# LibreChat

## Project Overview

LibreChat is a monorepo with the following key workspaces:

| Workspace | Language | Side | Dependency | Purpose |
|---|---|---|---|---|
| `/api` | JS (legacy) | Backend | `packages/api`, `packages/data-schemas`, `packages/data-provider`, `@librechat/agents` | Express server — minimize changes here |
| `/packages/api` | **TypeScript** | Backend | `packages/data-schemas`, `packages/data-provider` | New backend code lives here (TS only, consumed by `/api`) |
| `/packages/data-schemas` | TypeScript | Backend | `packages/data-provider` | Database models/schemas, shareable across backend projects |
| `/packages/data-provider` | TypeScript | Shared | — | Shared API types, endpoints, data-service — used by both frontend and backend |
| `/client` | TypeScript/React | Frontend | `packages/data-provider`, `packages/client` | Frontend SPA |
| `/packages/client` | TypeScript | Frontend | `packages/data-provider` | Shared frontend utilities |

The source code for `@librechat/agents` (major backend dependency, same team) is at `/home/danny/agentus`.

---

## Workspace Boundaries

- **All new backend code must be TypeScript** in `/packages/api`.
- Keep `/api` changes to the absolute minimum (thin JS wrappers calling into `/packages/api`).
- Database-specific shared logic goes in `/packages/data-schemas`.
- Frontend/backend shared API logic (endpoints, types, data-service) goes in `/packages/data-provider`.
- Build data-provider from project root: `npm run build:data-provider`.

---

## Code Style

### Naming and File Organization

- **Single-word file names** whenever possible (e.g., `permissions.ts`, `capabilities.ts`, `service.ts`).
- When multiple words are needed, prefer grouping related modules under a **single-word directory** rather than using multi-word file names (e.g., `admin/capabilities.ts` not `adminCapabilities.ts`).
- The directory already provides context — `app/service.ts` not `app/appConfigService.ts`.

### Structure and Clarity

- **Never-nesting**: early returns, flat code, minimal indentation. Break complex operations into well-named helpers.
- **Functional first**: pure functions, immutable data, `map`/`filter`/`reduce` over imperative loops. Only reach for OOP when it clearly improves domain modeling or state encapsulation.
- **No dynamic imports** unless absolutely necessary.

### DRY

- Extract repeated logic into utility functions.
- Reusable hooks / higher-order components for UI patterns.
- Parameterized helpers instead of near-duplicate functions.
- Constants for repeated values; configuration objects over duplicated init code.
- Shared validators, centralized error handling, single source of truth for business rules.
- Shared typing system with interfaces/types extending common base definitions.
- Abstraction layers for external API interactions.

### Iteration and Performance

- **Minimize looping** — especially over shared data structures like message arrays, which are iterated frequently throughout the codebase. Every additional pass adds up at scale.
- Consolidate sequential O(n) operations into a single pass whenever possible; never loop over the same collection twice if the work can be combined.
- Choose data structures that reduce the need to iterate (e.g., `Map`/`Set` for lookups instead of `Array.find`/`Array.includes`).
- Avoid unnecessary object creation; consider space-time tradeoffs.
- Prevent memory leaks: careful with closures, dispose resources/event listeners, no circular references.

### Backend Database Performance

- On request startup and first page load paths, watch for serial database reads.
  Multiple round trips to MongoDB can add significant latency when the database
  is far from the app server.
- Prefer passing already-loaded request/user/config data through helper
  functions instead of re-reading the same user, role, tenant, or principal data.
- When two reads are independent, start them in parallel and gate the response
  on the authorization or validation result before returning data.
- Keep authorization, permission, and tenant checks semantically identical when
  parallelizing reads. Speculative reads must remain scoped to the authenticated
  user or tenant and must not write to the response before validation succeeds.

### Type Safety

- **Never use `any`**. Explicit types for all parameters, return values, and variables.
- **Limit `unknown`** — avoid `unknown`, `Record<string, unknown>`, and `as unknown as T` assertions. A `Record<string, unknown>` almost always signals a missing explicit type definition.
- **Don't duplicate types** — before defining a new type, check whether it already exists in the project (especially `packages/data-provider`). Reuse and extend existing types rather than creating redundant definitions.
- Use union types, generics, and interfaces appropriately.
- All TypeScript and ESLint warnings/errors must be addressed — do not leave unresolved diagnostics.

### Comments and Documentation

- Write self-documenting code; no inline comments narrating what code does.
- JSDoc only for complex/non-obvious logic or intellisense on public APIs.
- Single-line JSDoc for brief docs, multi-line for complex cases.
- Avoid standalone `//` comments unless absolutely necessary.

### Import Order

Imports are organized into three sections:

1. **Package imports** — sorted shortest to longest line length (`react` always first).
2. **`import type` imports** — sorted longest to shortest (package types first, then local types; length resets between sub-groups).
3. **Local/project imports** — sorted longest to shortest.

Multi-line imports count total character length across all lines. Consolidate value imports from the same module. Always use standalone `import type { ... }` — never inline `type` inside value imports.

### JS/TS Loop Preferences

- **Limit looping as much as possible.** Prefer single-pass transformations and avoid re-iterating the same data.
- `for (let i = 0; ...)` for performance-critical or index-dependent operations.
- `for...of` for simple array iteration.
- `for...in` only for object property enumeration.

---

## Frontend Rules (`client/src/**/*`)

### Localization

- All user-facing text must use `useLocalize()`.
- Only update English keys in `client/src/locales/en/translation.json` (other languages are automated externally).
- Semantic key prefixes: `com_ui_`, `com_assistants_`, etc.

### Components

- TypeScript for all React components with proper type imports.
- Semantic HTML with ARIA labels (`role`, `aria-label`) for accessibility.
- Group related components in feature directories (e.g., `SidePanel/Memories/`).
- Use index files for clean exports.

### Data Management

- Feature hooks: `client/src/data-provider/[Feature]/queries.ts` → `[Feature]/index.ts` → `client/src/data-provider/index.ts`.
- React Query (`@tanstack/react-query`) for all API interactions; proper query invalidation on mutations.
- QueryKeys and MutationKeys in `packages/data-provider/src/keys.ts`.

### Data-Provider Integration

- Endpoints: `packages/data-provider/src/api-endpoints.ts`
- Data service: `packages/data-provider/src/data-service.ts`
- Types: `packages/data-provider/src/types/queries.ts`
- Use `encodeURIComponent` for dynamic URL parameters.

### Performance

- Prioritize memory and speed efficiency at scale.
- Cursor pagination for large datasets.
- Proper dependency arrays to avoid unnecessary re-renders.
- Leverage React Query caching and background refetching.

---

## Development Commands

| Command | Purpose |
|---|---|
| `npm run smart-reinstall` | Install deps (if lockfile changed) + build via Turborepo |
| `npm run reinstall` | Clean install — wipe `node_modules` and reinstall from scratch |
| `npm run backend` | Start the backend server |
| `npm run backend:dev` | Start backend with file watching (development) |
| `npm run build` | Build all compiled code via Turborepo (parallel, cached) |
| `npm run frontend` | Build all compiled code sequentially (legacy fallback) |
| `npm run frontend:dev` | Start frontend dev server with HMR (port 3090, requires backend running) |
| `npm run build:data-provider` | Rebuild `packages/data-provider` after changes |

- Node.js: v24.16.0
- Database: MongoDB
- Backend runs on `http://localhost:3080/`; frontend dev server on `http://localhost:3090/`

---

## Testing

- Framework: **Jest**, run per-workspace.
- Run tests from their workspace directory: `cd api && npx jest <pattern>`, `cd packages/api && npx jest <pattern>`, etc.
- Frontend tests: `__tests__` directories alongside components; use `test/layout-test-utils` for rendering.
- Cover loading, success, and error states for UI/data flows.

### Philosophy

- **Real logic over mocks.** Exercise actual code paths with real dependencies. Mocking is a last resort.
- **Spies over mocks.** Assert that real functions are called with expected arguments and frequency without replacing underlying logic.
- **MongoDB**: use `mongodb-memory-server` for a real in-memory MongoDB instance. Test actual queries and schema validation, not mocked DB calls.
- **MCP**: use real `@modelcontextprotocol/sdk` exports for servers, transports, and tool definitions. Mirror real scenarios, don't stub SDK internals.
- Only mock what you cannot control: external HTTP APIs, rate-limited services, non-deterministic system calls.
- Heavy mocking is a code smell, not a testing strategy.

---

## Formatting

Fix all formatting lint errors (trailing spaces, tabs, newlines, indentation) using auto-fix when available. All TypeScript/ESLint warnings and errors **must** be resolved.
