Lately I keep getting asked about legacy replacements.
It's become one of the more common conversations I have, and the framing is almost always the same: AI can rewrite this now, can't it? The demand is clearly rising, and I don't think the premise is wrong. A coding agent can rewrite a legacy application startlingly fast. Producing the new code has stopped being the bottleneck.
What hasn't moved is the ability to say the new system behaves like the old one. That turns out to be a claim nobody has the evidence to make — about either system.
So when one of these comes up now, analyzing the old code is no longer where I'd start. That's not the obvious move: the default is to understand the old system first, then rebuild from that understanding. It's the order every plan I've seen begins in, and it's the order we tried, with a coding agent doing the analysis. The honest summary is that it didn't converge.
The approach I've drifted into instead:
