DevsAI — Autonomous AI Code-Generation Platform

A full-stack Next.js app that turns a natural-language prompt into a multi-file React project, streamed back to the user in real time and rendered live inside a Sandpack iframe.

Status: local development working. Production deployment runbook is in deployment.md — replace this line with a live URL once deployed.

Stack: Next.js 16 · TypeScript · PostgreSQL (Prisma + Neon serverless adapter) · Gemini + Mistral via SSE streaming · Tailwind · Vercel

What I built

• A multi-step code-generation flow that prompts an LLM (Gemini 2.5 or Mistral), parses the streamed response, and renders the running app in a Sandpack iframe (lib/sandpack-config.ts, lib/stream-parser.ts).
• Server-Sent Events streaming via a Next.js Route Handler (app/api/get-next-completion-stream-promise/route.ts) using ReadableStream — each request gets its own controller in a closure, so concurrent users don't share buffers or module-level state.
• Prisma schema for Chat / Message / GeneratedApp with composite indexes for the dashboard query path (prisma/schema.prisma).
• GitHub Actions CI on every push and PR — runs next lint && tsc --noEmit --noUnusedLocals (.github/workflows/ci.yml).

What broke & what I figured out

1. Slow dashboard queries

Listing the messages of an open chat was taking ~400 ms — PostgreSQL was doing a Seq Scan and sorting in memory.

Fix: Added @@index([chatId, createdAt]) on Message. The query plan switched to Index Scan and the fetch dropped by ~25 % (Chrome DevTools → Network, before vs after on the same dataset). The reasoning, the actual EXPLAIN ANALYZE query, and an honesty note about the measurement live in prisma/optimization.md.

2. Schema accidentally deleted by a "perf" commit

While adding the index above (commit b85d4f5), the Chat / Message / GeneratedApp block got dropped from prisma/schema.prisma. npx prisma validate failed; the app stopped booting.

Fix: Recovered the previous-revision schema from the parent commit, validated, and re-committed:

git show b85d4f5^:prisma/schema.prisma > prisma/schema.prisma
npx prisma validate
git add prisma/schema.prisma
git commit -m "fix: restore prisma schema (Chat/Message/GeneratedApp) accidentally removed in b85d4f5"

Recovery commit is 34b9565. The full rollback playbook (including how to handle migrations that already ran in production) is in deployment.md.

3. Real-time streaming without cross-session leaks

SSE responses are constructed inside the route handler with a ReadableStream whose controller lives in a per-request closure — there is no module-scope buffer, so two simultaneous prompts can't bleed into each other's responses. The handler also gracefully closes the controller on errors so the client EventSource gets a clean shutdown.

Architecture

[Client (Next.js / React)]  <-- SSE -->  [Next.js Route Handlers]  <-- Prisma + Neon -->  [PostgreSQL]
                                                |
                                                +-- Gemini API
                                                +-- Mistral API

Run locally

git clone https://github.com/Eswar809/devsai.git
cd devsai
pnpm install
cp .env.example .env       # fill in DATABASE_URL + GEMINI_API_KEY (MISTRAL_API_KEY is optional)
npx prisma migrate dev
pnpm dev

Open http://localhost:3000.

Deploy

See deployment.md — covers Vercel + Neon setup, environment-variable wiring, automatic prisma migrate deploy during build, and a step-by-step rollback path for both code-only and code + schema rollbacks.

Project structure

devsai/
├── app/                # Next.js app-router pages + API routes (SSE streaming, S3 upload, OG image)
├── components/         # React components (UI, error boundary)
├── hooks/              # Custom React hooks
├── lib/                # Prisma client, prompt templates, sandpack config, stream parser
├── prisma/             # schema.prisma, migrations/, optimization.md
└── .github/workflows/  # CI: lint + type-check

Author

Deevi Eswar — GitHub · LinkedIn · Portfolio