older python version deployment with vercel

[vladlenadev]

Discussion Type

Product Feedback

Discussion Content

Hello everyone.
now I develop a simple serverless fastapi app with python 3.9.13.
but errors occured when deploy this app on vercel dashboard.
let me know how to solve this problem?
thank you.

[mecodeatlas]

Thanks for posting in the GitHub Community, @solunagit!

We're happy you're here. You are more likely to get a useful response if you are posting your question in the applicable category, the Discussions category is solely related to conversations around the GitHub product Discussions. This question should be in the Programming Help category. I've gone ahead and moved it for you. Good luck!

[vladlenadev]

thank you for your help.
and I am sorry about my mistake.

[tejashinde]

You'll need these specific files in your project root:

1. vercel.json - Configure Vercel to handle FastAPI:

json{
  "functions": {
    "api/index.py": {
      "runtime": "python3.9"
    }
  },
  "routes": [
    {
      "src": "/(.*)",
      "dest": "/api/index.py"
    }
  ]
}

2. requirements.txt - List your dependencies:
   fastapi
   uvicorn
   Project Structure
   Your project should look like this:
   your-project/
   ├── api/
   │ └── index.py # Your FastAPI app
   ├── vercel.json
   └── requirements.txt
   FastAPI App Configuration
   In api/index.py, structure your app like this:FastAPI Vercel ConfigurationCode from fastapi import FastAPI

from mangum import Mangum

# Create FastAPI app
app = FastAPI()

@app.get("/")
async def root():
    return {"message": "Hello World"}

@app.get("/api/hello")
async def hello():
    return {"message": "Hello from FastAPI oCommon Solutions

1. Add Mangum - Update your requirements.txt:

fastapi
uvicorn
mangum

2. Python Version - If you're using Python 3.9.13 specifically, update vercel.json:

json{
  "functions": {
    "api/index.py": {
      "runtime": "python3.9"
    }
  }
}

3. Environment Variables - If you have secrets, add them in Vercel dashboard under Settings → Environment Variables.

4. File Size Limits - Vercel has deployment limits. Keep your total deployment under 250MB.
   Debugging Steps

Check the Vercel deployment logs in your dashboard
Ensure all files are properly committed to your Git repository
Verify your FastAPI app works locally first
Test the /api/ endpoints after deployment

What specific error messages are you seeing in the Vercel dashboard?

[R41CY]

---

Hey @solunagit — Vercel doesn’t officially support Python for backend/serverless functions the way it does for Node.js. That’s likely why you're hitting errors, especially with older Python versions like 3.9.

✅ Best options:

1. Use a custom Dockerfile

   • Create a Dockerfile with Python 3.9 and FastAPI setup
   • Set it up with Vercel's [Custom Deployment](https://vercel.com/docs/concepts/projects/custom-deployments/overview)
   • This gives you full control over the runtime

2. Use a platform made for Python backends

   • [Render](https://render.com), [Fly.io](https://fly.io), or [Railway](https://railway.app) work great with FastAPI and Python 3.9+
   • Then host just the frontend (if needed) on Vercel

---

Let me know if you want a working Dockerfile example for Python 3.9 + FastAPI.

[AnMoreNight]

Hello.

1. Commit and push these files to GitHub.
2. In Vercel dashboard, link the repo project.
3. Vercel will detect the config, install dependencies, and deploy the serverless function.
4. Open your deployed URL—e.g. https://.vercel.app/—and see your FastAPI app respond.

-If deployment still fails, ensure:
1. All three files (api/index.py, vercel.json, requirements.txt) are in the repo root and committed.
2. Your api/index.py really references app (FastAPI instance) or handler (Mangum handler).
-Vercel's Python support is “beta”: issues may persist. If you face unresolved errors, consider using a Docker-based custom deploy or moving to platforms like Render, Fly.io, or Railway

Good luck solunagit!

[vladlenadev]

Thank you for your help.