A design is not a complete interaction specification
A desktop frame does not say what happens when a heading wraps, the navigation opens on a phone, or a user submits an invalid form. Those states are identified in the first review. Where the creative does not answer a question, your agency gets a concrete decision to make instead of an unnoticed invention.
The acceptance condition moves
“Make it match” can mean visual fidelity, a particular CMS editing experience, a performance budget or all three. List the required page types, breakpoints, interactions and integrations separately. That makes feedback reviewable: a missed agreed behavior is a defect, while a newly requested feature changes scope.
A successful preview hides a broken handover
The developer’s machine is not the agency’s deployment environment. Build commands, environment-variable names, third-party ownership and launch responsibilities belong in the handover. Secret values stay out of source. The agency should not need to reconstruct these details from a chat transcript when it takes over.