Effect 4.0
Effect 4.0
Rebuilt from the ground up, with a dependency-free core and long-term support
Today, we’re releasing Effect 4.0.
It’s the most ambitious release we’ve ever shipped. We rebuilt Effect from the ground up. It’s faster, uses a fraction of the memory, and tree-shakes to far smaller bundles.
Effect 3 vs. Effect 4 3.x 4.x
Smaller bundle size 5×
Concurrent task throughput 6.4×
Less memory per fiber 86%
Minimal program size lower is better
3.x 35.6 kB
4.x 7.1 kB
Tasks per second higher is better
3.x 0.71M
4.x 4.57M
Heap for 50,000 fibers lower is better
3.x 157.5 MB
4.x 21.8 MB
Bundles are built from identical source, minified and gzipped. Runtime figures are medians of nine fresh-process runs.
[](https://effect.website/blog/releases/effect/40#one-ecosystem-zero-dependencies)One ecosystem, zero dependencies
Many previously separate packages now live in `effect` itself. The packages around it share a single version and release in lockstep.
The core `effect` package has zero runtime dependencies. We control every line we ship, so there are no third-party dependency chains and less exposure to supply chain attacks.
[](https://effect.website/blog/releases/effect/40#from-functions-to-clusters)From functions to clusters
Effect started with a much smaller scope. Today, it covers the full range, from a single function to a distributed system.
People came for the programming model: an effect system with built-in typed errors, dependency injection, resource management, structured concurrency, and observability. The standard library and platform integrations grew around it. Then came higher-level primitives for distributed systems, including durable workflows and clustering.
Every layer is built on that same model, with the same guarantees all the way up.
[](https://effect.website/blog/releases/effect/40#mainstream-adoption)Mainstream adoption
Effect now runs in production at companies of all sizes, including large enterprises.
Weekly npm downloads
Downloads in the week of September 21, 2026 43.9M
Growth since 3.x 179×
4.x adoption already outsizes 3.x last 7 days
4.x 56%
3.x 44%
0 10M 20M 30M 40M 50M Apr 2024 Jan 2025 Jan 2026 Sep 2026
Projects with communities of their own have grown around it: Alchemy for cloud infrastructure, and Foldkit for the frontend.
If your team runs Effect in production, let’s talk. Tell us what you’re building and how we can help.
[](https://effect.website/blog/releases/effect/40#long-term-support)Long-term support
Teams are betting their products on Effect, and they deserve commitments they can plan around. Starting with this release, every major version of Effect has a long-term support policy.
For Effect 4.x, that means:
- Bug fixes until September 2029, or one year after 5.0 ships, whichever is later
- Security fixes until September 2029, or two years after 5.0 ships, whichever is later
Long-term support for Effect 4.x Bug and security fixes Security fixes only
If 5.0 ships in
2026 2027 2028 2029 2030 2031 2032
Sep 2027
4.x gets bug and security fixes until Sep 2029
Sep 2029
5.0 ships
Sep 2028
4.x gets bug fixes until Sep 2029 and security fixes until Sep 2030
Sep 2029 Sep 2030
5.0 ships
Sep 2030
4.x gets bug fixes until Sep 2031 and security fixes until Sep 2032
Sep 2031 Sep 2032
5.0 ships
Support lasts at least three years. If 5.0 ships late enough, bug and security fixes run past that minimum.
We’ll share the full policy and our release process in a follow-up post.
[](https://effect.website/blog/releases/effect/40#whats-next)What’s next
Our first priority is stabilizing more of the ecosystem. Some of the newer modules are still tagged `@stability unstable` or `@stability experimental`, which means they may change in minor or patch releases. We’ll use production feedback to refine them and promote them to stable.
Beyond that, we’ll continue expanding Effect into a single programming model for your entire application, with broader native platform support and more higher-level primitives.
[](https://effect.website/blog/releases/effect/40#over-to-you)Over to you
Migrating? Start with the migration guide. Hand it to your coding agent, and it should do most of the work on its own.
Curious? Check out our documentation or ask your agent why you should be building with Effect.
Thank you to everyone who contributed, tested releases, reported bugs, and helped others get started. Effect is better because of you.
Spread the word and celebrate with us on X, Bluesky, and Discord.
Thank you for your attention to this matter.
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Last updated Sep 30, 2026
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