xstate-v5

What this skill does

# XState v5

Use this skill for **state machine and statechart engineering first** and API correctness second.

This skill is **v5-only**. When examples, blog posts, answers, or local code smell v4-ish, translate them rather than mixing versions. Prefer local repo code and official v5 docs over generic memory.

Your job:

- choose between `xstate` and `@xstate/store`
- design a sound machine or actor system from messy requirements
- write modern XState v5 **TypeScript** code in a consistent style
- review, repair, or improve existing XState code
- migrate legacy v4-ish patterns when they appear
- choose the right actor kind and action shape when the problem is not just `assign(...)` plus `fromPromise(...)`
- connect machines and actors to `@xstate/react`, `@xstate/vue`, `@xstate/svelte`, or `@xstate/solid` when needed

## First pass

In an existing codebase:

1. Inspect local `package.json` files and imports.
2. Read nearby machines, stores, actors, and adapter usage.
3. Distinguish between new code, local edits, and migration work before choosing how opinionated to be.
4. Preserve surrounding style by default.

## Task mode

Choose a mode before writing code:

- **New code**: prefer the modern v5 patterns in this skill.
- **Local edit**: preserve local structure and naming unless the current code is mixed, broken, or cleanup was requested.
- **Migration**: prefer the **smallest safe translation** to v5. Preserve structure and semantics unless broader normalization was requested.

For migration and local edits, do not introduce `setup(...)`, named implementations, object-form guards/actions, tags, or actor decomposition unless they are required for correctness, significantly reduce local complexity, or were explicitly requested.

## Choose the tool

Prefer `@xstate/store` when the domain is simple event-based state management:

- no meaningful finite modes that need coordination, orchestration, or explicit lifecycle modeling
- no invoked async processes
- no actor communication
- no need for state machine or statechart concepts such as guarded transitions, delayed transitions, parallel states, or history

Simple fetching or mutation logic can still fit `@xstate/store` if it does not need machine-level orchestration.

Prefer XState when the domain has one or more of these:

- finite modes that matter to behavior or UI
- async workflows, retries, cancellation, or background processes
- multiple interacting processes or child actors
- explicit guards, delays, tags, or transition rules
- a need to model business process flow, not just update state

If the problem is too simple for a machine, say so plainly and recommend `@xstate/store`.

## Workflow

When requirements are fuzzy:

1. Sketch the machine shape first.
2. Name the important states, events, context, actors, and tags.
3. Call out uncertainty or tradeoffs briefly.
4. Then write the code.

When requirements are already clear, keep the sketch short or implicit and move to code.

Do not over-refactor existing code for conceptual purity. Improve the model where it matters, but preserve working structure when a larger rewrite is not justified.

## Modeling questions

Use these questions before writing or revising code:

- What are the finite states? Put **modes** in states, not in context.
- What belongs in context? Keep only durable data, not derivable booleans or duplicated mode.
- What are the domain events? Prefer meaningful names, often dot-separated, such as `form.submitted` or `order.confirmed`.
- What should be a guard versus a separate branch state?
- What side effects should be invoked actors versus transition actions?
- Can an event carry the needed data instead of stashing temporary relay data in context for a later state?
- Is `fromPromise(...)` actually the right actor, or is this a callback/subscription protocol that should send events back over time?
- Should the machine emit events to the surrounding system instead of forcing consumers to infer everything from context?
- Should a child concern be a spawned/invoked actor instead of more parent complexity?
- Does the parent really need these extra intermediate states, or should a child actor own that internal detail?
- Which states need tags for UI semantics such as `'loading'`, `'error'`, or `'dirty'`?
- Can the UI use `snapshot.matches(...)`, tags, or `snapshot.can(...)` instead of extra booleans?

## Preferred v5 patterns

For **new code**, prefer these patterns unless the local codebase has a strong reason not to:

- TypeScript only.
- Prefer `setup({...}).createMachine({...})`.
- Define named `actions`, `guards`, `actors`, and `delays` in `setup(...)`.
- Prefer transition objects like `{ target: 'next' }` over shorthand when it improves consistency.
- Prefer arrays for `actions`, even for a single action, when that keeps the shape consistent.
- Prefer action and guard objects such as `{ type: 'track' }` or `{ type: 'isValid', params: ... }` when the intent is reusable or named.
- Prefer `tags: []` for UI semantics and cross-cutting state meaning.
- Prefer domain-oriented event names; preserve local naming conventions if they are already established.
- In parallel states, keep transitions local to the region or substate whose lifecycle should change. Avoid region-root transitions that target sibling regions or use `reenter: true` unless resetting that whole source region is intentional.
- Prefer plain functions for derived values instead of storing derivable data in context.
- Prefer event payloads over temporary context relay when one state only needs to pass data to the next step.
- Prefer passing data into named actions and guards via `params` instead of reaching into `event` directly.
- Do not treat `assign(...)` as the only action pattern. For typed reusable logic beyond simple context updates, consider setup-scoped helpers from a `const machineSetup = setup(...)` object, such as `machineSetup.createAction(...)`, `machineSetup.enqueueActions(...)`, and `machineSetup.emit(...)`, or named action objects when they fit the behavior better.
- When a named `assign(...)` action updates multiple context properties, every property updater shares the same `params` type. Use one coherent params object for all updated fields, or split the work into separate named actions. Do not mix incompatible params shapes inside one assigner.
- When an implementation must inspect `event` and the type is not obvious, prefer `assertEvent(...)` for narrowing.
- Avoid `as any` and other loose casts in examples and final code unless there is no cleaner local option.
- Choose actor logic intentionally: prefer `fromPromise(...)` for one request/one result, and prefer `fromCallback(...)` for subscriptions, timers, external callbacks, and multi-event protocols that send events back over time.
- Consider `emit(...)` when the machine should notify the surrounding system directly.
- Keep parent states coarse when possible. If extra intermediate states mainly model an implementation detail, prefer letting a child actor own that internal behavior.
- If you show multiple files, wire them completely. Include real imports/exports for referenced modules and components, or keep the example smaller.

Keep event naming consistent. The `.` in event names is useful and meaningful, including for partial event descriptors and clearer domain grouping. See the official docs on [events and transitions](https://stately.ai/docs/transitions) and [TypeScript narrowing with `assertEvent(...)`](https://stately.ai/docs/typescript).

See `references/examples.md` for canonical code shapes, `references/advanced-patterns.md` when the task involves typed actions beyond `assign(...)`, callback actors, emitted events, or persistence/hydration, and `references/observables-and-inspection.md` for `fromObservable(...)`, inspection, and browser inspector patterns.

## UI integration

Prefer letting the machine drive UI behavior:

- use `snapshot.matches(...)` for finite mode checks
- use tags f