Simon Willison’s Weblog

How streaming LLM APIs work. New TIL. I used curl to explore the streaming APIs provided by OpenAI, Anthropic and Google Gemini and wrote up detailed notes on what I learned.

Also includes example code for receiving streaming events in Python with HTTPX and receiving streaming events in client-side JavaScript using fetch().

# 3:48 am / apis, http, json, llms

Jiter (via) One of the challenges in dealing with LLM streaming APIs is the need to parse partial JSON - until the stream has ended you won't have a complete valid JSON object, but you may want to display components of that JSON as they become available.

I've solved this previously using the ijson streaming JSON library, see my previous TIL.

Today I found out about Jiter, a new option from the team behind Pydantic. It's written in Rust and extracted from pydantic-core, so the Python wrapper for it can be installed using:

pip install jiter

You can feed it an incomplete JSON bytes object and use partial_mode="on" to parse the valid subset:

import jiter
partial_json = b'{"name": "John", "age": 30, "city": "New Yor'
jiter.from_json(partial_json, partial_mode="on")
# {'name': 'John', 'age': 30}

Or use partial_mode="trailing-strings" to include incomplete string fields too:

jiter.from_json(partial_json, partial_mode="trailing-strings")
# {'name': 'John', 'age': 30, 'city': 'New Yor'}

The current README was a little thin, so I submiitted a PR with some extra examples. I got some help from files-to-prompt and Claude 3.5 Sonnet):

cd crates/jiter-python/ && files-to-prompt -c README.md tests | llm -m claude-3.5-sonnet --system 'write a new README with comprehensive documentation'

# 8:03 pm / json, python