Does this basement stair stringer look safe or did I waste my money?

That notch you're describing is a massive red flag and a total structural failure waiting to happen. You never want to remove that much meat from the top of a stringer beacuse the structural integrity of the entire run depends on the depth of the uncut wood, which we call the **throat**. By cutting a huge chunk out to make it flush, your guy basically turned a 2x12 into a 2x4 at the most critical stress point. If there are only two stringers, that’s another major issue. For a standard width staircase, you realy need at least three stringers to prevent the treads from flexing or cupping over time. If the treads are running long past the outside stringer, they're going to act like a lever and eventually split the wood right along the grain. It sounds like he was prioritizing a quick finish over framing it to any kind of actual code. How the assembly is tied into the rim joist is usually where these budget builds fail first. If he just ran some #10 wood screws through a 2x6 into the floor joists, that's not going to hold up to the dynamic load of a person walking down. You really want to see **stair hangers** or a proper ledger board that’s lagged into the framing. The fact that there are no risers makes it even sketchier because risers acutally help stiffen the whole assembly and prevent the stringers from spreading. I wouldn't trust those stairs for anything more than a light load. You’re likely going to need to pull those stringers and start over with a proper header mount or use a sloped hanger that supports the full bottom edge of the wood. Don't wait for the wood to split; once that notch gives way, the whole thing is coming down.

That wiring part caught my eye too. If those wires are getting pinched or even just rubbing against the wood as it shifts, isn't that a fire risk on top of the structural stuff? I’m wondering if you coud just bolt a 4x4 post right under that notch to act as a leg instead of tearing the whole thing out. Or does the angle make that impossible? I'm just starting to learn about framing and it's wild how much can go wrong with just a few bad cuts.

Adding a 4x4 post might stop a collapse, but it won’t fix the bounce if those stringers are alredy undersized. One thing people forget is the landing. If he hacked the top that bad, I’d bet money he didn’t use pressure-treated wood at the base. If those stringers are sitting directly on basement concrete, they’ll rot out just like your old ones did.

Also, check the gap between those treads. If it’s an open riser and that gap is over 4 inches, it’s a major safety hazard for kids or pets. For a few hundred bucks, you basially got a temporary ladder. Check if your rise and run are even consistent before you bother bracing it.