In the age of agents, I feel like BDD is more important than ever since it describes the behavior, and if it does so well it’ll make it much easier for these tools to pick up correct behavior.

Gherkin style tests also come to mind

The best part about LLMs is that, if you write gherkin style tests, you don't even need to automate them yet to get value, the AI can "manually" validate initially and then only codify them into deterministic execution after they've been nailed down. That way you have a bulk set of tests that run automatically, and a few candidate tests that are not automatically run yet, and the boundaries expand over time.