---
description: Build Python Workers APIs using FastAPI with the built-in ASGI server.
title: FastAPI
image: https://developers.cloudflare.com/og-docs.png
---

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# FastAPI

Last updated Aug 25, 2026|Copy as Markdown|[View as Markdown](https://sm-rtk-allowlist-page.previews.developers.cloudflare.com/workers/languages/python/packages/fastapi/index.md)|[Agent setup](https://sm-rtk-allowlist-page.previews.developers.cloudflare.com/agent-setup/)

The FastAPI package is supported in Python Workers.

FastAPI applications use a protocol called the [Asynchronous Server Gateway Interface (ASGI) ↗](https://asgi.readthedocs.io/en/latest/). This means that FastAPI never reads from or writes to a socket itself. An ASGI application expects to be hooked up to an ASGI server, typically [uvicorn ↗](https://uvicorn.dev/). The ASGI server handles all of the raw sockets on the application’s behalf.

The Python Workers provide [an ASGI server ↗](https://github.com/cloudflare/workers-py/blob/main/packages/runtime-sdk/src/asgi.py)that you can use directly in your Python Worker, which lets you use FastAPI in Python Workers.

## Quick Start

To get started with FastAPI in Python Workers, follow these steps:

1. Create a `src/main.py` file with your FastAPI application:

```python
from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

import asgi
from workers import WorkerEntrypoint

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        return await asgi.fetch(app, request, self.env)
```

1. Create a `wrangler.jsonc` file to configure your Worker:

```jsonc
{
	"name": "my-fastapi-app",
	"main": "src/main.py",
	// Set this to today's date
	"compatibility_date": "2026-08-25",
	"compatibility_flags": ["python_workers"],
}
```

```toml
name = "my-fastapi-app"
main = "src/main.py"
# Set this to today's date
compatibility_date = "2026-08-25"
compatibility_flags = [ "python_workers" ]
```

1. Create a `pyproject.toml` file to manage your dependencies:

```toml
[project]
name = "my-fastapi-app"
version = "0.1.0"
requires-python = ">=3.13"
dependencies = [
    "fastapi",
]

[dependency-groups]
dev = [
    "workers-py",
    "workers-runtime-sdk"
]
```

1. Run your Worker locally:

```bash
uv run pywrangler dev
```

## Serve a frontend

You can serve a single-page application (SPA) or any static frontend alongside your FastAPI backend by using [Workers Static Assets](https://sm-rtk-allowlist-page.previews.developers.cloudflare.com/workers/static-assets/).

This is equivalent to FastAPI's native [app.frontend() ↗](https://fastapi.tiangolo.com/tutorial/frontend/) method, which serves a static build directory as low-priority routes so that API path operations are checked first. The difference is where the files live: `app.frontend()` reads files from the local filesystem, while on Workers the static assets are served from Cloudflare's globally distributed asset store through the `ASSETS` binding. This means your frontend files are not bundled inside the Worker itself, keeping the bundle small.

Place your frontend build output (for example, HTML, CSS, and JavaScript files) in a directory such as `./public/`. Then configure your Wrangler file with an `assets` block that includes a `binding` and sets `run_worker_first` to `true`. This ensures every request reaches your FastAPI Worker first, so your API routes take priority over static files.

Add a catch-all route at the end of your FastAPI app that proxies unmatched requests to the assets binding:

```jsonc
{
	"name": "my-fastapi-app",
	"main": "src/worker.py",
	// Set this to today's date
	"compatibility_date": "2026-08-25",
	"compatibility_flags": ["python_workers"],
	"assets": {
		"directory": "./public/",
		"binding": "ASSETS",
		"run_worker_first": true
	}
}
```

```toml
name = "my-fastapi-app"
main = "src/worker.py"
# Set this to today's date
compatibility_date = "2026-08-25"
compatibility_flags = [ "python_workers" ]

[assets]
directory = "./public/"
binding = "ASSETS"
run_worker_first = true
```

Be sure to create a `pyproject.toml` file to manage your dependencies:

```toml
[project]
name = "my-fastapi-app"
version = "0.1.0"
requires-python = ">=3.13"
dependencies = [
    "fastapi",
]

[dependency-groups]
dev = [
    "workers-py",
    "workers-runtime-sdk"
]
```

Then write your worker:

```python
from workers import WorkerEntrypoint
from fastapi import FastAPI, Request
from fastapi.responses import Response
import asgi

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        return await asgi.fetch(app, request, self.env)

app = FastAPI()

@app.get("/api/hello")
async def api_hello():
    return {"message": "Hello from the API"}

# Catch-all: proxy everything else to Workers Static Assets.
# This is the Workers equivalent of app.frontend("/", directory="dist").
@app.get("/{path:path}")
async def frontend(path: str, request: Request):
    env = request.scope["env"]
    asset_url = f"https://assets.local/{path}"
    resp = await env.ASSETS.fetch(asset_url)
    body = await resp.bytes()
    headers = dict(resp.headers)
    return Response(content=body, status_code=resp.status, headers=headers)
```

You can run this worker locally using `uv run pywrangler dev`.

With this setup, a request to `/api/hello` is handled by FastAPI, while a request to `/index.html` or any other path is served from the `./public/` directory through the assets binding.

For more information on configuring static assets, refer to the [Workers Static Assets documentation](https://sm-rtk-allowlist-page.previews.developers.cloudflare.com/workers/static-assets/).

## More examples

Clone the `cloudflare/python-workers-examples` repository and run the FastAPI examples there:

```bash
git clone https://github.com/cloudflare/python-workers-examples
cd python-workers-examples/03-fastapi
uv run pywrangler dev
```

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