There is no reason for detailed tech specs except for putting blame on people and covering ass. (Critical industry with strict standards excluded)

It should be a high level overview.

Then you start working on it and then review small pieces in code review and make modifications as more edge cases surface.

I would rather have specs than "make it like the old one, but better". Then I spend a lot of time doing that. Then they say things like: "the old one was localhost only, but we need it to be remote too". Like wtf? Then I find in the code handed to me they made assumptions that everything if fucking local! So the person asking for it to be remote assumed it would be local to start with. Like file io shit on same machine.

Yeah, I usually end up implementing like 70% of the new implementation while writing a tech spec. I don't really get how you can even write a good tech spec without looking and testing constantly, especially when it's code I didn't even see before