Effect v4 (Stable) - Development & Migration

Effect 4.0 is the stable major release of Effect, published from Effect-TS/effect (v4 was developed as effect-smol during beta). This skill covers:

Writing v4 code with the patterns the Effect team recommends for AI agents (references/llms.md, references/examples/)
Migrating v3 → v4: official migration guides plus a per-module API rename map

Ground Rules for v4 Code

Version alignment: every Effect package shares one version. Use the same version of effect and every @effect/* package (e.g. effect@4.0.1 + @effect/platform-node@4.0.1).
Imports: @effect/platform, rpc, cluster, cli, ai, sql, workflow, experimental were merged into effect. Import from effect/<area>: effect/http, effect/http-api, effect/rpc, effect/cluster, effect/workflow, effect/ai, effect/cli, effect/sql, effect/schema, effect/socket, effect/process, effect/encoding, effect/observability, effect/persistence, effect/reactivity, effect/net, effect/testing. effect/unstable/* paths no longer exist (no compat exports).
Separate packages remain only for platforms (@effect/platform-node|bun|deno|browser), SQL drivers (@effect/sql-*), AI providers (@effect/ai-*), @effect/opentelemetry, @effect/atom-*, @effect/vitest.
Stability: untagged APIs follow semver. @stability unstable APIs may break in minor releases; @stability experimental may break in patches. Most area modules (http, ai, cli, sql, rpc, cluster, ...) are still unstable even though they moved out of unstable/.
Requirements: TypeScript ≥ 5.9 (TS 7 recommended); @effect/vitest needs Vitest 5; @effect/atom-react needs React 19.