I'd argue that you 100% need a spec --ideally tied to some high-level unit tests-- and should have a plan, but I don't think there's any strict need to write any of those three by hand; the critical step is where you review it and request changes if needed.

Doing it all manually better guarantees quality of course, so makes sense for a lot of problems. But if you're looking to move a bit faster with a bit more risk, I'd sacrifice the writing step without sacrificing the reading & validating step.

I tend to agree with bbor. I use the agent to build the spec, usually from unfiltered audio to text data dump. The spec also allows you to manage context by pre-defining scope. I would argue managing context is more important than a spec at a certain quality level of the spec. A high-quality spec should inherently help manage context. If you are leaning into full end to end automation, there is no substitute. I'm not sure "better" is the lens. A more capable model also has more paths of interpretation that it can conceivably take based on the framing of your rough goal.