# Shared Layer Architecture (`src/shared` / `@shared`)

This is the canonical reference for what belongs in `@shared`, how it is organized, and the rules that keep its top level from sprawling. It owns the `@shared`-internal rules; [Architecture Overview](./architecture-overview.md), [Renderer Architecture](./renderer-architecture.md), and [Naming Conventions](./naming-conventions.md) reference it.

`@shared` is the **cross-process primitive layer** — Layer 4 in [Renderer Architecture §2](./renderer-architecture.md). It depends on no app code and is importable by `main`, `renderer`, and `preload`.

## 1. Two Invariants

Everything in `@shared` must satisfy **both**, or it does not belong here.

### 1.1 Cross-process

A module belongs in `@shared` only if **both** `main` and `renderer` actually use it — types included, with one deliberate carve-out: the Cache schema registry (§1.1.1).

- **Why**: `@shared` is the single source of truth shared across the process boundary; single-process code already has a process to live in.
- Reachable from only one process → it lives in that process's own layer (`src/main/*` or `src/renderer/{utils,hooks,services}`).
- **No speculative placement.** If something only *might* become cross-process, write it in `main`/`renderer` first and move it here once it actually crosses. Do not park it in `@shared` for a possibility — the common failure is a type or util added "in case", then never used cross-process and left as cruft.

#### 1.1.1 Carve-out: the Cache schema registry

The Cache subsystem is the one exception to §1.1. Every Cache **key schema and its value type** lives in `@shared/data/cache/` (`cacheSchemas.ts` + `cacheValueTypes.ts`) **regardless of which process consumes it** — including renderer-only types (`Tab`, `ChatScrollAnchor`, `AgentOpenExternalAppTarget`, …). A renderer-only cache value type here is **compliant, not a §1.1 violation** — do not flag or relocate it. Cache subsystem only; §1.1 holds everywhere else.

### 1.2 No mutable runtime state

`@shared` exports **types, pure functions, and immutable data only**. It exports **no class-instance singletons** (services / managers / registries) and nothing that holds runtime-mutable state.

- **Why**: `main` and `renderer` are separate V8 realms; an `@shared` module is loaded **once per process**. A "shared singleton" is a fiction — it silently becomes N per-process instances that diverge. Mutable state has no coherent shared owner; it belongs to the process whose lifecycle and context it reflects.
- **`new` is not the test — runtime mutability + identity is.** `new` is allowed only to build immutable data that is then frozen and exported (a `Map` / `Set` / `RegExp` lookup built once from static data and never mutated, e.g. the private `commandMap` in `command/definitions.ts`).
- A stateful class ships only its **definition (blueprint)** from `@shared`; its **instance** is created per-process. Example: `ContextKeyService` is defined cross-process, but `new ContextKeyService()` lives in the renderer's `CommandContextKeyProvider`.

| Allowed | Banned |
|---|---|
| `type` / `interface` / `enum`, schema-derived types | `export const x = new XService()` (any exported instance singleton) |
| pure functions, predicates, converters | a registry / manager / service instance |
| immutable data — consts, definitions, frozen lookups built via `new Map`/`Set` | any module-level value holding runtime-mutable state |
| stateful-class **definitions** (blueprints) | a live **instance** of such a class |

## 2. The Closed Top-Level Set

The top level is a **closed set** — this is [Naming Conventions §4.8](./naming-conventions.md) (top-level closed by default) applied to `@shared`. Exactly these five, by three principled categories:

| Dir | Category | Why it earns a top-level home |
|---|---|---|
| `ai` | **Core domain** | Cherry Studio *is* an AI product; AI's cross-process contracts and pure logic are first-class (mirrors `src/main/ai/`). Holds AI's cross-process slice only — not AI UI or per-process services. |
| `data` | **Cross-process infra** | The data layer's cross-process contracts: API entity/request types, cache/preference/bootConfig schemas, migration mappings, presets. Framework-like, domain-agnostic. |
| `ipc` | **Cross-process infra** | The IpcApi framework: route `define` helpers, request + event schemas, error model, and shared types (`IpcContext`, `WindowId`). Domain-agnostic. |
| `types` | **Shape bucket** | Cross-process type declarations with no single owner. |
| `utils` | **Shape bucket** | Cross-process pure logic plus its supporting constants and class blueprints. |

**Governance rule**: a new capability **never** earns a new top-level dir. It is either (a) the core domain [only `ai`], (b) genuine cross-process infrastructure, or (c) decomposed **by shape** into `types` / `utils`. Anything else → `types` / `utils`.

Naming respects [§4.9](./naming-conventions.md): `ai` / `data` / `ipc` are singular namespaces, `types` / `utils` are plural buckets.

## 3. Shape: `types` vs `utils`

`@shared` has only **two** shape buckets. With no UI, no React, and no per-process runtime, the renderer's rich shapes (`components` / `hooks` / `services` / `pages`) collapse to **declarations vs pure logic**.

| `types/` | `utils/` |
|---|---|
| type aliases, interfaces, enums, schema-derived types | pure functions, predicates, converters |
| (plus the small consts a type needs) | plus the constants / static data those functions need, and stateful-class blueprints |

Routing between the two follows the [Naming Conventions §5.2](./naming-conventions.md) route-by-shape table.

### 3.1 File vs subdirectory, and barrels

- **A single `.ts` file is the default.** Most topics are one file — `types/<topic>.ts`, `utils/<topic>.ts`, imported directly. Promote to a subdirectory only when the topic actually owns multiple files ([Naming Conventions §4.4](./naming-conventions.md)); never pre-create one.
- **A topic subdirectory has exactly one `index.ts`** as its public API — `types/<topic>/index.ts`, `utils/<topic>/index.ts`, explicit named exports, no `export *`. The import surface is then identical whether the topic is a file or a subdir (`@shared/utils/<topic>` either way), and the subdir's other files stay private behind it.
- **The bucket roots `types/` and `utils/` have no `index.ts`.** A bucket is a category, not a module — a root barrel re-exporting every file buys no aggregate API and only adds churn and import-cycle risk on every addition. Import the specific file or topic, never the bucket.
- **`types/` has no runtime tests.** A declarations bucket has no runtime behavior to test, so a *behavioral* test under `types/` (`expect(fn(...))…`) signals the file holds logic — a predicate, type guard, converter, factory, or function — that belongs in `utils/` (route-by-shape, §3). Move the logic to `utils/<topic>.ts` (importing the types it needs from `types/`, the blessed `utils → types` direction) and the test follows it. Type guards (`x is T`) are runtime predicates too — co-locate them with the logic in `utils/`, not with the interface in `types/`. Schemas built from a validator function (`z.custom(isFoo)`) follow the function to `utils/`; a purely declarative schema (`z.object({…})`) may stay in `types/`. The **one** test that does belong in `types/` is a **type-level test** (`expectTypeOf` / `assertType`): it asserts a type contract itself and has no runtime to relocate. Such a test is a **transitional guard** — it earns its place only while a hand-written type is the source of truth; once a runtime schema (Zod / IpcApi) owns the contract and the type is `z.infer`-derived, the schema's own validation subsumes it and the type-level test retires with that migration.

### 3.2 Constants & static data

- **Default: a constant lives in its domain/topic single-file**, beside the logic it serves (AI model defaults → `ai/`; a file-type list → `utils/file/`).
- **`utils/constants.ts` is NOT a bucket.** It carries only the genuinely **global, cross-process** residue (`KB`/`MB`/`GB`, `APP_NAME`). Add to it only when you are 100% certain a constant is app-global and cross-cutting; if it belongs to any domain, it goes in that domain's file. — **Why**: this is precisely the guardrail the old `config/constant.ts` lacked, which is how it grew into an 82-importer junk drawer (now dissolved — §6).
- **Single-process constants → leave `@shared`** (Invariant 1.1).
- **There is no `config/` bucket.** A constant is data; a frozen value in its domain file (or `utils/`) expresses everything a `config/` dir would, without inviting unrelated globals.

### 3.3 Stateful-class blueprints

A stateful class's **definition** is pure code, so it rides in its topic module under `utils/` — precedent: the stateful `MatchPatternMap` class in `utils/blacklistMatchPattern.ts`. `@shared` has **no `services/` bucket**, because services are per-process (Invariant 1.2).

## 4. Placement Decision

Two gates, in order, then categorize:

1. **Cross-process?** Reached by both processes — no → it goes to a process layer (`src/main/*` or `src/renderer/*`). *(Carve-out: a Cache key's schema entry + value type stay in `@shared/data/cache/` even when single-process — §1.1.1.)*
2. **Stateless / immutable?** No exported instance, no mutable state — no → only the blueprint and static data stay; the **instance** goes per-process.
3. **Categorize**: core domain (`ai`) / infra (`data`, `ipc`) / shape (`types`, `utils`). Not one of the first two → decompose by shape into `types` / `utils`; **never** open a new top-level dir.

## 5. Anti-Patterns

- **Exported instance singleton** — `export const x = new XService()`, or any registry / manager / service instance. Violates Invariant 1.2.
- **Single-process code in `@shared`** — main-only or renderer-only logic placed here for convenience. Violates Invariant 1.1. *(Former epicenter: the now-dissolved `config/constant.ts` — §6. The Cache schema registry is the one sanctioned exception — §1.1.1.)*
- **Junk-drawer file or dir** — a `config/` bucket or a `constant.ts` accumulating unrelated globals across domains and processes. Decompose by domain + process; do not relocate as a blob.
- **A new top-level dir per capability** — every capability decomposes by shape; the top level is closed (§2).
- **A stateful "service" in `@shared`** — state has no coherent shared owner; it belongs to `main` or `renderer`.

## 6. Migration (target vs current — deferred, tracked)

The structural decomposition is **done**: `command`, `file`, `shortcuts`, `externalApp`, and `config` were dissolved out of the top level — cross-process slices into `types/` + `utils/` by shape, single-process code back into `main`/`renderer` (Invariant 1.1) — and `menuRegistry`'s exported instance was replaced by the pure `resolveMenu` (Invariant 1.2). `config`'s ~82-importer `constant.ts` was decomposed by domain + process (file-ext lists → `utils/file/`, `KB`/`MB`/`GB`/`APP_NAME` → `utils/constants.ts`, terminal/update/OAuth/timeout/window-sizing blocks back to their owning `main`/`renderer` modules); the actual consumer process was confirmed per item rather than trusted from a directional plan (e.g. `API_SERVER_DEFAULTS` proved renderer-only, `MIN_WINDOW_*` cross-process, `providers.ts` renderer-only). The `utils/index.ts` bucket-root barrel was later split into topic files (§3.1). A subsequent `types`/`utils` audit confirmed the single-process residue in the table below and removed dead code (`types/codeTools.ts`'s unused `LoaderReturn`, which also dragged a `@types` renderer import into `@shared` — a layering violation now gone); `keywordSearch` and `SerializableSchema` looked main-only/dead on `main` but proved cross-process against the `feat/chat-page` truth branch (renderer `GlobalSearch`, `renderer/types/serialize.ts`) and correctly stay. A later pass cleared three of the rows below — `searchSnippet.ts` and `pdf.ts` moved to `src/main/utils/` (searchSnippet is a generic, DB-free text helper, so it landed in `main/utils/` rather than the `data/services/utils/` first pencilled in), and `externalApp.ts`'s `EXTERNAL_APPS` const was inlined into `ExternalAppsService` — and switched the `types/file`, `utils/file`, `utils/command`, and `utils/api` topic barrels from `export *` to explicit named exports (§3.1). The error/serializable cluster was then resolved by routing each part to its real home: the genuinely cross-process `SerializedError` shape stays in `@shared/types/error.ts`, the write-side `serializeError` moved to `src/main/ai/utils/`, and the AI-SDK subtype/guard family — dead on the `@shared` side and ~98% duplicated by the renderer's own copy — was deleted along with the unused `ProviderSpecificError` (the renderer keeps its live parallel copy). Remaining deviations:

| Area | Current | Target |
|---|---|---|
| `data/types/` converters/guards — `coerceSearchRole`, `deriveRootSpanId`, `readCherryMeta`/`withCherryMeta`, `knowledge.ts` string helpers | logic living in the `data` type bucket; behavioral tests under `data/types/__tests__/` flag it (§3.1, last paragraph) | open question: route-by-shape would move them to a `utils` location, but schema-derived guards are conventionally co-located — decision deferred |
| `IpcChannel.ts` | 18 KB v1 channel enum at the root | v1 legacy; folded into `ipc/` as the IpcApi migration retires channels — not part of this governance |

## 7. Related

- [Architecture Overview](./architecture-overview.md) — process model and the `@shared` one-line summary.
- [Renderer Architecture §2–§3](./renderer-architecture.md) — the layer model and how the renderer depends on `@shared`; §6 owns command's **renderer-side** cells (this doc owns its `@shared` cell).
- [Naming Conventions §4.8](./naming-conventions.md) — top-level closed by default (this doc is its `@shared` application); §4.9 singular vs plural; §5.2 route-by-shape.