HTTP Protocols¶

Uvicorn provides several HTTP protocol implementations that you can choose from using the --http option:

uvicorn main:app --http <auto|h11|httptools|zttp|zttp1|zttp2>

By default, Uvicorn uses --http auto, which automatically selects:

1. httptools - If httptools is installed, Uvicorn will use it for maximum performance
2. h11 - If httptools is not available, Uvicorn falls back to h11

h11¶

h11 is a pure Python HTTP/1.1 implementation. It is a required dependency of Uvicorn, so it is always available, and it is the only implementation compatible with PyPy.

httptools¶

httptools is a Python binding for the Node.js HTTP parser. It is installed as part of the uvicorn[standard] optional extras, and provides greater performance than h11, but is not compatible with PyPy.

zttp¶

zttp is a sans-IO HTTP parser for Python with a core written in Zig. Prebuilt wheels are available for CPython on Linux, macOS and Windows.

You can install it with:

pip install zttp

uv add zttp

You can run uvicorn with zttp with the following command:

uvicorn main:app --http zttp

zttp is the only implementation with HTTP/2 support. It comes in three variants: zttp serves both HTTP/1.1 and HTTP/2, zttp1 serves HTTP/1.1 only, and zttp2 serves HTTP/2 only. See the HTTP/2 documentation for details.