Effect 4.0 rebuilds the TypeScript effect system from the ground up: zero-dependency core, ~5× smaller bundles, ~6.4× concurrent throughput, 86% less memory per fiber, and a long-term support policy through at least 2029.

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.

Compared with Effect 3.x on identical source (minified and gzipped; runtime medians of nine fresh-process runs):

Minimal program size: 35.6 kB → 7.1 kB (~5× smaller)
Concurrent task throughput: 0.71M → 4.57M tasks/sec (~6.4×)
Heap for 50,000 fibers: 157.5 MB → 21.8 MB (~86% less memory per fiber)

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.

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.

Mainstream adoption

Effect now runs in production at companies of all sizes, including large enterprises. Weekly npm downloads in the week of September 21, 2026 were about 43.9M, with 179× growth since 3.x. In the last 7 days, 4.x adoption already outsizes 3.x (about 56% vs 44%). Projects with communities of their own have grown around it: Alchemy for cloud infrastructure, and Foldkit for the frontend.

Long-term support

Starting with this release, every major version of Effect has a long-term support policy. For Effect 4.x:

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

Support lasts at least three years. If 5.0 ships late enough, bug and security fixes run past that minimum.

What's next

The first priority is stabilizing more of the ecosystem. Some newer modules are still tagged @stability unstable or @stability experimental. Beyond that, broader native platform support and more higher-level primitives. Migrating? Start with the migration guide—hand it to your coding agent, and it should do most of the work on its own.

Thank you to everyone who contributed, tested releases, reported bugs, and helped others get started.