# Effect 4.0 rebuilds the TypeScript runtime around a zero-dependency core and LTS

Effect 4.0 is a major rebuild of the typed-concurrency library with a dependency-free core, consolidated ecosystem and a minimum three-year support commitment; the team reports sharply smaller bundles and lower per-fiber memory.

Effect 4 changes runtime architecture and maintenance guarantees, not just APIs. Its reported 5x smaller bundles and 86% lower fiber memory are vendor benchmarks requiring workload-specific validation.

- Status: Active
- Published: 2026-10-08T17:55:58+13:00
- Updated: 2026-10-08T17:55:58+13:00
- Categories: Web Development, JavaScript, Frameworks
- Tags: Effect, long-term support, runtime performance, structured concurrency, TypeScript
- Canonical HTML: https://beyondthe.news/dossiers/effect-4-zero-dependency-runtime-lts-performance

## What changed

Effect 4.0 shipped September 30, 2026 as a major rebuild. It folds formerly separate packages into a shared-version ecosystem, removes runtime dependencies from the core effect package, and publishes vendor benchmarks comparing a 35.6 KB to 7.1 KB minimal program, 0.71M to 4.57M tasks/s and 157.5 MB to 21.8 MB heap for 50,000 fibers. The project also introduces a formal LTS policy: Effect 4 receives bug and security fixes until at least September 2029, with extensions tied to the timing of a future 5.0 release.

## Why it matters

Effect underpins typed errors, resource management, structured concurrency and workflows in TypeScript services. Changes to its runtime, package graph and support lifecycle affect build size, memory budgets, dependency security and long-term maintenance planning. The support policy is an operational commitment; the performance claims are benchmarks under defined vendor conditions, not universal results.

## A smaller zero-dependency core

Effect 4 consolidates packages into the effect distribution and removes third-party runtime dependencies from its core, reducing dependency-chain and bundle overhead.

## Vendor benchmarks show large gains under defined tests

The team reports 5x smaller minimum bundles, 6.4x concurrent task throughput and 86% lower memory per fiber. Benchmarks use identical source for bundles and nine fresh-process runs for runtime medians.

## LTS becomes an explicit contract

The 4.x line receives bug/security fixes until at least September 2029; depending on the next major release date, support can extend beyond that floor.

## Migration and unstable modules still need review

A migration guide exists, but some new modules remain marked unstable or experimental and may change in smaller releases.

## Key details

- Effect 4.0 released September 30, 2026.
- The core effect package has zero runtime dependencies.
- Vendor reports minimal bundles shrinking from 35.6 KB to 7.1 KB.
- Vendor reports throughput rising 0.71M to 4.57M tasks/s and 86% lower memory for 50,000 fibers.
- 4.x has a minimum support floor through September 2029.
- Some newer modules remain unstable or experimental.

## Builder takeaways

- Use the official migration guide and run application-specific type/build tests.
- Rebenchmark memory and concurrency under real workloads before planning capacity reductions.
- Review consolidated package imports and dependency lockfiles.
- Record the LTS support window in your maintenance policy.

## What to watch

- Independent production benchmark results and regressions.
- Effect 4 module stability promotions.
- Timing and support-policy details for Effect 5.

## Uncertainties

- Performance figures are vendor measurements, not independent cross-application guarantees.
- Some modules remain experimental and may evolve.
- Support extensions depend on the future 5.0 release date.

## Sources

- [Effect 4.0](https://effect.website/blog/releases/effect/40) — Effect · primary · 2026-09-30T00:00:00+13:00. Primary architecture, benchmark methods and LTS commitments.
- [Effect v4 RC: September 2026 Updates](https://effect.website/blog/effect-v4-rc-september-recap) — Effect · primary · 2026-10-01T00:00:00+13:00. Implementation and correctness sweep background.