feature-sliced-design

Feature-Sliced Design (FSD) v2.1

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

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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
same code is currently being used in multiple places (not hypothetically),
the usages do not always change together, and the boundary has a focused
responsibility.

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, listed here from highest to lowest:

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.

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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 currently used in multiple places,
with stable boundaries?

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

Step 4: Is it a business domain model currently used in multiple places,
with stable boundaries?

• 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
same code is actively used in multiple places and the boundary is clear.

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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/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

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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 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";

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.

Environment-specific public APIs

A slice should normally expose its public API through a single index.ts.
Ad-hoc customization is not recommended.

If a single index.ts cannot preserve a runtime boundary, add an
environment-specific 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. 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) => { ... };

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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/. When the
same user data is being consumed by another page (not hypothetically),
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.

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 same code is currently used by multiple
   consumers and the boundary is stable.
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. Place CRUD in shared/api/. CRUD is infrastructure, not entities.
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

// ✅ 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:
// + widgets/   ← UI blocks currently reused across multiple pages
// + features/  ← User interactions currently reused across multiple pages
// + entities/  ← Domain models currently reused across pages or 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

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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 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–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, 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/).
• Do not create a top-level assets/ segment. Place static assets next
  to the code that uses them. See references/asset-handling.md.

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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. Shared domain logic → move to
  entities/, keep UI in features/widgets.
• 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–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.

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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:

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).

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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/: Branding assets shared across the app (use sparingly; see
  references/asset-handling.md)

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.

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10. Quick Reference

• Import direction: app → pages → widgets → features → entities → shared
• Minimal FSD: app/ + pages/ + shared/
• Create entities when: the same business domain model is currently
  used across multiple pages, features, or widgets, with stable
  boundaries.
• Create features when: the same user interaction is currently used
  across multiple pages or widgets, with stable boundaries.
• Breaking rules: Only as an intentional design choice. Document the
  reason in code (comment or ADR).
• Cross-import resolution (entities): Merge boundaries first; @x is a
  necessary compromise, not recommended.
• Cross-import resolution (features/widgets): Strategy A (merge), B
  (push to entities), C (compose from upper layer), or D (Public API).
  The @x notation is for entities only.
• File naming: Domain-based (user.ts, order.ts). Never technical-role
  (types.ts, utils.ts).
• Asset placement: Place next to the code that uses them; reuse goes to
  shared/ui/; global stylesheets and fonts go to app/.
• Slice groups: Optional navigation aid for large layers; group folder
  has no segments and no public API.
• Processes layer: Deprecated. See references/migration-guide.md.

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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, including grouping closely related slices into a
  parent folder for navigation (e.g., "set up project structure", "where does
  this folder go", "how do I group these payment entities"):
  → Read references/layer-structure.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, deciding where authentication data belongs, 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, 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, Nuxt, Vite, CRA, Astro) for wiring routes to FSD pages,
  placing middleware/instrumentation files, structuring API route handlers,
  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, TanStack Query /
  React Query, including query factories, infinite scroll, Suspense mode,
  and useMutationState) 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.