Effect

Use this skill when work touches Effect v4 beta usage, service/layer architecture, runtime entrypoints, Schema v4, platform modules, testing, or migrations from Effect v3.

Workflow

Inspect the local Effect surface before changing code:
- Package versions for
```
effect
```
  ,
```
@effect/*
```
  , TypeScript, runtime platform packages, test libraries, and adapters.
- Imports: root
```
effect
```
  , direct modules like
```
effect/Effect
```
  ,
```
effect/Schema
```
  ,
```
effect/unstable/*
```
  , platform packages, or old v3 packages.
- Runtime boundary: CLI, HTTP handler, worker, server, browser, test, library, or framework-managed entrypoint.
- Dependency shape: services, layers, config providers, platform layers, managed runtimes, and scope ownership.
Clone
```
Effect-TS/effect-smol
```
into
```
.temp/effect-smol
```
(if missing) and always browse that checkout when referencing Effect v4 code — APIs, migration guides, package READMEs, and tests. Do not rely on memory or stale docs alone. Details: source-map.md.
Refresh current docs whenever the task asks for latest behavior or the local beta version is different. Start from source-map.md.
For install/version, core Effect usage, generator style, typed errors, async interop, config, and runtime boundaries, use setup-core.md.
For services,
```
Context.Service
```
, references, layers, memoization, scopes, and
```
ManagedRuntime
```
, use services-layers-runtime.md.
For Schema v4 shapes, validation, classes, tagged errors, transformations, codecs, serialization, and JSON Schema generation, use schema-v4.md.
For
```
@effect/vitest
```
(runners, TestClock, layers, flaky/live, good/bad patterns), use vitest-testing.md.
For v3 to v4 migration, unstable modules, and HTTP/platform packages, use migration-platform-testing.md.
Implement in the existing project style:
- Match the installed beta version and local import style.
- Prefer explicit service and layer composition over hidden globals.
- Keep framework/process edges thin; push business logic into Effects, services, and layers.
- Treat
```
effect/unstable/*
```
  APIs as beta-plus-unstable. Check installed declarations before using or recommending them.

Effect Judgment

Effect v4 is beta. Be honest about API drift and verify local declarations before making broad changes.

Use

Context.Service

for v4 services. Do not introduce v3

Context.Tag

Context.GenericTag

Effect.Tag

, or

Effect.Service

patterns in v4 code.

Prefer
```
yield* Service
```
inside
```
Effect.gen
```
for service access. Use
```
Service.use
```
and
```
useSync
```
only for small, local one-liners.

Define layers explicitly with

Layer.succeed

Layer.effect

, or

Layer.effectDiscard

; wire dependencies with

Layer.provide

and

Layer.provideMerge

Compose and provide layers once when possible. Use
```
Layer.fresh
```
or
```
Effect.provide(..., { local: true })
```
only for intentional isolation.
Use
```
ManagedRuntime.make(layer)
```
for repeated JS entrypoint runs against shared services, and dispose it when done.
Use
```
Effect.runPromise
```
,
```
runPromiseExit
```
,
```
runFork
```
, or their
```
With
```
variants only at application edges.
Inside
```
Effect.gen
```
,
```
yield*
```
works with Yieldable values. Outside generators, convert non-Effect Yieldables explicitly or use module functions such as
```
Ref.get
```
,
```
Deferred.await
```
, and
```
Fiber.join
```
.
Prefer typed failures and tagged error classes over throwing. Use defects only for unrecoverable programmer errors.
Keep Schema v4 encode/decode direction visible at boundaries; do not assume decoded
```
Type
```
and encoded input are the same.
For tests: install v4
```
@effect/vitest@beta
```
(→
```
4.0.0-beta.x
```
; never bare/
```
latest
```
→
```
0.30.x
```
v3). Prefer
```
it.effect
```
(already scoped + TestClock/TestConsole). Import
```
TestClock
```
from
```
effect/testing
```
. Do not use
```
it.scoped
```
/
```
it.scopedLive
```
for Effect Scope. Treat
```
layer()
```
as shared across tests unless you
```
Effect.provide
```
per test.

Verification

Prefer the repo's existing checks. For meaningful Effect v4 work, include the relevant subset:

Typecheck for
```
Effect<A, E, R>
```
requirements, service availability, layer composition, Schema encoded/type sides, and beta API names.
Focused tests with
```
@effect/vitest@beta
```
(
```
it.effect
```
, TestClock, scoped resources, typed Exit/Result failures). Use per-test
```
Effect.provide
```
when isolation matters; treat shared
```
layer()
```
as suite-scoped.
Runtime smoke at JS/framework edges where Effects are run or managed runtimes are disposed.
Schema decode/encode tests for valid input, invalid input, defaults, excess properties, transformations, classes, tagged errors, and generated JSON Schema.
Migration scans for v3-only APIs, old package imports, old catch/fork names, old runtime patterns,
```
FiberRef
```
,
```
Either
```
, and removed Schema APIs.

effect

NPX Install

Tags

SKILL.md Content

Effect

Workflow

Effect Judgment

Verification