I think a lot of people are building their harnesses this way more recently, but last year, Claude, Codex (and other harnesses) were storing everything on a big json file on disk.
It really feels like I'm being gaslight here into believing "durable agents" are novel. All I'm seeing is the API that is behind any Chat Web UI. Persistence, resumability, recovery from errors and branching and forking have been table stakes there since the release of the first ChatGPT.