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/ or app/

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

Vercel supports two methods for serving frontend and static assets with FastAPI.

Place files in a public/ directory at your project root. Vercel serves them from the CDN at the matching root URL paths. For example, public/logo.svg is served at /logo.svg. Default response headers apply 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 served automatically from public/vercel.svg.
    return RedirectResponse("/vercel.svg", status_code=307)

When using app.frontend() or app.mount() with StaticFiles, files are promoted to the CDN at build time:

from fastapi import FastAPI
 
app = FastAPI()
 
@app.get("/api/hello")
def hello():
    return {"message": "Hello"}
 
app.frontend("/", directory="dist")

from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles
 
app = FastAPI()
 
app.mount("/assets", StaticFiles(directory="assets"), name="assets")

Promoted source directories are kept in the function bundle by default so the app can read from them at runtime. To exclude them and serve files from the CDN only, set exclude = true:

[tool.vercel.fastapi.static]
exclude = true

See Configuration for all available settings.

Files are served from the CDN at the mount's URL prefix. Route declaration order determines which wins when a route and a CDN file share the same path:

• A route declared before the mount takes priority over CDN files. Matching requests reach the function.
• A route declared after the mount does not take priority. Matching requests are served from the CDN.

from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles
 
app = FastAPI()
 
# Declared before the mount, so this route wins over any CDN file at this path.
@app.get("/static/protected.json")
def protected():
    return {"access": "denied"}
 
app.mount("/static", StaticFiles(directory="static"))

The bare mount root (for example, /static) always reaches the function. StaticFiles redirects it to the trailing-slash form.

app.frontend() registers a low-priority frontend build. Every API route takes priority over frontend files regardless of declaration order.

The fallback parameter controls CDN behavior when no file matches a request under the mount:

An index.html fallback applies to navigation requests only: requests with an explicit text/html or application/xhtml+xml Accept header and an extension-less final URL segment (for example, /dashboard but not /dashboard/app.js). All other misses reach the function. A 404.html fallback applies to every miss regardless of the Accept header. If "auto" is set but neither file exists in the build directory, no fallback is applied.

Fallback routes apply to GET and HEAD requests only. All other methods always reach the function.

CDN-served files bypass the function entirely, so middleware handlers and Depends() guards do not run for them.

Vercel detects these cases and keeps the affected static files and frontends in the function rather than promoting them to the CDN:

• Top-level middleware: All static mounts and frontends stay in the function.
• Sub-app middleware: Only that sub-app's mounts stay in the function. Mounts elsewhere are still promoted.
• Frontend dependencies: Frontends with Depends() guards stay in the function.

Set cdn = true to promote files to the CDN even when middleware or dependency guards are present:

[tool.vercel.fastapi.static]
cdn = true

All settings go under [tool.vercel.fastapi.static] in pyproject.toml.

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).

For more about deploying FastAPI on Vercel, see:

• FastAPI official documentation
• Build with a FastAPI starter template
• How to ship a FastAPI app on Vercel
• Vercel Functions documentation
• Backend templates on Vercel