---
name: deploy-fullstack-vercel
description: Build and deploy a full-stack app (React frontend + Python/FastAPI backend) or a Vellum app to Vercel as a serverless demo with seeded data
compatibility: "Designed for Vellum personal assistants"
metadata:
  emoji: "🚀"
  vellum:
    category: "development"
    display-name: "Deploy Fullstack to Vercel"
---

# Deploy Fullstack to Vercel

Deploy a full-stack app with a React/Vite frontend and Python/FastAPI backend to Vercel as a serverless demo, OR deploy a Vellum-built app from the library. No auth required - meant for demos, portfolio pieces, and quick showcases.

## When to Use

- User says "deploy this to Vercel", "host this", "publish this"
- User has a project with a frontend + backend they want live
- User wants to deploy a Vellum app that uses backend features (data store, custom routes)
- User wants a quick demo deployment (no persistent database needed)

## Authentication

### Vellum App Publishing

For publishing Vellum apps from the library, use the built-in `publish_page` tool. This is the preferred path — it uses the stored Vercel API token (`vercel/api_token`) via the brokered publish flow without exposing the token to shell commands.

**The stored Vercel API token is reserved for brokered `publish_page` and `unpublish_page` actions only.** Do not pass it to `bash`, `curl`, Vercel CLI commands, or proxy credential injection. Do not use `network_mode: "proxied"` with `credential_ids` for Vercel deployments.

### Custom Full-Stack Deployments

For custom projects that need Vercel deployment (not Vellum app publishing):

1. Install the Vercel CLI with `bun install -g vercel` (not npm — npm is not available in the sandbox).
2. Use `vercel login` to authenticate interactively (opens browser for the user).
3. If the user does not want to use CLI auth, stop and ask them for an approved deployment path. Do not extract, inject, or shell with the stored API token.

## Deploying a Vellum App

When the user asks to deploy a Vellum app from their library (from `/workspace/data/apps/<app-name>/`):

### 1. Detect Vellum Bridge Usage

Check the compiled app for Vellum bridge API usage:

```bash
grep -l "window\.vellum\.\|vellum\.fetch\|vellum\.data\|vellum\.sendAction" /workspace/data/apps/<app-name>/dist/*.js /workspace/data/apps/<app-name>/dist/*.html 2>/dev/null
```

If found, the app depends on the Vellum bridge and needs a shim to work standalone.

### 2. Create Vellum Bridge Shim

The app uses `window.vellum.*` APIs that are normally injected by the Vellum viewer. For standalone deployment, create a `vellum-shim.js` file in the app's `dist/` directory that provides browser-native replacements.

**Before writing the shim, read the app's compiled JavaScript** (`dist/main.js` or equivalent) to understand exactly which `window.vellum.*` APIs the app calls and what data shapes it expects. The shim must match the app's actual usage — don't guess at signatures.

**Common APIs to shim (implement only what the app actually uses):**

| Bridge API                    | Standalone replacement                                | Notes                                                                                                                    |
| ----------------------------- | ----------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------ |
| `vellum.data.query()`         | localStorage-backed store                             | Read the app code to determine the record shape — some apps expect `{id, data: {...}}` wrappers, others use flat records |
| `vellum.data.create(...)`     | localStorage insert with `crypto.randomUUID()`        | Match the argument signature the app passes (some pass a payload, others pass `{id, ...fields}`)                         |
| `vellum.data.update(...)`     | localStorage update                                   | Match the argument signature (typically `(id, payload)`)                                                                 |
| `vellum.data.delete(...)`     | localStorage delete                                   | Typically `(id)`                                                                                                         |
| `vellum.fetch(path, opts)`    | `console.warn` + return empty success Response        | Custom routes aren't available standalone                                                                                |
| `vellum.sendAction(id, data)` | No-op with `console.warn`                             | Surface actions aren't available standalone                                                                              |
| `vellum.openLink(url)`        | `window.open(url, '_blank')`                          |                                                                                                                          |
| `vellum.widgets.toast(msg)`   | Create a temporary styled `<div>` that auto-dismisses |                                                                                                                          |
| `vellum.route`                | `null`                                                | Deep-link routes aren't available standalone                                                                             |

**Structure:** Wrap everything in an IIFE that guards against the real bridge: `(function() { if (window.vellum) return; ... })();`

### 3. Inject the Shim into index.html

Add a `<script src="vellum-shim.js"></script>` tag in `dist/index.html` BEFORE any `<script type="module">` tags:

```bash
sed -i 's|<script type="module"|<script src="vellum-shim.js"></script>\n<script type="module"|' dist/index.html
```

### 4. Deploy the App

```bash
cd /workspace/data/apps/<app-name>/dist
```

Create a `vercel.json` in the dist directory:

```json
{
  "rewrites": [
    {
      "source": "/((?!main\\.js|main\\.css|vellum-shim\\.js|assets/).*)",
      "destination": "/index.html"
    }
  ]
}
```

Then deploy using the `publish_page` tool (preferred). For Vellum apps, use the built-in app publish flow rather than raw Vercel API calls from shell.

## Deploying a Custom Full-Stack Project

### 1. Build the Frontend

```bash
cd <project>/frontend
bun install
bunx vite build
```

This produces static files in `frontend/dist/`.

### 2. Create the Vercel Deploy Directory

```
<project>/vercel-deploy/
├── api/
│   ├── index.py          ← FastAPI app wrapper (entry point)
│   ├── database.py        ← DB config (use /tmp for SQLite)
│   ├── models.py
│   ├── schemas.py
│   ├── seed_data.py       ← Must seed ALL required data (users, etc.)
│   ├── routers/
│   │   ├── __init__.py
│   │   └── *.py
│   └── requirements.txt   ← Python deps (fastapi, sqlalchemy, pydantic)
├── index.html             ← From frontend/dist/
├── assets/                ← From frontend/dist/assets/
└── vercel.json
```

**Key steps:**

```bash
mkdir -p <project>/vercel-deploy/api

# Copy frontend build output to deploy root
cp -r <project>/frontend/dist/* <project>/vercel-deploy/

# Copy backend files into api/
cp <project>/backend/models.py <project>/vercel-deploy/api/
cp <project>/backend/database.py <project>/vercel-deploy/api/
cp <project>/backend/schemas.py <project>/vercel-deploy/api/
cp <project>/backend/seed_data.py <project>/vercel-deploy/api/
cp -r <project>/backend/routers <project>/vercel-deploy/api/
cp <project>/backend/requirements.txt <project>/vercel-deploy/api/
```

### 3. Create api/index.py (Serverless Entry Point)

```python
import sys, os
sys.path.insert(0, os.path.dirname(__file__))

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware
from database import engine, Base, SessionLocal
from seed_data import seed_exercises, seed_default_user  # all seed functions
from routers import users, exercises, workouts, schedule, progress

# Create tables and seed on EVERY cold start
Base.metadata.create_all(bind=engine)
db = SessionLocal()
try:
    seed_exercises(db)
    seed_default_user(db)  # IMPORTANT: seed all required data
finally:
    db.close()

app = FastAPI(title="MyApp")

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

app.include_router(users.router)
# ... other routers

@app.get("/api/health")
def health_check():
    return {"status": "ok"}
```

### 4. Update database.py for Vercel

**Critical:** Vercel serverless functions can only write to `/tmp`. Update the SQLite path:

```python
SQLALCHEMY_DATABASE_URL = "sqlite:////tmp/app.db"
```

### 5. Seed ALL Required Data

**This is the #1 gotcha.** Since `/tmp` is ephemeral, every cold start gets a fresh database. If your frontend assumes certain data exists (like user ID 1), you MUST seed it:

```python
def seed_default_user(db: Session):
    count = db.query(UserProfile).count()
    if count > 0:
        return
    user = UserProfile(name="Demo User", ...)
    db.add(user)
    db.commit()
```

### 6. Create vercel.json

```json
{
  "rewrites": [
    { "source": "/api/(.*)", "destination": "/api/index.py" },
    { "source": "/((?!assets/).*)", "destination": "/index.html" }
  ]
}
```

This routes:

- `/api/*` → Python serverless function
- Everything else → React SPA (index.html)

### 7. Deploy

```bash
cd <project>/vercel-deploy
vercel --yes --prod
```

### 8. Verify

```bash
curl -s <deployed-url>/api/health
# Should return: {"status":"ok"}
```

## Gotchas & Limitations

| Issue                       | Solution                                                                 |
| --------------------------- | ------------------------------------------------------------------------ |
| SQLite resets on cold start | Seed ALL required data in index.py startup                               |
| No persistent storage       | Acceptable for demos. For production, use Vercel Postgres or Supabase    |
| No auth                     | Fine for demos/portfolios. Add auth layer for real apps                  |
| `requirements.txt` location | Must be inside `api/` folder (next to index.py)                          |
| Module imports in routers   | Use `sys.path.insert(0, os.path.dirname(__file__))` in index.py          |
| CORS                        | Set `allow_origins=["*"]` for demo deployments                           |
| `--name` flag deprecated    | Don't use `--name` with Vercel CLI, just deploy from the directory       |
| Vellum bridge APIs          | Use the vellum-shim.js to provide localStorage-backed data + no-op stubs |
| npm not available           | Use `bun install -g vercel` to install Vercel CLI in sandbox             |

## Vercel CLI Quick Reference

```bash
bun install -g vercel        # Install
vercel login                 # Authenticate (opens browser for user-mediated auth)
vercel --yes --prod          # Deploy to production (skip prompts)
vercel logs --project <name> # Check function logs
```