FastHTML — Build AI Web Apps in Pure Python

What it is

FastHTML is a Python web framework that lets you build modern, interactive web applications entirely in Python. It uses HTMX for client-side interactivity, so you write Python functions that return HTML components without touching JavaScript. Server-side rendering is the default, making it naturally suited for AI-integrated applications where the backend does the heavy lifting.

FastHTML targets Python developers, data scientists, and AI engineers who want to build web UIs for their models and tools without learning a frontend framework.

How it saves time or tokens

Traditional web development requires separate frontend and backend codebases, typically involving React or Vue plus a Python API. FastHTML eliminates the frontend build step entirely. You write Python functions decorated with routes, return HTML components, and HTMX handles dynamic updates. This cuts the technology stack in half and removes the need for API serialization between frontend and backend.

How to use

1. Install FastHTML:

pip install python-fasthtml

1. Create a minimal app:

from fasthtml.common import *

app, rt = fast_app()

@rt('/')
def get():
    return Titled('Hello', P('Welcome to FastHTML'))

serve()

1. Run your app:

python main.py
# Open http://localhost:5001

Example

An AI chat interface built with FastHTML:

from fasthtml.common import *
import openai

app, rt = fast_app()
messages = []

@rt('/')
def get():
    chat_list = Ul(*[Li(m) for m in messages], id='chat')
    form = Form(
        Input(name='msg', placeholder='Ask anything...'),
        Button('Send'),
        hx_post='/chat', hx_target='#chat', hx_swap='innerHTML'
    )
    return Titled('AI Chat', chat_list, form)

@rt('/chat')
def post(msg: str):
    messages.append(f'You: {msg}')
    response = openai.chat.completions.create(
        model='gpt-4o-mini',
        messages=[{'role': 'user', 'content': msg}]
    )
    messages.append(f'AI: {response.choices[0].message.content}')
    return Ul(*[Li(m) for m in messages])

serve()

Related on TokRepo

• Coding tools — developer tools for building AI applications
• AI tools for content — content generation and web tools

Common pitfalls

• FastHTML uses HTMX for interactivity; complex client-side state management (like drag-and-drop) still requires JavaScript
• The framework is relatively new; community resources and third-party plugins are limited compared to Flask or Django
• Server-side rendering means every interaction hits the server; for high-traffic applications, consider adding caching or rate limiting