Deploy a FastAPI app on Vercel

Deploy a FastAPI app to Vercel with the Python runtime and Vercel Functions. Vercel looks for a FastAPI instance named app at supported entrypoints in your repository.

Create a FastAPI app or use an existing one:

Initialize a new FastAPI project with the Vercel CLI init command:

vc init fastapi

This clones the FastAPI example repository in a directory called fastapi.

To run a FastAPI application on Vercel, define an app instance that initializes FastAPI at a supported entrypoint:

• app.py, index.py, server.py, main.py, wsgi.py, or asgi.py
• the same filenames inside src/, app/, or api/

For example:

from fastapi import FastAPI
 
app = FastAPI()
 
@app.get("/")
def read_root():
    return {"Python": "on Vercel"}

To point Vercel to a FastAPI app in a custom module, set tool.vercel.entrypoint in pyproject.toml:

[tool.vercel]
entrypoint = "backend.server:app"

The tool.vercel.entrypoint value tells Vercel to look for a FastAPI instance named app in ./backend/server.py.

The build property in [tool.vercel.scripts] defines the Build Command for FastAPI deployments. It runs after dependencies are installed and before your application is deployed.

[tool.vercel.scripts]
build = "python build.py"

For example:

def main():
    print("Running build command...")
    with open("build.txt", "w") as f:
        f.write("BUILD_COMMAND")
 
if __name__ == "__main__":
    main()

Use vercel dev to run your application locally.

python -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt
vercel dev

Deploy the project by connecting your Git repository or by using the Vercel CLI:

vc deploy

To serve static assets, place them in the public/** directory. Vercel serves those files from the CDN with default headers unless you override them in vercel.json.

from fastapi import FastAPI
from fastapi.responses import RedirectResponse
 
app = FastAPI()
 
@app.get("/favicon.ico", include_in_schema=False)
async def favicon():
    # /vercel.svg is automatically served when included in the public/** directory.
    return RedirectResponse("/vercel.svg", status_code=307)

You can use FastAPI lifespan events to manage startup and shutdown logic, such as initializing and closing database connections.

from contextlib import asynccontextmanager
from fastapi import FastAPI
 
@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup logic
    print("Starting up...")
    await startup_tasks()
    yield
    # Shutdown logic
    await cleanup_tasks()
 
app = FastAPI(lifespan=lifespan)

When you deploy a FastAPI app to Vercel, it becomes a single Vercel Function. Vercel uses Fluid compute by default, so the function scales with traffic.

To configure that function, add an entry to the functions object in vercel.json keyed by your resolved entrypoint file. For example, to let an app defined in app/main.py run for up to 60 seconds, set maxDuration:

{
  "$schema": "https://openapi.vercel.sh/vercel.json",
  "functions": {
    "app/main.py": {
      "maxDuration": 60
    }
  }
}

For more options, see Configuring functions and the functions property.

All Vercel Functions limitations apply to FastAPI applications, including:

• Application size: The FastAPI application becomes a single bundle, which has a standard bundle size limit of 500MB. Large Functions support Python bundles up to 5GB on Fluid compute when enabled (public beta). Our bundling process removes __pycache__ and .pyc files from the deployment's bundle to reduce size, but does not perform application bundling.

For more about deploying FastAPI on Vercel, see:

• FastAPI official documentation
• Vercel Functions documentation
• Backend templates on Vercel