Flask

• Flask's deployment docs explicitly say the built-in development server should not be used in production, which means the paid layer is your production serving stack.
• Gunicorn, mod_wsgi, containers, or managed platforms each create different cost and operational tradeoffs even though Flask itself stays free.
• Async support exists, but teams expecting websockets or full ASGI patterns still need to choose the rest of the serving architecture carefully.
• Extensions or admin tooling may have their own licenses, but Flask core remains a zero-license-cost framework.

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