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.

1. 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.

1. 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.

1. Place CRUD in shared/api/. CRUD is infrastructure, not entities.
2. 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 entitie

…

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Bbar0n234
• Source: Bbar0n234/learnflow-ai
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.