1# Migrating from Effect v3 to Effect v4
2
3 **Note:** If you run into a migration issue this guide doesn't cover, let us
4 know on [Discord](https://discord.gg/effect-ts) or open an
5 [issue](https://github.com/Effect-TS/effect/issues).
6
7## Background
8
9Effect v4 is a major release with structural and organizational changes across
10the ecosystem. The core programming model — `Effect`, `Layer`, `Schema`,
11`Stream`, etc. — remains the same, but how packages are organized, versioned,
12and imported has changed significantly.
13
14### Versioning
15
16All Effect ecosystem packages now share a **single version number** and are
17released together. In v3, packages were versioned independently (e.g.
18`effect@3.x`, `@effect/platform@0.x`, `@effect/sql@0.x`), making compatibility
19between packages difficult to track. In v4, if you use `effect@4.0.0`,
20the matching SQL package is `@effect/sql-pg@4.0.0`.
21
22### Package Consolidation
23
24Many previously separate packages have been merged into the core `effect`
25package. Functionality from `@effect/platform`, `@effect/rpc`,
26`@effect/cluster`, and others now lives directly in `effect`.
27
28Packages that remain separate are platform-specific, provider-specific, or
29technology-specific:
30
31`@effect/platform-*` — platform packages
32`@effect/sql-*` — SQL driver packages
33`@effect/ai-*` — AI provider packages
34`@effect/opentelemetry` — OpenTelemetry integration
35`@effect/atom-*` — framework-specific atom bindings
36`@effect/vitest` — Vitest testing utilities
37
38These packages must be bumped to matching v4 versions alongside `effect`.
39
40### Unstable Module System
41
42v4 includes **unstable modules** under `effect/*` import paths. Their paths no
43longer contain an `unstable` segment, but their API documentation is marked
44with `@stability unstable`.
45
46`@stability unstable` means an API may receive new features in patch releases
47and breaking changes in minor releases. `@stability experimental` means it may
48receive new features and breaking changes across patch versions. `@stability
49stable` and APIs without a stability tag follow strict semver. Every module and
50directly importable export declares its stability explicitly. Modules in
51packages other than `effect` are currently `@stability unstable`.
52
53APIs that expose a third-party dependency are also marked `@stability
54unstable`, because that dependency's own releases can change them. This covers
55accessors to underlying clients (for example `NodeRedis`'s `client` and `use`),
56options typed as the dependency's options, and re-exports such as
57`@effect/platform-node/Undici`. It includes the generated provider schemas in
58the `@effect/ai-*` packages and the `@effect/opentelemetry` integration.
59
60Imports using `effect/unstable/<module>` must drop the `unstable` segment.
61For example, replace `effect/unstable/http` with `effect/http` and
62`effect/unstable/ai/LanguageModel` with `effect/ai/LanguageModel`. There
63are no compatibility exports for the old paths.
64
65Unstable modules include: `ai`, `cli`, `cluster`, `devtools`, `eventlog`,
66`http`, `http-api`, `jsonschema`, `observability`, `persistence`, `process`,
67`reactivity`, `rpc`, `schema`, `socket`, `sql`, `workflow`, `workers`.
68
69Moving these modules does not stabilize their APIs.
70
71### Performance and Bundle Size
72
73The fiber runtime has been rewritten for reduced memory overhead and faster
74execution. The core `effect` package supports aggressive tree-shaking. A
75minimal Effect program bundles to 7.1 kB (minified + gzipped).
76
77---
78
79## Migration Guides
80
81### Import and API Rename Maps
82
83[v3 to v4 Import and API Rename Maps](./migration/v3-to-v4.md)
84
85### Core
86
87[Services: `Context.Tag` → `Context.Service`](./migration/services.md)
88[Cause: Flattened Structure](./migration/cause.md)
89[Error Handling: `catch*` Renamings](./migration/error-handling.md)
90[Forking: Renamed Combinators and New Options](./migration/forking.md)
91[Effect Subtyping Changes](./migration/yieldable.md)
92[Fiber Keep-Alive: Automatic Process Lifetime Management](./migration/fiber-keep-alive.md)
93[Layer Memoization Across `Effect.provide` Calls](./migration/layer-memoization.md)
94[FiberRef: `FiberRef` → `Context.Reference`](./migration/fiberref.md)
95[Runtime: `Runtime<R>` Removed](./migration/runtime.md)
96[Scope](./migration/scope.md)
97[Equality](./migration/equality.md)
98
99### Modules
100
101[Schema v4 Migration Guide](./migration/schema.md)