Effect V4

Effect v4

The CLI, @composio/cli-keyring, and @composio/json-schema-to-effect-schema run on Effect v4. 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 4.0.3 — never ^, @next, or a mismatched version across packages. @effect/cli and @effect/platform no longer exist as dependencies; their surfaces are consolidated into effect and effect/*. 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, Option combinators in place of isSome ternaries, driving time in tests with TestClock (fork, adjust until the fiber settles, join) instead of it.live, and the v3→v4 rename table (labeled historical, for recognizing stale patterns).
• references/cli-surface.md — effect/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 release.

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@4.0.3 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.