Does Flask Work With Vercel?
Flask can run on Vercel, but you need to use Vercel's serverless functions API rather than traditional WSGI hosting.
Quick Facts
How Flask Works With Vercel
Flask doesn't natively deploy to Vercel's serverless platform because Vercel is optimized for frontend frameworks and stateless serverless functions, not long-running WSGI servers. However, you can use Flask with Vercel's Python runtime by converting your Flask app to work as serverless functions. Each route becomes a separate function handler in a `api/` directory, and Vercel automatically routes requests. This approach works well for APIs and server-side logic, but you lose Flask's traditional request handling model—no middleware stacks, app context, or session management work the same way. The developer experience involves restructuring your app into function-based handlers or using a thin wrapper. Performance is excellent for infrequent requests due to Vercel's edge network, but cold starts can impact latency on the first invocation. This setup is ideal for building REST APIs alongside Next.js or other frontend frameworks, but less suitable for complex Flask applications with heavy session management, background tasks, or database connection pooling.
Best Use Cases
Quick Setup
pip install flaskfrom flask import Flask, jsonify
from vercel_python_runtime import Response
app = Flask(__name__)
@app.route('/api/hello', methods=['GET'])
def hello():
return jsonify({"message": "Hello from Flask on Vercel"})
@app.route('/api/users/<int:user_id>', methods=['GET'])
def get_user(user_id):
return jsonify({"id": user_id, "name": "John Doe"})
def handler(request):
return app(request.environ, lambda *args: None)
Known Issues & Gotchas
Cold starts cause 1-5 second latency on first request after inactivity
Fix: Use Vercel's cron jobs to keep functions warm, or accept latency for low-traffic APIs
Flask middleware and request context behave differently in serverless; session management breaks
Fix: Use stateless authentication (JWT), store sessions in external databases (Redis/DynamoDB), avoid Flask-Session
File uploads and temporary file storage don't persist between requests
Fix: Upload files directly to S3 or Vercel's Blob storage instead of local filesystem
Background tasks and long-running processes timeout (10-60 second limit)
Fix: Use external task queues (Celery with Redis), or split into multiple shorter functions
Alternatives
- •FastAPI + Vercel: Modern async Python framework with better serverless support and automatic OpenAPI docs
- •Next.js API Routes + Node.js: Purpose-built for Vercel with superior cold start performance and native integration
- •AWS Lambda + Flask: More control over serverless environment with better long-running task support via Step Functions
Resources
Related Compatibility Guides
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