Better Chat

A modern AI chat application with multi-model support, MCP integration, and per-user data isolation using Cloudflare Workers and Durable Objects.

Features

Multi-Model Support

• Free Models: GPT-4o mini, GPT-4.1 mini/nano, Gemini 2.5 Flash Lite
• BYOK (Bring Your Own Key) Models:
  • OpenAI: GPT-4o, GPT-4.1, GPT-5, o3/o4-mini, o3, o3-pro
  • Anthropic: Claude Opus 4, Sonnet 4.5/4/3.7, Haiku 3.5
  • Google: Gemini 2.5 Flash/Pro
• Advanced AI Capabilities: Streaming responses, extended thinking/reasoning mode, tool calling, vision/image support

MCP (Model Context Protocol) Integration

• Built-in MCP servers with enable/disable controls
• Custom MCP server support (HTTP/SSE)
• MCP tools exposed to AI models during conversations
• Visual tool call rendering with results

User Features

• Chat Management: Create, view, and manage conversations with persistent history
• API Key Management: Securely store encrypted BYOK API keys
• Usage Tracking: Monitor token usage and quotas across models
• Customizable Settings: Configure models, providers, MCP servers, and appearance preferences
• Profile Management: Account settings, session management, and deletion

Functional Feature-Based Architecture

Better Chat uses a functional, feature-based architecture with clear separation between routes, business logic, and data access. Each feature is self-contained with a consistent structure, making it easy to locate and modify code.

Standard Feature Structure

features/[feature]/
  ├── routes.ts      # API routes (thin, validation only)
  ├── handlers.ts    # Business logic orchestration (optional)
  ├── queries.ts     # Database reads
  ├── mutations.ts   # Database writes
  ├── utils.ts       # Pure utility functions
  ├── types.ts       # TypeScript types
  └── constants.ts   # Constants (optional)

Architecture Principles:

• 100% Functional — No classes, only pure functions
• Thin Routes — Routes validate input, call handlers, return response
• Query/Mutation Split — Clear separation between reads and writes
• Feature Colocation — All related code lives together in one feature directory
• Consistent Naming — Same file names across all features

Data Flow: AI Chat Completion

1. Frontend Request
   └─ POST /api/ai/ with messages and conversationId
      ↓

2. Routes Layer (features/ai/routes.ts)
   ├─ Validates authentication (requireUserDO)
   ├─ Parses request body (Zod schema)
   └─ Calls streamCompletion(userId, stub, body)
      ↓

3. Handler Layer (features/ai/handlers.ts)
   ├─ validateIncomingMessages() — validates messages
   ├─ getUserSettings() — loads user preferences
   ├─ requireAvailableQuota() — checks usage limits
   ├─ resolveModelProvider() — determines provider
   ├─ createUserProviderRegistry() — sets up AI SDK
   ├─ getCustomMcpServers() — fetches MCP configs
   ├─ getMCPTools() — connects to MCP servers
   ├─ buildSystemPrompt() — constructs prompt
   ├─ streamText() — streams AI response
   ├─ onFinish:
   │   ├─ userDOStub.appendMessages() — saves to DO
   │   ├─ maybeGenerateConversationTitle() — create title
   │   ├─ closeMCPClients() — cleanup
   │   └─ recordUsage() — tracks tokens
   └─ Returns streaming response
      ↓

4. Data Layer
   ├─ settings/queries.ts: getUserSettings() → D1
   ├─ usage/mutations.ts: recordUsage() → D1
   ├─ tools/mcp/queries.ts: getCustomMcpServers() → D1
   └─ DO: appendMessages(), listMessages() → per-user SQLite
      ↓

5. Infrastructure
   ├─ D1: Settings, usage tracking, MCP configs
   ├─ Durable Objects: Per-user conversation storage
   ├─ AI Providers: OpenAI, Anthropic, Google APIs
   ├─ MCP Servers: External tool providers
   └─ Crypto: API key encryption

Example: Updating User Settings

Most operations use oRPC for type-safe API calls:

1. Frontend: api.settings.update({ theme: "dark" })
2. Routes: settingsRouter.update validates input via Zod schema
3. Handlers: updateUserSettings(userId, input) orchestrates logic
4. Mutations: Writes updated settings to D1 database
5. Response: Typed response back to frontend via oRPC

Why This Architecture?

Benefits of functional feature-based architecture:

• Simplicity: No layers, classes, or abstractions — just functions
• Predictability: Same structure across all features
• Discoverability: Need usage logic? Check features/usage/
• Maintainability: Changes isolated to one feature directory
• LLM-Friendly: Clear boundaries, consistent patterns, easy to reason about
• Performance: Thin routes and pure functions optimize cold starts

This architecture strikes a balance: structured enough to scale, simple enough to understand quickly. Features are self-documenting through consistent file naming, making it easy for both humans and LLMs to navigate the codebase and make targeted changes without unintended side effects.

Database Architecture

• D1 (SQLite): Shared/global data (auth, usage quotas, user settings)
• Durable Objects SQLite: Per-user isolated storage (conversations, messages)
• Per-User Isolation: Each user gets their own Durable Object instance with dedicated SQLite database
• Cloudflare Workers: Edge-native serverless runtime
• Better Auth: Email OTP and social authentication (Google, GitHub)
• KV Sessions: Distributed session storage with rate limiting

Tech Stack

Frontend

• React 19 with React Server Components support
• TanStack Router for file-based routing
• TanStack Query for data fetching and caching
• oRPC for type-safe API calls
• Tailwind CSS 4 for styling
• shadcn/ui for UI components
• Vercel AI SDK for streaming AI responses
• React Markdown with syntax highlighting (Shiki)

Backend

• Cloudflare Workers for serverless compute
• Hono for HTTP routing
• oRPC for type-safe RPC
• Drizzle ORM for database operations
• Better Auth for authentication
• Vercel AI SDK for AI provider integration
• Resend for email delivery (production)

Infrastructure

• Cloudflare D1: Central SQLite database
• Cloudflare Durable Objects: Per-user stateful storage
• Cloudflare KV: Session and cache storage
• Alchemy: Multi-stage deployment and orchestration

Development

See CLAUDE.md for detailed development instructions, architecture documentation, and code conventions.

Quick Start

# Install dependencies
bun install

# Start development environment (uses .env.dev)
bun a:dev

# Individual apps
bun dev:web     # Web app only (port 3001)
bun dev:server  # Server only (port 3000)

Database Management

# D1 (Central Database)
bun db:generate  # Generate migrations
bun db:migrate   # Run migrations
bun db:push      # Push schema changes
bun db:studio    # Open Drizzle Studio (dev)

# Durable Objects (Per-User Database)
cd apps/server && bun do:generate  # Generate DO migrations

Deployment

# Deploy to production (uses .env.prod)
bun a:deploy

# Destroy deployment
bun a:destroy

Environment Setup

Create stage-specific environment files:

• .env.dev (development)
• .env.prod (production)

Required variables: BETTER_AUTH_URL, BETTER_AUTH_SECRET, CORS_ORIGIN, API_ENCRYPTION_KEY, and provider-specific API keys.

See .env.example files in root and app directories for complete configuration.

License

MIT