# effect

A guide for building TypeScript applications with Effect v4, covering services, schemas, configuration, HTTP clients, streams, and tests.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 2500 installs · ★ 352
- Author: kitlangton
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/kitlangton/skills/tree/HEAD/skills/effect/SKILL.md
- Updated: 2026-10-02
- HTML: https://affpapa.org/en/nevibecode/skills/effect

## What it does

This skill guides TypeScript application work with Effect v4. It recommends patterns such as `Effect.gen`, `Effect.fn`, `Context.Service`, `Layer.effect`, `Schema.Struct`, `Schema.TaggedError`, `Config`, `Schedule`, `Stream`, and the Effect HTTP client. For tests, it points to `TestClock`, `Deferred`, `Queue`, `Latch`, and `Ref` instead of relying on real sleeps.

The guide branches into references for schemas, services and layers, configuration, scheduling, caching, streams, HTTP clients, and testing. It also sets boundaries for HTTP handlers, external integrations, and retries. Check the Effect version before applying a suggestion: the guidance is reviewed against `4.0.0`, while the installed package takes precedence because prerelease APIs and import paths can differ.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add kitlangton/skills --skill effect -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/effect/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/effect/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add kitlangton/skills --skill effect -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/effect/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add kitlangton/skills --skill effect -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/effect/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install kitlangton/skills effect
```

2. **Manually: put the files in place**
- File: `.github/skills/effect/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add kitlangton/skills --skill effect -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/kitlangton/skills --path skills/effect
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/effect/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add kitlangton/skills --skill effect -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/kitlangton/skills/tree/HEAD/skills/effect/SKILL.md

## When to use it
- choose a schema for unknown input
- build an Effect service and layer
- configure retries with a bounded schedule
- wire an event stream through a queue
- test time-sensitive code without real sleeps

## Example prompts
- Build a `UserService` for Effect v4 with `Context.Service`, `Layer.effect`, and typed errors.
- Decode this JSON with `Schema.decodeUnknownEffect(...)` and define the domain schema it needs.
- Add a bounded retry `Schedule` to the HTTP call and test it with `TestClock`.

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 4a7ea77b46efee05.

BEGIN THIRD-PARTY FILE 4a7ea77b46efee05
```text
---
name: effect
description: |
  Opinionated guide for building production TypeScript applications with Effect v4. Use when implementing Effect workflows, services, layers, schemas, configuration, schedules, caches, streams, HTTP clients, or tests.
license: MIT
compatibility: Requires Effect v4. Examples are reviewed against the version documented in this repository.
---

# Effect

Use current Effect v4 APIs and the production defaults in this skill. Established project conventions still take precedence unless the task is explicitly changing them.

## Source Rule

Reviewed against Effect **4.0.0**, upstream commit
[`6389d9ac6`](https://github.com/Effect-TS/effect/tree/6389d9ac64c0f62ccc8b575fb9afc65fc104e814).
Read `references/V4_APIS.md` when upgrading from v3 or a v4 prerelease.
The installed version takes precedence: prerelease APIs and import paths can differ.

Check these before guessing:

- the nearest `AGENTS.md` and any project-local Effect practices doc
- the project-pinned `effect` package source and version
- current upstream Effect source when the installed package does not answer the question

## Branch Chooser

Read only the branch references that match the task.

- Data models, schemas, brands, variants, optional keys, or decoders: read `references/SCHEMA.md`.
- Services, module surfaces, layers, runtime wiring, errors, `Effect.fn`, or test services: read `references/SERVICES_LAYERS.md`.
- Runtime config, env variables, `ConfigProvider`, or `layerConfig`: read `references/CONFIG.md`.
- Retry, repeat, polling, backoff, jitter, rate-limit-aware policies, or pass loops: read `references/SCHEDULING.md`.
- Memoization, per-key TTL caches, deduplicating concurrent lookups, or request batching: read `references/CACHING.md`.
- Streams, event sources, async iterables, queues/pubsubs, pagination, backpressure, or stream consumers: read `references/STREAMS.md`.
- Outgoing HTTP calls, Effect HttpClient, status handling, or HTTP rate limiting: read `references/HTTP_CLIENTS.md`.
- Effect tests, time, sleeps, concurrency synchronization, or fakes: read `references/TESTING.md`.

If a task spans several branches, read all matching files before editing.

## Core Defaults

- Compose workflows with `Effect.gen(function* () { ... })`.
- Define public service methods and non-trivial internal service methods with `Effect.fn("Domain.operation")`.
- Use `Effect.fnUntraced` only for internal helpers where stack-frame/span metadata is intentionally unnecessary.
- Prefer `Context.Service` for application services when the codebase has not standardized on another current service-tag style.
- Build real service implementations with `Layer.effect(Service, Effect.gen(...))` and return `Service.of({ ... })`.
- Model records with `Schema.Struct(...)` plus a same-name `interface`.
- Model typed Effect errors with `Schema.TaggedError`.
- Read runtime config through `Config`, not direct `process.env` access in application logic.
- Use `Schedule` for retry, repeat, polling, pacing, and backoff policies.
```
END THIRD-PARTY FILE 4a7ea77b46efee05

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