the project type). "Won't customize administration tools or workflows of WordPress / Magento / ABC Platform," to, "Will utilize Foundation / AWS / blah blah blah."

You can always do work that is in a gray area, or clearly out of scope of the contract, if you want to be nice to the client. The contracts are for clients that you don't want to be nice to. They are levers you can pull to get the client back into a manageable space. If you don't give yourself a lot of levers, you're left with just "walk away" when the shit hits the fan... and typically that option doesn't include getting paid.

Even for "simple" projects now my Statements of Work are 25+ pages. Templatized as best I can, and all reviewed by a lawyer I trust. I don't sign contracts that clients send me, without stipulating that if that contract and the one I sent over are ever at odds, then the contract I sent over wins. You have to protect yourself, especially as a one-man-shop. You can't afford to get bogged down in shitty squabbles, so like with most things... "a stitch in time saves nine" here. Longer, more explicit and detailed contracts save you time.