LibreChat is a self-hosted, multi-tenant AI chat platform that unifies access to major LLM providers (OpenAI, Anthropic, Google, AWS Bedrock, Azure, and custom OpenAI-compatible endpoints) through a single web interface. The architecture is a monolithic Express.js/Node.js backend serving a React (Vite) SPA, backed by MongoDB (Mongoose), Redis for caching/rate-limiting, MeiliSearch for full-text search, and PostgreSQL (pgvector) for RAG embeddings. The codebase is organized as a monorepo with shared packages: `data-schemas` (Mongoose models/methods with multi-tenant isolation), `data-provider` (Zod schemas, API types, React Query hooks), `api` (MCP management, streaming, auth, SSRF protection), and `client` (reusable React component library). Key data flows include: user requests hitting Express routes, middleware performing JWT/OAuth/LDAP/SAML auth and RBAC checks, agent orchestration via `@librechat/agents` with tool execution (MCP, code interpreter, web search), and SSE streaming with resumable stream support via Redis-backed job management. The system implements a sophisticated multi-tenant isolation layer using Mongoose plugins (`tenantIsolation`) that automatically scope queries by tenant, an ACL/permissions system with bit-flag-based permission checks, and SSRF protection with DNS resolution validation for MCP server URLs. The MCP (Model Context Protocol) subsystem is particularly well-developed, featuring OAuth flow management with reconnection tracking, tool discovery, and both stdio and HTTP/SSE transport support. The streaming architecture supports horizontal scaling with Redis-backed event transport and job stores, enabling resumable streams across multiple server instances.