Effect V4

Effect v4

The CLI, @composio/cli-keyring, and @composio/json-schema-to-effect-schema run on Effect v4 (release candidate). Every claim below is grounded in the migrated source, not memory of v3.

Exact version matrix

effect, @effect/platform-bun, and @effect/vitest are pinned to the same exact 4.0.0-rc.112 — never ^, @next, or a mismatched prerelease across packages. @effect/cli and @effect/platform no longer exist as dependencies; their surfaces are consolidated into effect and effect/unstable/*. See versions.json for the full matrix (also typescript, vitest).

Read next

• references/core-patterns.md — services, layers, typed errors, Schema, choosing between Effect.gen and Effect.fn, and the v3→v4 rename table (labeled historical, for recognizing stale patterns).
• references/cli-surface.md — effect/unstable/cli: Command, Flag, Argument, GlobalFlag, CliConfig, the custom CliOutput.Formatter, and the runner's double-print rule.
• references/upgrade-workflow.md — procedure for bumping to a newer prerelease.

Code excerpts in those references are short quotes from real, currently-compiling repo files (path cited at each excerpt) — not standalone examples. The compile-checked source of truth is always the cited file itself; when it and a reference disagree, trust the file and fix the reference.

Non-negotiables

• Effect.gen(function* () {...}) for effect values — the dominant form, including named module consts — and (params) => Effect.gen(...) for parameterized helpers. Effect.fn(...) is the function form whose effects carry stack-frame annotations (its optional name string adds a per-call tracing span), worth it for service members and combinator callbacks that should be attributable in error reports. All forms re-run their body per execution. See "Effect.gen vs Effect.fn" in core-patterns.
• Define services with Context.Service and an explicit static readonly Default/layer layer built with Layer.succeed/Layer.effect/Layer.provide. V4 does not generate a layer for you.
• Model expected failures with Schema.TaggedError (or a plain Data.TaggedError when no Schema fields are needed) and recover with Effect.catchTag/catchTags/Match, never manual _tag string comparisons.
• Wrap fallible Promises with Effect.tryPromise({ try, catch }); Effect.promise turns rejection into a defect. No async/await or try/catch inside Effect workflows — ESLint bans them in ts/packages/cli/src.
• Treat every remembered v3 package name and API as wrong until verified against ts/vendor/effect (read-only source oracle — never edit or import from it) and the installed effect@4.0.0-rc.112 typings. Source may be ahead of the published package; the compiler is the compatibility gate.

Verification

pnpm typecheck
pnpm --filter @composio/cli test

Run pnpm validate:agent-skills and pnpm validate:skill-routing after editing this skill or its descriptions, and compile the TypeScript blocks in this skill and in typescript-testing/references/effect-v4-cli.md against the pinned packages with:

node .agents/skills/effect-v4/scripts/check-examples.mjs

Blocks that quote repo files with unresolvable imports carry a no-check fence info string.