What’s the best tool for edging grass around stepping stones in a lawn?

I have these stepping stones because the lawn is the main path to the house. What can I say… it seemed like an excellent idea at the time! I still like them but despite me changing the substrate twice, they like to sink a bit and the heat they retain make the grass suffer 🥵

I think I would be a lot happier with them if I can keep the edges really neat. I don’t have a great tool for doing tight circumference edges like this. What would you use?