Anthropic's dynamic workflows ported Bun from Zig to Rust. 750K lines. 11 days. 99.8% tests passing.

This is the story I keep turning over. Anthropic shipped dynamic workflows in Claude Code, where the model writes its own JavaScript orchestration script and fans work across tens to hundreds of background subagents. Agents attack a problem from independent angles while others try to refute the findings, and you let it run until the answers converge. (Anthropic)

The demo is the part that made me sit up. Jarred Sumner used it to port Bun from Zig to Rust. Roughly 750,000 lines. 99.8% of the existing test suite passing. Eleven days from first commit to merge. The structure they described: one workflow mapped Rust lifetimes per struct field, and the next emitted behavior-identical .rs files in parallel.

I've done migrations. Not at this scale, but enough to know that the hard part of a port isn't any single file. It's holding the whole system in your head while you change it piece by piece, and the way a change in one corner breaks an assumption three modules away. The claim here is that you can decompose that into independent units, run them concurrently, and use a verification swarm to catch the breakage. The existing test suite is doing enormous work as the oracle. 99.8% passing only means something because Bun had a serious test suite to begin with.

That's the lesson for builders, and it's a humbling one. The workflows didn't replace good engineering practice. They amplified it. If your codebase has thin test coverage, you can't run this play. The agent fleet has nothing to converge against. The teams that invested in tests for years are the ones who can now parallelize migrations across hundreds of agents. The teams that skipped tests still can't.

I'm skeptical of the eleven-days number as a generalizable claim. Bun is a specific kind of codebase: a runtime with sharp, well-defined behavior and a test suite built to pin that behavior down. A CRUD app with implicit business logic baked into UI handlers and no tests is a different animal. But as a proof that large-scale migration parallelizes across an agent fleet, this is the clearest evidence I've seen. Connect it to the token story below and you understand why nobody's budget survives contact with this kind of work. Hundreds of subagents refuting each other for eleven days is a lot of tokens.

If you want to try the pattern, start small. Pick a self-contained module with good tests. Let a workflow map the transformation, run the conversion in parallel, and let the test suite be the judge. Then look at the bill before you scale it up.