Stage
The word that jumps out at me is "working." Plenty of tools handle review of static mockups, but signing off on a live prototype or a near-shipped product is where feedback loops usually fall apart. Stage is aiming at that messy middle, and that's a smart place to build.
The review, fix, sign off loop is basically the entire job of a product team before launch, yet it's usually scattered across comment threads, screenshots, and hallway conversations. If Stage can make the final approval explicit, with a clear record of who said yes and when, it saves teams the awkward "I thought you approved that" conversation.
I'd want to know how it handles versioning and whether the sign-off lives in the tool or just gets exported back to wherever the team already works. The pitch is simple enough that execution will decide it. But the focus on working prototypes rather than polished decks feels like the right instinct.