Anthropic rewrote its Messages API in Rust with no Rust devs
Somi's summary of the Latent Space episode says humans owned the plan and the verification, and the model owned the code.
Anthropic rewrote the Messages API in Rust with zero Rust engineers on the team, and CPU usage dropped 40 to 50%. That comes from Somi's summary of @katelyn_lesse on the Latent Space podcast. Claude wrote the code. Humans wrote the plan and checked that old and new matched on real traffic.
Pretty sure "never do the big rewrite" is dead advice.
swyx, who hosts the show, backed the account when people pushed back, including, he says, people quoting his own show notes at him.
The verification is the story
Strip out the Rust and what is left is an old technique applied to a new problem. You do not merge a rewrite because the tests pass. You run the new implementation against production traffic and compare its output to the existing service until you stop finding differences. That turns a rewrite from a bet into something you can watch converge. Somi's summary is explicit that this is the part humans kept, along with the plan.
The no Rust devs detail is the part that will get quoted, and it is also the part most likely to be misread. Nobody in the sources claims the team did not need to understand what the model produced. The claim is narrower, that the team did not already have Rust engineers, and that a plan plus a traffic level diff was enough scaffolding to ship.
The claim underneath
"Never do the big rewrite" is conventional wisdom because rewrites historically cost more than anyone budgeted and shipped behavior changes nobody asked for. The cost side of that argument is what changes when a model writes the code, and the behavior change side is what shadow comparison is supposed to catch. Somi's conclusion is that the old advice is now dead. That is one engineer's read on one rewrite at one company, reported second hand from a podcast, and the sources do not include the migration timeline, the size of the codebase, or how long the comparison ran.
For anyone sitting on a service they have wanted to port for years, the transferable piece is not the language. It is whether you can diff old against new on live traffic at all.
its true, Anthropic really vibecoded their entire backend to Rust and none of you noticed
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