feature-sliced-design

Summary

---
name: feature-sliced-design
description: >
  Official Feature-Sliced Design (FSD) v2.1 skill for applying the methodology
  to frontend projects. Use when the task involves organizing project structure
  with FSD layers, deciding where code belongs, placing static assets (images,
  icons, fonts, PDFs), grouping closely related slices, defining public APIs
  and import boundaries, resolving cross-imports or evaluating the @x pattern,
  deciding whether to create or remove an entity, evaluating whether the
  entities layer is needed at all, deciding where page layouts belong or
  whether to use the widgets layer (discouraged), deciding whether logic
  should remain local or be extracted, migrating from FSD v2.0 or a non-FSD
  codebase, integrating FSD with frameworks (Next.js App Router and Pages
  Router, React Router, Nuxt, Vite, Astro), or implementing common patterns
  such as authentication, API handling, Redux, and TanStack Query
  (React Query) within FSD.
---

# Feature-Sliced Design (FSD) v2.1

> **Source**: [fsd.how](https://fsd.how) | Strictness can be adjusted based on
> project scale and team context.

**How to use this skill.** For placement decisions, start with the decision
tree in Section 2 and use the placement table in Section 3 as a quick
reference. To check a structure for violations, use the rules in Section 4.
To resolve same-layer cross-imports, use Section 7. For task-specific
guidance, load only the relevant reference files from Section 10; do not
preload the rest.

## 1. Core philosophy & layer overview

FSD v2.1 core principle: **"Start simple, extract when needed."**

### The extraction rule

Place code in `pages/` first. Duplication across pages is acceptable and
does not by itself require extraction to a lower layer. Extract only when
all three conditions hold:

1. The same code is used in multiple places right now, not hypothetically.
2. It has a reason to change that is independent of any one consumer.
3. The boundary has a focused responsibility.

### The six layers

**Not all layers are required.** Most projects can start with only `shared/`,
`pages/`, and `app/`. Add `features/` and `entities/` only when they provide
clear value. Do not create empty layer folders "just in case." The `widgets/`
layer is **discouraged** (see the callout below).

FSD uses 6 standardized layers, listed here from highest to lowest:

```text
app/       → App initialization, providers, routing
pages/     → Route-level composition, owns its own logic
widgets/   → Reusable UI blocks (discouraged, see the callout below)
features/  → Reusable user interactions (see the extraction rule above)
entities/  → Reusable business domain models (see the extraction rule above)
shared/    → Infrastructure with no business logic (UI kit, utils, API client)
```

**The official layer reference discourages using the Widgets layer**, and
this skill follows it. Widgets may seem useful for representing independent
UI blocks. However, in real frontend code, UI blocks often include logic
required for user flows, such as data fetching, state management, and event
handling. In this case, the responsibilities of Features, which handle user
flows, and Widgets, which handle UI blocks, can overlap, making the boundary
between the two layers unclear.

Not creating a widget does not mean moving the block elsewhere untouched.
A screen-specific composition stays in `pages`; a reused action and the UI
to perform it go to `features`; context-free UI goes to `shared`; an
app-wide layout goes to `app`.

Discouraged is not deprecated: an existing widgets layer stays valid. See
`references/layer-structure.md` for that case and for layout placement.

### The import rule

A module may only import from layers strictly below it. Cross-imports
between slices on the same layer are forbidden, with one narrow exception
in Section 7.

```typescript
// Allowed
import { Button } from "@/shared/ui/Button"; // features → shared
import { useUser } from "@/entities/user"; // pages → entities

// Violation
import { loginUser } from "@/features/auth"; // entities → features
import { likePost } from "@/features/like-post"; // features → features
```

**Note**: The `processes/` layer is **deprecated** in v2.1. For migration
details, read `references/migration-guide.md`.

## 2. Decision framework

When writing new code, follow this tree:

**Step 1: Where is this code used?**

- Used in only one page → keep it in that `pages/` slice.
- Used in 2+ pages but duplication is manageable → keeping separate copies
  in each page is also valid.
- An entity or feature with a single consumer → keep it there (Steiger
  flags this as `insignificant-slice`).

**Step 2: Is it reusable infrastructure with no business logic?**

The official layer reference draws the line for Shared like this: no
business logic, but business-themed is fine (a company logo, a page
layout), and so is UI logic (autocomplete, a search bar). Exchanging data
with the backend and CRUD boilerplate are not business logic either.
Business logic is a rule the product enforces on its own data, such as
applying a discount to an order. If the code fits none of the exclusions
and still does not clearly enforce a product rule, the term does not
decide; go back to Step 1 and place it by where it is used.

- UI components → `shared/ui/`
- Utility functions → `shared/lib/`
- API client, route constants → `shared/api/` or `shared/config/`
- Auth tokens, session management → `shared/auth/`
- CRUD once several slices call it → `shared/api/` (a single caller keeps
  it, see Step 1)

**Step 3: Is it a complete user action that several consumers share, with
a focused responsibility and a reason to change of its own?**

- Yes → `features/`
- Uncertain, single use, or speculative reuse → keep in the page.

**Step 4: Is it a business domain model that several consumers share, with
a focused responsibility and a reason to change of its own?**

- Yes → `entities/`
- Uncertain, single use, or speculative reuse → keep in the page.

**Step 5: Is it app-wide configuration?**

- Global providers, router, theme → `app/`

**Golden Rule: When in doubt, keep it in `pages/`. Extract only when the
extraction rule holds.**

## 3. Quick placement table

| Scenario                   | Single use                                  | Confirmed multi-use                   |
| -------------------------- | ------------------------------------------- | ------------------------------------- |
| User profile form          | `pages/profile/ui/ProfileForm.tsx`          | `features/profile-form/`              |
| Product card               | `pages/products/ui/ProductCard.tsx`         | `entities/product/ui/` if the entity owns it |
| API request (read or CRUD) | `pages/product-detail/api/fetch-product.ts` | `shared/api/` (no domain rules)       |
| Auth token/session         | `shared/auth/`                              | `shared/auth/`                        |
| Auth login form            | `pages/login/ui/LoginForm.tsx`              | `features/auth/`                      |
| Generic Card layout        |                                             | `shared/ui/Card/`                     |
| Modal manager              |                                             | `shared/ui/modal-manager/`            |
| Modal content              | `pages/[page]/ui/SomeModal.tsx`             |                                       |
| Date formatting util       |                                             | `shared/lib/format-date.ts`           |

"Confirmed multi-use" means the extraction rule holds, not that a second
consumer appeared: two similar copies that keep drifting apart stay in
their pages (`references/growth-walkthrough.md`, Snapshot 1). Entity UI
carries the Section 6 caution even when the rule does hold.

## 4. Architectural rules (MUST)

These rules are the foundation of FSD. Violations weaken the architecture.
If you must break a rule, ensure it is an intentional design decision and
document the reason in code (a comment or ADR).

### 4-1. Import only from lower layers

`app → pages → widgets → features → entities → shared`.
Upward imports are forbidden. So are cross-imports between slices on the
same layer, except through the other slice's public API as a last resort
(Section 7, Strategy D).

### 4-2. Public API: every slice exports through index.ts

External consumers may only import from a slice's `index.ts`. Direct imports
of internal files are forbidden.

```typescript
// Correct
import { LoginForm } from "@/features/auth";

// Violation: bypasses public API
import { LoginForm } from "@/features/auth/ui/LoginForm";
```

**Shared layer:** Shared has no slices. Define a separate public API per
segment (`shared/ui/index.ts`, `shared/api/index.ts`, etc.) rather than
one top-level `shared/index.ts`. This keeps imports from Shared
organized by intent.

Where one index over a segment's unrelated modules hurts bundling, give
each component, library, or controller folder its own index instead
(`shared/ui/Button/index.ts` as `@/shared/ui/Button`, `shared/api/post/`
as `@/shared/api/post`). That folder is then the boundary; reaching past
it (`@/shared/ui/Button/Button.tsx`) is still a violation. See
`references/layer-structure.md` for the shape.

**Environment-specific entry points:** a slice normally exposes one
`index.ts`, and ad-hoc variations are not recommended. If a single index
cannot preserve a runtime boundary, add an entry point such as
`index.server.ts`. See `references/framework-integration.md`.

### 4-3. No cross-imports between slices on the same layer

If two slices on the same layer need to share logic, follow the resolution
order in Section 7. Never reach into another slice's internals.

### 4-4. Domain-based file naming (no desegmentation)

Name files after what they are for, the domain or concern they serve, not
after their technical role. Technical-role names like `types.ts`,
`utils.ts`, `helpers.ts` mix unrelated concerns in a single file and
reduce cohesion.

```text
// BAD: technical-role naming
model/types.ts          ← Which types? User? Order? Mixed?
model/utils.ts

// GOOD: domain-based naming
model/user.ts           ← User types + related logic
model/order.ts          ← Order types + related logic
api/fetch-profile.ts    ← Clear purpose
```

### 4-5. No business logic in shared/

Shared contains only infrastructure: UI kit, utilities, API client setup,
route constants, assets. Business calculations, domain rules, and workflows
belong in `entities/` or higher layers. Section 2, Step 2 says what counts.

```typescript
// BAD: business logic in shared
// shared/lib/userHelpers.ts
export const calculateUserReputation = (user) => { ... };

// GOOD: move it to whoever owns the rule
// pages/profile/model/reputation.ts       ← while the profile page owns it
// entities/user/model/reputation.ts       ← once a user boundary is earned
export const calculateUserReputation = (user) => { ... };
```

## 5. Recommendations (SHOULD)

### 5-1. Pages first: place code where it is used

Place code in `pages/` first. Extract to lower layers only when truly needed.
Extraction is a design decision that affects the whole project, so the
threshold should be high.

**What stays in pages:**

- Large UI blocks used only in one page
- Page-specific forms, validation, data fetching, state management
- Page-specific business logic and API integrations
- Code that looks reusable but is simpler to keep local

**Evolution pattern:** Start with everything in `pages/profile/`. Extract
the shared model to `entities/user/` when a second page consumes it *and*
the extraction rule holds. A response type that several pages read is not
one of those cases: it stays in `shared/api`. Keep page-specific API calls
and UI in the page.

### 5-2. Be conservative with entities

The entities layer is highly accessible (almost every other layer can import
from it), so changes propagate widely.

1. **Start without entities.** `shared/` + `pages/` + `app/` is valid FSD.
   Thin-client apps rarely need entities.
2. **Do not split slices prematurely.** Keep code in pages. Extract to
   entities only when the extraction rule holds.
3. **Business logic does not automatically require an entity.** Keeping types
   in `shared/api` and logic in the current slice's `model/` segment may
   be sufficient.
4. **CRUD is infrastructure, not entities.** Place it by the request
   placement rule: with its consumer while there is one, in `shared/api/`
   once several slices call it.
5. **Place auth data in `shared/auth/` or `shared/api/`.** Tokens and login
   DTOs are auth-context-dependent and rarely reused outside authentication.

For detailed guidance on keeping the entities layer clean (when to skip
it entirely, how to isolate business contexts, why CRUD belongs in
`shared/api`), see `references/excessive-entities.md`.

### 5-3. Start with minimal layers

```text
// Valid minimal FSD project
src/
  app/         ← Providers, routing
  pages/       ← All page-level code
  shared/      ← UI kit, utils, API client

// Add layers only when an actual use case requires them:
// + features/  ← User-action boundaries that need one shared home
// + entities/  ← Domain boundaries that need one shared home
// (widgets/ is discouraged; see Section 1 for where that code goes instead)
```

### 5-4. Validate with the Steiger linter

[Steiger](https://github.com/feature-sliced/steiger) is the official FSD
linter. Key rules:

- **`insignificant-slice`**: Flags a slice with no references, or with one,
  and suggests merging it into the layer above. Pages may hold a single
  reference, and so may slices used only from `app/`.
- **`excessive-slicing`**: Suggests merging or grouping when a layer has too
  many slices.

```bash
npm install -D @feature-sliced/steiger
npx steiger src
```

## 6. Anti-patterns (AVOID)

- **Do not create entities prematurely.** Data structures used in only one
  place belong in that place.
- **Do not put CRUD in entities.** Plain CRUD is `shared/api/`. An
  operation that carries business rules is placed by who owns the rule,
  which may be an entity, a feature, or the page running the workflow.
- **Do not create a `user` entity just for auth data.** Tokens and login DTOs
  belong in `shared/auth/` or `shared/api/`.
- **Do not abuse `@x`.** It is a necessary compromise, not a recommended
  pattern. The notation is for the entities layer only, and only when
  boundary merge is genuinely impossible. Features and widgets handle
  cross-imports through strategies A through D (see Section 7).
- **Do not extract single-use code.** A feature or entity used by only one
  page should stay in that page.
- **Do not use technical-role file names.** Use domain-based names
  (see Rule 4-4).
- **Be cautious adding UI to entities.** Entity UI tempts cross-imports from
  other entities. If you add UI segments to entities, only import them from
  higher layers (features, pages, app), never from other entities.
- **Do not create god slices.** Slices with excessively broad responsibilities
  should be split into focused slices (e.g., split `user-management/` into
  `auth/`, `profile-edit/`, `password-reset/`).
- **Do not create a top-level `assets/` segment.** Place static assets next
  to the code that uses them; global stylesheets and fonts go to `app/`.
  See `references/asset-handling.md`.

## 7. Cross-import resolution

Cross-imports are a code smell, not an absolute prohibition. The right
strategy depends on the layer and the situation.

### Entities layer: prefer boundary merge, @x is last resort

Cross-imports in `entities` are usually caused by splitting entities too
granularly. Before reaching for `@x`, consider whether the boundaries
should be merged.

`@x` is a **necessary compromise, not a recommended approach**. Use it only
when boundaries genuinely cannot be merged, and document why. Overuse locks
entity boundaries together and increases refactoring cost.

### Features and widgets: four strategies (A, B, C, D)

In `features` and `widgets`, choose based on context:

- **Strategy A: slice merge.** Two slices always change together → merge.
- **Strategy B: push to entities.** A shared domain responsibility → move
  it to the entity that owns it, keep UI in the feature.
- **Strategy C: compose from upper layer (IoC).** The parent (pages or app)
  imports both slices and connects them via render props, slots, or DI.
- **Strategy D: Public API access.** When reuse is genuinely unavoidable,
  allow it only through the slice's `index.ts`. Never reach into `model/`,
  `store/`, or internal files.

The `@x` notation is for the entities layer only. Features and widgets use
strategies A through D above.

### Strictness depends on project context

Cross-imports are dependencies that are generally best avoided, but
sometimes used intentionally. Strictness varies by project context:

- **Early-stage products** with heavy experimentation: allowing some
  cross-imports may be a pragmatic speed trade-off.
- **Long-lived or regulated systems** (fintech, large-scale services):
  stricter boundaries pay off in maintainability and stability.

If a cross-import is introduced, treat it as a deliberate choice and
document the reasoning in code (a comment explaining why other strategies
do not apply).

For detailed code examples of each strategy, read
`references/cross-import-patterns.md`.

## 8. Segments & structure rules

### Standard segments

Segments group code within a slice by technical purpose:

- **`ui/`**: UI components, styles, display-related code
- **`model/`**: Data models, state stores, business logic, validation
- **`api/`**: Backend integration, request functions, API-specific types
- **`lib/`**: Internal utility functions for this slice
- **`config/`**: Configuration, feature flags

### Layer structure rules

- **App and Shared**: No slices, organized directly by segments. Segments
  within these layers may import from each other.
- **Pages, Widgets, Features, Entities**: Slices first, then segments inside
  each slice.
- **Slice groups (optional)**: A group folder may contain related slices on
  the same layer for navigation purposes only. The group has no segments and
  no public API. See `references/layer-structure.md` for details.

### File naming within segments

Always use domain-based names that describe what the code is about:

```text
model/user.ts            ← User types + logic + store
model/order.ts           ← Order types + logic + store
api/fetch-profile.ts     ← Profile fetching
api/update-settings.ts   ← Settings update
```

If a segment has only one domain concern, the filename may match the slice
name (e.g., `features/auth/model/auth.ts`).

## 9. Shared layer guide

Shared contains infrastructure with **no business logic**. It is organized by
segments only (no slices). Segments within shared may import from each other.

**Allowed in shared:**

- `ui/`: UI kit (Button, Input, Modal, Card)
- `lib/`: Utilities (formatDate, debounce, classnames)
- `api/`: API client, route constants, CRUD helpers, base types
- `auth/`: Auth tokens, login utilities, session management
- `config/`: Environment variables, app settings
- Assets live with the code that uses them, not in an `assets/` segment.
  See `references/asset-handling.md`.

Shared **may** contain application-aware code: route constants, API
endpoints, branding assets, and transport types such as `ProductDTO`.
It must **never** hold the business rules an entity or feature owns, nor
import from those layers.

## 10. Conditional references

Read the following reference files **only** when the specific situation applies.
Do **not** preload all references.

- **When reviewing or reorganizing folder and file structure** that already
  exists, deciding what goes inside a layer or slice, deciding where a page
  layout belongs, routing widget-like code to another layer, or grouping
  closely related slices into a parent folder for navigation (e.g., "where
  does this folder go", "how do I group these payment entities"):
  → Read `references/layer-structure.md`

- **When setting up a new project from scratch** (e.g., "set up an FSD
  project", "start a new app with FSD"), or when asked whether to add
  entities or features yet, or to show how a structure earns each layer
  over time rather than its finished shape:
  → Read `references/growth-walkthrough.md`

- **When resolving cross-import issues** between slices on the same layer,
  evaluating the `@x` pattern, choosing between Strategy A/B/C/D for
  features and widgets, or deciding whether boundaries should be merged:
  → Read `references/cross-import-patterns.md`

- **When deciding whether to create or remove an entity**, dealing with too
  many entities, evaluating whether to skip the entities layer entirely,
  placing CRUD operations, or isolating business contexts to avoid `@x`
  chains:
  → Read `references/excessive-entities.md`

- **When deciding where to place static assets** (images, icons, fonts,
  PDFs, stylesheets) for a single slice, for sharing across slices, or
  globally:
  → Read `references/asset-handling.md`

- **When migrating** from FSD v2.0 to v2.1, converting a non-FSD codebase to
  FSD, phasing out an existing widgets layer, or deprecating the processes
  layer:
  → Read `references/migration-guide.md`

- **When integrating FSD with a specific framework** (Next.js with App Router
  or Pages Router, React Router, Nuxt, Vite, Astro) for wiring routes to
  FSD pages, placing proxy/middleware and instrumentation files,
  structuring API
  route handlers, or configuring path aliases:
  → Read `references/framework-integration.md`

- **When implementing authentication, type definitions, or API request
  handling** as concrete code within FSD structure (token storage, login
  flow, DTO placement, where a request function lives):
  → Read `references/auth-and-api.md`

- **When wiring state management** (Redux slices, TanStack Query / React
  Query, including query factories, infinite scroll, Suspense mode, and
  `useMutationState`) into FSD structure:
  → Read `references/state-management.md`

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