---
name: effect-v4
description: Write, review, or upgrade Effect v4 code in the Composio CLI, cli-keyring, and json-schema-to-effect-schema packages, all using the matching range effect@^4.1.0 — Context.Service and explicit layers, Schema.TaggedError and typed recovery, the effect/cli command surface, and the vendored effect source oracle. Use when writing or reviewing Effect v4 code, answering a v4 API question, working in effect/cli, defining a Context.Service service, modeling an error with Schema.TaggedError, bumping the Effect pins, or verifying an unfamiliar API against ts/vendor/effect. Do not use for CLI command UX/wiring design (use cli-command) or CLI E2E tests (use cli-e2e).
---
# 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.
## Version matrix
`effect`, `@effect/platform-bun`, and `@effect/vitest`
use the **same range** `^4.1.0`; keep their lockfile resolutions aligned. Do not use
`@next` or mismatched versions across packages. `@effect/cli` and `@effect/platform` no longer exist as
dependencies; their surfaces are consolidated into `effect` and `effect/*`.
See [versions.json](versions.json) for the exact example-check matrix (also `typescript`, `vitest`).
## Read next
- [references/core-patterns.md](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](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](references/upgrade-workflow.md) — procedure for
bumping to a newer release.
References label source excerpts and standalone framework examples separately. Repo-linked
excerpts name the file checked by `validate:cli-skills`. The cited source remains authoritative
when an excerpt or explanation disagrees with the implementation.
## 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@^4.1.0` typings. Source may be ahead of the published package;
the compiler is the compatibility gate.
## Verification
```bash
pnpm typecheck
pnpm --filter @composio/cli run lint:effect
pnpm --filter @composio/cli test
```
Run `bun run validate:agent-skills` and `bun run 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:
```bash
node .agents/skills/effect-v4/scripts/check-examples.mjs
```
Blocks that quote repo files with unresolvable imports carry a `no-check` fence info string.
`bun run validate:cli-skills` also verifies repo-linked command examples, their native Effect client requests, and the selected invocation contracts in Agent Substrate CI. Standalone snippet compilation and routing probes alone do not certify command behavior or agent task performance.