Effect 4.0 Ships: Dependency-Free Rewrite with 5x Smaller Bundles and Long-Term Support

Effect 4.0, a TypeScript framework for building production-grade applications with typed errors, structured concurrency, and dependency injection, has been released. This release marks the first stable version of the framework, which underwent eight months of beta testing and six weeks of release candidate testing. The framework's core team aimed to achieve smaller bundles, lower memory usage, and higher throughput.

Comparing Effect 4.0 to its predecessor, 3.x, the new version promises a 5x reduction in bundle size, from 35.6 kB to 7.1 kB. The memory used by 50,000 fibers has decreased significantly, from 157.5 MB to 21.8 MB, a 86% reduction. Concurrent task throughput has also increased, from 0.71 million to 4.57 million tasks per second, approximately 6.4x.

The overhaul of the framework has also led to a consolidation of the ecosystem. Core modules from @effect/platform, @effect/rpc, and @effect/cluster have been integrated into the main effect package. Every @effect/* package now shares a single version, and the core now ships with zero runtime dependencies after the removal of pg (replaced by a native PgProtocol client) and MessagePack.

Effect 4.0 introduces a long-term support policy. The 4.x line will receive bug fixes until September 2029, or one year after the release of 5.0, whichever comes later. Security fixes will be provided until September 2029 or two years after the release of 5.0, whichever is later.

The framework's weekly download count at launch was approximately 43.9 million, covering the entire ecosystem across beta and RC tags. On its first day, the stable version of Effect 4.0 recorded around 150,000 downloads. One HackerNews commenter expressed a positive experience using the beta for several projects, praising the framework's painless experience and the team's efforts.

The team behind Effect 4.0 acknowledges that some modules were released without the intended @stability tag, which has now been corrected. Every exported API now declares its stability explicitly. Following the initial release, Effect 4.1 was published on 20 October 2026, introducing new effect/crypto modules, a Glob module, a streaming Csv module, and support for RFC 9457 Problem Details in HttpApi.