What this is

Agent frameworks and tool-execution runtimes increasingly need to run JavaScript the model itself wrote — a code-generation step, a small data transform, a "call this JS snippet to compute the answer" tool. That code is untrusted by construction. pydeno embeds a real V8 isolate per runtime (via Rust/PyO3) so that code runs with no filesystem, no network, and no ambient Node.js APIs by default, with heap and wall-clock limits you set, and a real way to kill it if it doesn't stop on its own.

It is not a general "embed any JS anywhere" library and not a JS interpreter reimplemented in Python — it's V8, the same engine behind Chrome and Node.js, so the JS your model writes actually behaves like JS.

import pydeno

pydeno.bind_function("add", lambda a, b: a + b)
print(pydeno.eval("add(2, 3)"))  # 5

Why it's different

• Real V8, not a toy interpreter. Every language quirk, every standard library method, every performance characteristic your model expects from "JavaScript" is the real thing.

• Cross-thread termination that actually works. A watchdog thread calling .terminate() on a runaway while(true){} used to panic instead of stopping the code (pyo3_runtime.PanicException: Runtime is unsendable, but sent to another thread) — a real bug in the upstream design that meant a stuck agent tool call couldn't be killed from outside. TerminationHandle fixes this: it exposes only the Send + Sync part of V8's isolate handle, so a watchdog on a separate thread can interrupt execution safely. See upstream-divergence.md for the full root cause and proof; tests/test_termination_handle.py is the regression test.

  As of 0.2.0 this works on every shape of stuck script, not just runaway loops. terminate() used to be a no-op against a runtime parked on a pending promise — V8 only acts on a termination when it next enters JavaScript, and new Promise(() => {}) never does, so it was unkillable. The dispatcher now reads the termination flag between polls, killing all such shapes in ~1.7 ms, while while(true){} still dies the cheap way at ~0.12 ms and a killed runtime keeps its bound functions. See BENCHMARKS.md and tests/test_parked_termination.py.

• A host-callback bridge for tool-calling from JS. bind_function/ bind_object let the sandboxed JS call back into Python — the primitive an agent needs when the model's JS wants to invoke a real tool, not just compute a value. ToolBridge builds the product shape on top: hand it a dict of callables and get a call budget, fail-closed name checking, and JS-catchable typed errors, instead of hand-rolling all three.

• Tool errors JS can branch on. A Python exception raised inside a bound tool reaches JavaScript as a real Error whose name is the exception's class name, so the model's JS can tell "bad arguments" from "rate limited" from "tool not found" with a plain catch (e) { if (e.name === ...) } — rather than string-matching one flattened TypeError. The same name/message shape a JS exception already carries into Python, so the two directions are symmetric.

• console.log you can actually read. Sandboxed console.* output can be routed back to a Python callback with RuntimeConfig(on_console=...), receiving structured (level, args) — the single most common thing you want when showing a model what its own script printed. Composes with enable_console rather than replacing it.

Get Started

pip install pydeno  # or uv pip install pydeno

Requires Python 3.10+ on macOS or Linux. pydeno is experimental — expect breaking changes between versions.

Quickstart: sandboxed eval, tool-calling, and a timeout

import asyncio

from pydeno import Runtime, RuntimeConfig

# Cap memory; the runtime terminates itself if JS tries to exceed it.
config = RuntimeConfig(max_heap_size=10 * 1024 * 1024)  # 10 MB


async def main():
    with Runtime(config) as runtime:
        # The tool-calling primitive: expose a Python function to the sandboxed JS.
        def get_weather(city: str) -> str:
            return f"72F and sunny in {city}"

        runtime.bind_function("getWeather", get_weather)

        # Model-generated JS calling back into your tool.
        result = runtime.eval("getWeather('Zurich')")
        print(result)  # "72F and sunny in Zurich"

        # Enforce a wall-clock timeout on code that might not terminate.
        try:
            await runtime.eval_async("while (true) {}", timeout=2.0)
        except Exception as exc:
            print(f"killed a runaway eval after 2s: {exc}")


asyncio.run(main())

Running an untrusted sync eval() you can't await? Grab a TerminationHandle before the call and .terminate() it from a watchdog thread — see tests/test_termination_handle.py for the exact pattern (and the bug it fixed, in docs/contributing/upstream-divergence.md).

Many tools, one budget

When the model gets more than one tool, ToolBridge gives you the whole surface — budget, name checking, typed errors — in one object:

from pydeno import Runtime, ToolBridge

bridge = ToolBridge(
    {"get_weather": get_weather, "send_email": send_email},
    max_calls=50,  # total tool calls this agent turn may make
)

with Runtime() as runtime:
    bridge.attach(runtime)
    runtime.eval("tools.get_weather('Zurich')")

The call that exceeds max_calls never reaches your Python function — JS gets a catchable ToolBudgetError.

Integrations

• FastMCP tool bridge - expose FastMCP tools to sandboxed JS via bind_function and an in-process fastmcp.Client
• pydantic-ai "code mode" agent - an agent tool where the model submits one JS batch script instead of many separate tool calls, run safely with a timeout
• ToolBridge - hand a sandbox several Python tools with a total call budget, typed errors the model's JS can branch on, and console.log routed back to Python
• Vendored npm libraries - run real npm document-generation libraries (pptxgenjs, pdf-lib) from their browser bundles inside the sandbox; see the guide for what makes a library a good candidate
• Arrow IPC dataframes - move 100k+ row tables into the sandbox as Arrow IPC bytes instead of JSON objects (5 ms vs 253 ms, and it works at the default 10 MB serialization limit that rejects the JSON payload); see the guide

Documentation

• Quick Start
• Concepts: runtimes, type conversion, resource controls
• Guides: binding functions, module loading, snapshots
• Use cases: AI agent sandboxes, workflow runners, plugin systems
• API reference
• Agent skill: an Agent Skills SKILL.md that teaches coding agents (Claude Code and others) to use pydeno correctly. Copy skills/pydeno/ into your agent's skills directory, e.g. .claude/skills/
• Benchmarks — measured, reproducible numbers, including the pooling comparison above