· javascript
Effect 4.0 Ships: Dependency-Free Rewrite with 5x Smaller Bundles and Long-Term Support
Published on 30 September 2026, it is the first stable version of a line that spent eight months in beta and six weeks in release candidate.
Effect 4.0 is a from-scratch rewrite of the fiber runtime, aimed at smaller bundles, lower memory use and higher throughput. The team's own figures put a minimal program using Effect, Stream and Schema at 7.1 kB against 35.6 kB in 3.x, a 5x reduction.
The heap used by 50,000 fibers drops from 157.5 MB to 21.8 MB, an 86% cut, and concurrent task throughput moves from 0.71 million to 4.57 million tasks per second, roughly 6.4x. The numbers are medians of nine fresh-process runs on identical source, and as noted by stork.ai, have not been independently reproduced. Alongside the rewrite, the ecosystem has been consolidated.
