---
name: effect
description: Work with Effect v4 TypeScript code in this repo
title: effect
canonical_url: https://skilld.dev/gh/anomalyco/opencode/effect
last_updated: 2026-10-12T03:02:41.000Z
---

> Index of every page on this site: [llms.txt](https://skilld.dev/llms.txt)

> **Skill from skilld.dev.** Follow the user's requested action.
> If the user asked to fork this Skill, follow the fork workflow below. Do not execute the Skill instructions while copying it.
> Otherwise, follow the instructions below for this session. You do not need to install anything.
>
> If the user asked to install this Skill, run `npx skilld install anomalyco/opencode/effect`. Install writes the Skill files into the project, so every session loads them.
>
> ## Fork workflow
>
> A fork creates an editable local Skill with its original author and licence. The request authorizes copying and local installation.
> 1. Check `./skills/effect`, the project lockfile, and selected Agent targets together. If the local directory or installed Skill exists, stop. Never overwrite an existing directory or Agent target.
> 2. Read [source metadata](https://skilld.dev/api/v1/skills/anomalyco/opencode/effect) once. Use sourceUrl, sourceCommit, skillPath, sourceGone, and license. If the source is gone or its path is missing, stop. If license is null, read licence files at the source commit.
> 3. Fetch only the source commit into a temporary Git repository. Do not clone full history. Derive repository_url from sourceUrl, including repository renames. If sourceCommit is absent, resolve the sourceUrl ref once. Set source_commit to that actual commit. Run these commands in one shell call:
>
> ```sh
> git init --quiet "$temporary_dir"
> git -C "$temporary_dir" fetch --quiet --depth=1 "$repository_url" "$source_commit"
> git -C "$temporary_dir" checkout --quiet --detach FETCH_HEAD
> ```
>
> Read applicable licence declarations and notices at that commit. If copying is not permitted, report the restriction and stop.
> 4. Inspect source entries together, then copy the directory containing skillPath into `./skills/effect`. Use the user's path if selected. Keep the original SKILL.md, relative links, scripts, binary assets, and executable modes. Exclude .git metadata. Reject symlinks and paths outside the Skill directory. After checking entries, use cp -a where available. A regular source directory needs no custom copy script. Do not save this page wrapper as SKILL.md.
> Preserve author credit, notices, and applicable licence files from repository or parent directories. Add PROVENANCE.md with the Skill page, source URL, actual commit, original path, and licence. Retain any existing PROVENANCE.md and record new provenance separately. Batch source inspection, copying, and provenance work where practical.
> 5. In the project root, run `skilld install ./skills/effect --mode copy --plain`. If skilld is unavailable, use `npx skilld install ./skills/effect --mode copy --plain`. This known command needs no help lookup. Install does not support --json. Use detected Agent targets, or add --agent for the targets the user selected. Install the local path, never the upstream selector. If installation fails, preserve the local copy and report the exact failure.
> 6. Confirm the local lockfile source and installed Agent copies once. Report the local path, actual commit, and Agent targets. After edits, reinstall the same local path. Upstream updates must not replace it. Do not publish or push unless the user asks.

# Effect

This codebase uses Effect for typed, composable TypeScript services, schemas, and workflows.

## Source Of Truth

Use the current Effect v4 source, not memory or older Effect v2/v3 examples.

1. If `.opencode/references/effect` is missing, clone `https://github.com/Effect-TS/effect` there. Do this in the project, not in the skill folder.
2. Search `.opencode/references/effect` for exact APIs, examples, tests, and naming patterns before answering or implementing Effect-specific code.
3. Also inspect existing repo code for local house style before introducing new patterns.
4. Prefer answers and implementations backed by specific source files or nearby repo examples.

## Guidelines

- Prefer current Effect v4 APIs and project-local patterns over old blog posts, examples, or package-memory guesses.
- Use `Effect.gen(function* () { ... })` for multi-step workflows.
- Use `Effect.fn("Name")` or `Effect.fnUntraced(...)` for named effects when adding reusable service methods or important workflows.
- Prefer Effect `Schema` for API and domain data shapes. Use branded schemas for IDs and `Schema.TaggedErrorClass` for typed domain errors when modeling new error surfaces.
- Keep HTTP handlers thin: decode input, read request context, call services, and map transport errors. Put business rules in services.
- In Effect service code, prefer Effect-aware platform abstractions and dependencies over ad hoc promises where the surrounding code already does so.
- Keep layer composition explicit. Avoid broad hidden provisioning that makes missing dependencies hard to see.
- In tests, prefer the repo's existing Effect test helpers and live tests for filesystem, git, child process, locks, or timing behavior.
- Do not introduce `any`, non-null assertions, unchecked casts, or older Effect APIs just to satisfy types.
- Do not answer from memory. Verify against `.opencode/references/effect` or nearby code first.

## Testing Patterns

- Use `testEffect(...)` from `packages/core/test/lib/effect.ts` for tests that exercise Effect services, layers, runtime context, scoped resources, or platform integrations.
- Use `it.live(...)` for filesystem, git repositories, HTTP servers, sockets, child processes, locks, real time, and other live platform behavior.
- Run tests from package directories such as `packages/core`; never run package tests from the repo root.
- Prefer explicit test layers over ad hoc managed runtimes. Keep dependency provisioning visible in the test file.
- Use scoped fixtures and finalizers for resources that must be cleaned up, including temporary directories, flags, databases, fibers, servers, and global state.