File	What changed
`cause.md`	`Cause<E>` is now a flat list of failures instead of a recursive 6-variant tree
`equality.md`	`Equal.equals` now uses structural equality for plain objects/arrays (v3 used reference)
`error-handling.md`	`catchAll` → `catch`; `catchSome*` → `catchFilter` / `catchCauseFilter`
`fiber-keep-alive.md`	Process now auto-stays alive while fibers are suspended; v3 didn't
`fiberref.md`	`FiberRef` / `FiberRefs` / `FiberRefsPatch` / `Differ` removed → use `Context.Reference`
`forking.md`	`fork*` combinators renamed and accept startup-behavior options
`generators.md`	`Effect.gen` no longer takes a `this`/`self` first argument
`layer-memoization.md`	Layer dedup now shared across `Effect.provide` calls (v3 was per-call)
`runtime.md`	`Runtime<R>` value removed; runtime context handled differently
`schema.md`	Big v3 → v4 Schema rename table + complex migrations (1000+ lines). For opinionated patterns, use `effect-schema-v4` skill
`scope.md`	`Scope.extend` → `Scope.provide`
`services.md`	`Context.Tag` / `GenericTag` / `Effect.Tag` / `Effect.Service` → unified `Context.Service`
`yieldable.md`	`Ref` / `Deferred` / `Fiber` / `Config` / `Option` / etc. are no longer `Effect` subtypes; use the new `Yieldable` abstraction

How to use

Find the relevant file by what you're migrating (the table above maps concept → file).
Read it from disk — these are full migration guides with code examples, not summaries: repos/effect/migration/<name>.md.
For schema-specific migration, the dedicated effect-schema-v4 skill in this bundle has the opinionated rename table + classes/unions/transformations patterns. migration/schema.md is the raw upstream reference; effect-schema-v4 is the editorialized cheat sheet.
For non-migration API questions (idiom hunts, examples), use effect-ai-docs to navigate ai-docs/ instead.

Cross-references

effect-schema-v4 skill — Schema rename table, structured patterns
effect-ai-docs skill — module-level docs and idiomatic examples
effect-error-handling skill — opinionated patterns on top of the v4 error API
effect-services-implementation and effect-layer-design skills — patterns for the new Context.Service model

If repos/effect/ doesn't exist, add the Effect source subtree from the consuming project root:

git subtree add --prefix=repos/effect https://github.com/Effect-TS/effect-smol.git main --squash

Do not WebFetch upstream URLs when the subtree is available — read from disk.