Open Chat

[!TIP] Try it live at oss.chat — no sign-up required, you get a guest budget and can start chatting right away.

A local-first, multi-model AI chat app — and a working demonstration of how Prisma Streams, Prisma Postgres, and Prisma Next fit together in a real application.

Every assistant token is appended to a durable stream before the browser renders it. Close the tab mid-answer, reopen it, and the response replays and finishes exactly where it left off. No message is ever lost to a dropped connection.

What it demonstrates

• Prisma Streams as the system of record for chat messages: an append-only event log per user, with a routing key per chat. Appends are durable before delivery; reads resume from any offset.
• Prisma Postgres (via prisma dev) for everything relational: users, sessions, chat metadata, and usage accounting — fully local.
• Prisma Next as the typed data layer: the contract in src/prisma/contract.prisma drives an end-to-end typed client, sharing one pg.Pool with Better Auth.

Features

• Streaming chat with any text model on OpenRouter — switch models mid-conversation; each message records which model wrote it
• Durable, resumable streams: refresh, reconnect, or restart the server without losing tokens
• Anonymous guest sessions with a lifetime budget; existing email/password accounts get a monthly one (public sign-up is disabled)
• Usage metering in micro-USD with a per-chat, per-model cost breakdown
• Markdown rendering, message permalinks, chat search, rename, and delete
• One process, no build step: Bun serves the API, the SSE proxy, and the React client

How it works

Browser (React + TanStack DB)
   │  fetch /api/*            ▲ SSE /api/chats/:id/events
   ▼                          │
Bun server ── Better Auth (sessions, guests)
   │
   ├── Prisma Next ──► Prisma Postgres   chats, users, usage
   │
   ├── Prisma Streams                    durable message events
   │     one stream per user, routing key per chat
   │
   └── OpenRouter                        model streaming (only external call)

Sending a message appends a durable message.created event, then streams the model's reply as message.delta events into the same stream. The browser consumes them over an authenticated SSE proxy and folds them into messages with the same pure function the server uses for history replay (src/shared/messages.ts).

The full design is written up in docs/architecture.md.

Getting started

You need Bun ≥ 1.2 and an OpenRouter API key.

# 1. Install dependencies
bun install

# 2. Start local Prisma Postgres + Prisma Streams
bun run db:dev

# 3. Configure the environment
cp .env.example .env
#    – set DATABASE_URL and STREAMS_URL to the URLs printed by:
#      DATABASE_URL=... bunx prisma dev ls
#    – set OPENROUTER_API_KEY and a random BETTER_AUTH_SECRET

# 4. Create the database tables
bun run db:init

# 5. Run the app
bun run dev

Open http://localhost:3000 — you'll be signed in as a guest automatically and can start chatting.

Deploy to Prisma Compute

The live instance at oss.chat runs on Prisma Compute as two apps in one project: the chat server and the durable Streams service it talks to. Deploying your own takes a few minutes with the Prisma CLI.

# 1. Sign in and create a project (this also provisions a Prisma Postgres database)
bunx @prisma/cli@latest auth login
bunx @prisma/cli@latest project create my-open-chat

# 2. Create the tables in the project's primary database
bunx @prisma/cli@latest database connection create <database-id>   # prints DATABASE_URL
DATABASE_URL=<that-url> bun run db:init

# 3. Deploy the Streams service (pick any long random key)
bunx @prisma/cli@latest app deploy --app Streams \
  --framework bun --entry src/streams-app/index.ts --http-port 8080 \
  --env STREAMS_API_KEY=<random-key> --no-db --prod --yes

# 4. Deploy the chat app, pointing it at the database and the Streams URL from step 3
bunx @prisma/cli@latest app deploy --app open-chat \
  --framework bun --entry src/start.ts --http-port 3000 \
  --env DATABASE_URL=<that-url> \
  --env STREAMS_URL=<streams-app-url> \
  --env STREAMS_API_KEY=<random-key> \
  --env BETTER_AUTH_SECRET=<random-secret> \
  --env OPENROUTER_API_KEY=<your-openrouter-key> \
  --env APP_ORIGIN=<chat-app-url> \
  --no-db --prod --yes

That's it — the CLI builds locally, uploads, and the deployment is live in seconds. Secrets live only in Compute's env config, never in the repo. (On the very first deploy you don't know the app URL yet: deploy once, then set APP_ORIGIN to the printed URL and deploy again. Subsequent deploys keep their env vars.)

Project layout

Scripts

Learn more

• docs/architecture.md — the stream-per-user / routing-key-per-chat pattern, data ownership, and the durable streaming path
• docs/features.md — every user flow, written as a verification checklist
• prisma-next.md — how the Prisma Next contract workflow operates in this repo

License

MIT