“It was about 136 billion tokens broken up in such a way that if you were to use the public pricing of those models at those times, it came out to approximately $120,000. This is not factoring any discounts… plus my time… about three weeks of my time spread out over three months plus the token costs. And it's only getting cheaper.”
The Developing Dev · Software Engineering & Infra · November 2026
Context
Asked about the cost of rewriting the Copilot runtime in Rust with agents, Toub totals the token spend at public list prices and estimates his own time. It is a rare concrete price tag on an agent-driven migration of several hundred thousand lines.
Transcript
…different paths and looking at different things. And one of the biggest changes you can make is completely changing the model. And so the best way to get different opinions and focusing on different things that I found is by having different models. Especially than the one that I used initially.
So, you know, after you finish the rewrite, what was the concrete impact and the cost? So, if you look at it from a business perspective, what was the benefit and the cost of this project?
…previous world that I was talking about. So, from a business perspective, it's worth a lot to do this port. In terms of what it actually costs us, it was about if I, so I stored all of my session logs or 99% of my session logs from this effort. And from those session logs, I was then able to sort of deduce the token counts for fresh tokens, cash writes, cash reads, output tokens. And all told, it was about 136 billion tokens broken up in such a way that if you were to use the public pricing of those models at those times, it came out to approximately $120,000. This is not factoring any discounts or things that companies might have for what those tokens cost. And this is just sort of what's on the website of OpenAI and Anthropic, plus my time, which it took, it was about 14, 15 weeks, but I was doing other things at the same time. A lot of these agentic projects, if I look at my co-pilot app that I currently have on my screen, I've got about 35 sessions right now for things that I'm…
Wow. So three weeks of, I guess, of developers' time plus $120,000, you're able to rewrite the entire code base. If you were to scope this out pre-AI, let's say you wanted to undertake this, what would that look like?
First, it wouldn't have happened. And I'm actually serious, like for this particular thing, it wouldn't have happened because by the time we were done, we would have ported the wrong thing. This space is moving so fast that we would have basically never finished, I think. In fact, you can kind of…
It sounds like, at least for this type of work where it's super what we want is very obvious and encoded somewhere in the existing functionality. So like this type of work is at least one order of magnitude cheaper, maybe even more. I mean, if you had a team of engineers and they're, you know…
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