langchain-quickjs

langchain-quickjs provides a QuickJS-backed REPL middleware for Deep Agents. It adds a repl tool that can evaluate small JavaScript snippets for computation, control flow, JSON manipulation, and calls to exposed Python foreign functions.

Basic usage

from deepagents import create_deep_agent
from langchain_quickjs import QuickJSMiddleware


def normalize_name(name: str) -> str:
    return name.strip().lower()


agent = create_deep_agent(
    model="openai:gpt-4.1",
    tools=[],
    middleware=[
        QuickJSMiddleware(
            ptc=[normalize_name],
            add_ptc_docs=True,
        )
    ],
)

With this middleware installed, the agent receives a repl tool that runs each JavaScript evaluation in a fresh QuickJS context. If you expose Python callables through ptc, they are available inside the REPL as foreign functions.

REPL behavior

• The REPL is stateless. Each call starts from a fresh QuickJS context, so variables, functions, and other values defined in one repl call are not available in the next one.
• Execution uses QuickJS, so JavaScript support is limited to what QuickJS provides. It is good for small computations, control flow, JSON manipulation, and calling exposed foreign functions, but it is not a browser or Node.js runtime and does not provide their APIs.
• Foreign functions support passing primitive values between JavaScript and Python, including int, float, bool, str, and None. Lists and dictionaries returned from Python are also supported and are bridged back into JavaScript arrays and objects.
• Async foreign functions are supported. Because QuickJS callbacks are synchronous, awaitables are delegated to a dedicated daemon-thread event loop and their resolved results are returned back into the REPL call.

Current limitations

• Does not work with HIL in the REPL.
• Does not support ToolRuntime yet.