Composable AI Stack (CAS) | (Bun + Turborepo + Convex-first)

A modular AI stack that scales from local tools to enterprise systems

• Name: Composable AI Stack
• Short: CAS
• Repo slug: cas or composable-ai-stack
• Package scope (if you publish): @cas/* or @composable-ai/*
• Docs title line: Composable AI Stack (CAS)
• One-liner tagline: A modular AI stack that scales from local tools to enterprise systems.

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Overview

A reusable monorepo template for AI-Integrated SaaS apps:

• apps/web: Next.js + Tailwind + shadcn/ui
• apps/convex: Convex backend (schema + functions + HTTP endpoints)
• apps/api: Optional ElysiaJS (Bun) API gateway
• apps/worker: background tasks + eval runner CLI (Bun)
• packages/: shared prompts, schemas, AI utilities, eval harness, configs

Goal: be your modular AI stack that scales from local tools to enterprise systems

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Requirements

• Bun
• A Convex project (free tier works for prototyping)
• Optional: Stripe account for memberships

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Quick Start

1) Install dependencies (root)

bun install

2) Copy env and fill values

cp .env.example .env

3) Run development

In separate terminals:

bun run dev:convex
bun run dev:web
# optional
bun run dev:api

Or run everything:

bun run dev

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Convex Setup

From apps/convex:

cd apps/convex
bunx convex dev

Convex will output a deployment URL. Set:

• NEXT_PUBLIC_CONVEX_URL in .env (and/or apps/web/.env.local)

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shadcn/ui + Tailwind

apps/web is preconfigured with Tailwind and a minimal shadcn/ui setup.

Add a shadcn component (from within apps/web):

cd apps/web
bunx shadcn@latest add button

Where to tweak:

• apps/web/components.json
• apps/web/tailwind.config.ts
• apps/web/app/globals.css

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Memberships / Billing (Convex-first)

Template includes placeholders for:

• membership fields in apps/convex/convex/schema.ts
• a webhook route in apps/convex/convex/http.ts (/stripe/webhook)

Typical flow:

1. Create checkout session in a Convex action (server-side)
2. Stripe redirects user back to your app
3. Stripe sends webhook events to /stripe/webhook
4. Convex updates your users.planStatus

If you want a dedicated webhook gateway with custom middleware/rate limiting, use apps/api (ElysiaJS).

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Scaling the rate limiter

apps/api ships an in-memory rate limiter (apps/api/src/middleware/rate-limit.ts). It tracks request counts in a single-process Map. For multi-worker or multi-instance deployments, replace the backing store with Redis or Upstash:

1. Install: bun add --filter=api @upstash/ratelimit @upstash/redis
2. Swap the Map in rate-limit.ts for an Upstash Ratelimit instance.
3. Set UPSTASH_REDIS_REST_URL and UPSTASH_REDIS_REST_TOKEN in .env.

The middleware exposes X-RateLimit-Backend: memory so you can verify which store is in use at runtime.

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Scripts (root)

• bun run dev — dev for all apps
• bun run dev:web — Next.js only
• bun run dev:convex — Convex only
• bun run dev:api — Elysia only
• bun run build — build all buildable apps
• bun run eval — run eval harness

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Repo Layout

apps/
  web/
  convex/
  api/
  worker/

packages/
  shared/
  schemas/
  prompts/
  ai/
  evals/
  config/

See use-cases.md and apps/api/use-cases.md.

UI Stack

• Web UI uses Tailwind CSS v4.1 + shadcn/ui.

Versions (template defaults)

Pinned to your requested versions:

• Bun: 1.3.7
• Turborepo (turbo): 2.7.6
• TypeScript: 5.9.3
• Prettier: 3.8.1
• Next.js: 16.1.6 (App Router)
• React: 19.2.4
• Tailwind CSS: 4.1.18
• Elysia: 1.4.22
• Convex: 1.31.6
• ESLint: 9.39.2

Keeping versions pinned

Run:

bun run check:versions

CI will fail if any pinned dependency versions drift.

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Guardrail Pattern

The @acme/ai package provides a guardrail pattern for safe, validated LLM calls:

┌─────────────┐     ┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│   Input     │────▶│    Zod      │────▶│ Moderation  │────▶│  LLM Call   │
│   (raw)     │     │ Validation  │     │   Hook      │     │             │
└─────────────┘     └─────────────┘     └─────────────┘     └──────┬──────┘
                                                                   │
                    ┌─────────────┐     ┌─────────────┐           │
                    │   Output    │◀────│    Zod      │◀──────────┘
                    │  (typed)    │     │ Validation  │
                    └─────────────┘     └─────────────┘

Usage Example

import { z } from 'zod';
import { runWithGuardrails, openai, DEFAULT_MODELS } from '@acme/ai';

const inputSchema = z.object({ message: z.string().max(500) });
const outputSchema = z.object({ reply: z.string(), confidence: z.number() });

const result = await runWithGuardrails(
  openai(DEFAULT_MODELS.openai),
  'Generate a helpful reply',
  { message: 'Hello, I need help!' },
  {
    name: 'support-reply',
    inputSchema,
    outputSchema,
    moderationHook: async (input) => input.message.length > 0,
  }
);

console.log(result.output); // { reply: '...', confidence: 0.95 }
console.log(result.traceId); // Langfuse trace ID

All guardrail runs are automatically traced to Langfuse with full input/output logging.

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Observability

This stack uses Langfuse for LLM observability and tracing:

Required Environment Variables

# LLM Providers
OPENAI_API_KEY=sk-...
ANTHROPIC_API_KEY=sk-...

# Langfuse (Observability)
LANGFUSE_PUBLIC_KEY=pk-...
LANGFUSE_SECRET_KEY=sk-...
LANGFUSE_BASE_URL=https://cloud.langfuse.com  # Optional: use your self-hosted instance

Features

• Automatic tracing: Every LLM call via tracedGenerate() or runWithGuardrails() is traced
• Generation tracking: Prompts, outputs, token usage, and latency are captured
• Score logging: Eval scores are sent to Langfuse for monitoring
• Self-hosted option: Run your own Langfuse instance by changing LANGFUSE_BASE_URL

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Author

Created by Bashar Ayyash — yabasha.dev