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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>

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

The zttp implementation serves HTTP/1.1 by default. This keeps experimental HTTP/2 support opt-in. You can enable HTTP/2 negotiation with:

uvicorn main:app --http zttp --http2

See the HTTP/2 documentation for details.

Experimental

zttp support is currently experimental and not suited for production usage. If you try it out, please report any issues or feedback on the issue tracker.