Feature-Sliced Design (FSD) v2.1

Source: fsd.how | Strictness can be adjusted based on project scale and team context.

1. Core Philosophy & Layer Overview

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

Place code in pages/ first. Duplication across pages is acceptable and does not automatically require extraction to a lower layer. Extract only when the team agrees it is necessary.

Not all layers are required. Most projects can start with only shared/, pages/, and app/. Add widgets/, features/, entities/ only when they provide clear value. Do not create empty layer folders "just in case."

FSD uses 6 standardized layers with a strict top-down import direction:

app/       → App initialization, providers, routing
pages/     → Route-level composition, owns its own logic
widgets/   → Large composite UI blocks reused across multiple pages
features/  → Reusable user interactions (only when used in 2+ places)
entities/  → Reusable business domain models (only when used in 2+ places)
shared/    → Infrastructure with no business logic (UI kit, utils, API client)

Import rule: A module may only import from layers strictly below it. Cross-imports between slices on the same layer are forbidden.

// ✅ 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 used in only one page → keep it in that page (Steiger: insignificant-slice).

Step 2 — Is it reusable infrastructure with no business logic?

UI components → shared/ui/
Utility functions → shared/lib/
API client, route constants → shared/api/ or shared/config/
Auth tokens, session management → shared/auth/
CRUD operations → shared/api/

Step 3 — Is it a complete user action reused in 2+ places, and does the team agree to extract it?

Yes → features/
Uncertain or single use → keep in the page.

Step 4 — Is it a business domain model reused in 2+ places, and does the team agree to extract it?

Yes → entities/
Uncertain or single use → 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 team agrees.

3. Quick Placement Table

Scenario	Single use	Multi-use (with team agreement)
User profile form	`pages/profile/ui/ProfileForm.tsx`	`features/profile-form/`
Product card	`pages/products/ui/ProductCard.tsx`	`entities/product/ui/ProductCard.tsx`
Product data fetching	`pages/product-detail/api/fetch-product.ts`	`entities/product/api/`
Auth token/session	`shared/auth/` (always)	`shared/auth/` (always)
Auth login form	`pages/login/ui/LoginForm.tsx`	`features/auth/`
CRUD operations	`shared/api/` (always)	`shared/api/` (always)
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`

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 — document the reason and obtain team consensus.

4-1. Import only from lower layers

app → pages → widgets → features → entities → shared. Upward imports and cross-imports between slices on the same layer are forbidden.

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.

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

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

RSC / meta-framework exception: In environments with distinct client/server boundaries, split entry points are permitted (index.client.ts, index.server.ts). See references/framework-integration.md for details.

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. Do not create direct imports.

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

Name files after the business domain they represent, not their technical role. Technical-role names like types.ts, utils.ts, helpers.ts mix unrelated domains in a single file and reduce cohesion.

// ❌ Technical-role naming
model/types.ts          ← Which types? User? Order? Mixed?
model/utils.ts

// ✅ 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.

// ❌ Business logic in shared
// shared/lib/userHelpers.ts
export const calculateUserReputation = (user) => { ... };

// ✅ Move to the owning domain
// entities/user/lib/reputation.ts
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. When extraction seems worthwhile, discuss with the team — this is a design decision that affects the whole project.

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/. When another page needs the same user data and the team agrees, extract the shared model to entities/user/. 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.

Start without entities. shared/ + pages/ + app/ is valid FSD. Thin-client apps rarely need entities.
Do not split slices prematurely. Keep code in pages. Extract to entities only when 2+ consumers are confirmed and the team agrees.
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.
Place CRUD in shared/api/. CRUD is infrastructure, not entities.
Place auth data in shared/auth/ or shared/api/. Tokens and login DTOs are auth-context-dependent and rarely reused outside authentication.

5-3. Start with minimal layers

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

// Add layers only when the team decides they are needed:
// + widgets/   ← UI blocks reused in 2+ pages
// + features/  ← User interactions reused in 2+ pages
// + entities/  ← Domain models reused in 2+ pages/features

5-4. Validate with the Steiger linter

Steiger is the official FSD linter. Key rules:

insignificant-slice: Suggests merging an entity/feature into its page if only one page uses it.
excessive-slicing: Suggests merging or grouping when a layer has too many slices.

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. Use shared/api/. Consider entities only for complex transactional logic.
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 last resort, not a recommended pattern (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, widgets, pages) — 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/).

7. Cross-Import Resolution Order

When two slices on the same layer need to share code, try these strategies in order. Always attempt earlier strategies first.

Merge slices — If two slices always change together, they likely belong in one slice.
Extract shared logic to entities — If multiple features/widgets share domain logic, move that logic to entities/. Keep UI in features/widgets.
Compose in a higher layer (IoC) — Use inversion of control. The parent layer (pages or app) imports both slices and connects them via render props, slots, or dependency injection.
@x notation (last resort) — Create explicit, controlled cross-imports between entities only. Document why other strategies do not apply. Review periodically.

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.

File naming within segments

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

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/ — Images, fonts, icons

Shared may contain application-aware code (route constants, API endpoints, branding assets, common types). It must never contain business logic, feature-specific code, or entity-specific code.

10. Quick Reference

Import direction: app → pages → widgets → features → entities → shared
Minimal FSD: app/ + pages/ + shared/
Create entities when: 2+ pages/features/widgets share the same business domain model, and the team agrees.
Create features when: 2+ pages/widgets share the same user interaction, and the team agrees.
Breaking rules: Only as an intentional design choice — document the reason, get team consensus.
Cross-import resolution: Merge → Extract to entities → Compose in higher layer → @x (last resort)
File naming: Domain-based (user.ts, order.ts). Never technical-role (types.ts, utils.ts).
Processes layer: Deprecated. See references/migration-guide.md.

11. Conditional References

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

When creating, reviewing, or reorganizing folder and file structure for FSD layers and slices (e.g., "set up project structure", "where does this folder go"): → Read references/layer-structure.md
When resolving cross-import issues between slices on the same layer, evaluating the @x pattern, or dealing with excessive entity coupling: → Read references/cross-import-patterns.md
When migrating from FSD v2.0 to v2.1, converting a non-FSD codebase to FSD, or deprecating the processes layer: → Read references/migration-guide.md
When integrating FSD with a specific framework (Next.js, Nuxt, Vite, CRA) or configuring path aliases: → Read references/framework-integration.md
When implementing concrete code patterns for authentication, API request handling, type definitions, or state management (Redux, React Query) within FSD structure: → Read references/practical-examples.md Note: If you already loaded layer-structure.md in this conversation, avoid loading this file simultaneously. Address structure first, then load patterns in a follow-up step if needed.

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