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

Effect 4.0, a from-scratch rewrite of the TypeScript framework's fiber runtime, shipped on 30 September 2026 after eight months in beta and six weeks in release candidate, with the team reporting a minimal Effect, Stream and Schema program at 7.1 kB versus 35.6 kB in 3.x. The team's figures, medians of nine fresh-process runs and not independently reproduced, put 50,000-fiber heap use at 21.8 MB versus 157.5 MB and concurrent throughput at 4.57 million tasks per second versus 0.71 million. Effect 4.0 consolidates @effect/platform, @effect/rpc and @effect/cluster into the main package, drops all runtime dependencies, and introduces long-term support with bug fixes until September 2029 or one year after 5.0 ships, whichever is later.

Effect https://effect.website/ , the TypeScript framework for building production-grade applications with typed errors, structured concurrency and dependency injection, has released Effect 4.0 https://effect.website/blog/releases/effect/40 . 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 https://effect.website/blog/releases/effect/40 , 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 https://www.stork.ai/blog/effect-40-is-powerfuland-riskier-than-it-looks , have not been independently reproduced. Alongside the rewrite, the ecosystem has been consolidated. Core modules from @effect/platform , @effect/rpc and @effect/cluster now live inside the main effect package, every @effect/ package shares a single version, and the core ships with zero runtime dependencies after the final removal of pg replaced by a native PgProtocol client and MessagePack. With 4.0, the team has introduced a long-term support policy. Effect 4.x receives bug fixes until September 2029, or one year after 5.0 ships, whichever is later. Security fixes run until September 2029 or two years after 5.0, again whichever is later. Stork.ai https://www.stork.ai/blog/effect-40-is-powerfuland-riskier-than-it-looks points out that the 43.9 million weekly downloads cited at launch covers the whole ecosystem across beta and RC tags, while stable Effect 4.0 itself recorded "approximately 150,000 downloads on its first day". On HackerNews, one commenter has been using the beta for some time and has had a very positive experience: I’ve been using the beta on three separate projects for quite a while now and it has been totally pain-free. I mostly use the v3x core library which has expanded to contain other libraries in v4 and Schema so I can’t speak to much outside of those, but I trust they’ve done a great job all around. I don’t see it mentioned here, but 4 also comes with a much simpler API in some modules. Schema is much better. It’s an exceptional update. I followed many of the video logs on YouTube and was impressed by the team. This is one of my favourite software projects, period. This rebuild is incredibly well done, and I hope a lot of people are willing to try it out and adopt it, especially with the LTS. The team's own Effect 4.0: The First Week https://effect.website/blog/effect-4-first-week post, published alongside patch releases 4.0.1 and 4.0.2 on 8 October, admitted that several modules intended to ship as unstable went out without a @stability tag, implicitly promising they were stable. The missing tags were added, and every exported API now declares its stability explicitly. The same window moved HTTP, SQL, RPC and AI spans to the OpenTelemetry semantic conventions. Effect 4.1 https://effect.website/blog/releases/effect/41 followed on 20 October 2026 with new effect/crypto modules covering randomness, hashing and signing, a Glob module backed by FileSystem.glob on Node, Bun and Deno, a streaming Csv module in effect/encoding , and support for RFC 9457 Problem Details https://www.rfc-editor.org/rfc/rfc9457 in HttpApi . For teams on 3.x, the v3 to v4 migration guide https://github.com/Effect-TS/effect/blob/main/MIGRATION.md is the starting point, with the team suggesting coding agents can handle most of the mechanical work. Effect sits in similar territory to ZIO https://zio.dev/ in the Scala world, alongside TypeScript libraries such as Effection 4.0 https://frontside.com/effection that target structured concurrency with a deliberately smaller surface. Effect is an open-source TypeScript framework maintained by Effectful Technologies https://effectful.co/ , providing typed error handling, structured concurrency, dependency injection and observability primitives for both frontend and backend applications.