@pydantic/monty

Run untrusted Python safely from JavaScript. In Node.js this uses a pool of crash-isolated monty interpreter subprocesses; browser bundlers resolve the same public API to a Web Worker pool backed by a lean wasm build.

Monty is a sandboxed Python interpreter written in Rust. A sandbox process can never be made fully crash-proof against memory errors (stack overflow, allocator aborts), so this package only runs the interpreter in worker subprocesses: a worker that crashes raises MontyCrashedError, is replaced by the pool, and your Node.js process is never at risk.

The native binding and the monty binary ship together via platform-specific npm packages installed automatically (like esbuild). Browser builds use the package browser export and never import the napi loader; they run the sandbox in a Web Worker (wasm32-wasip1) with the same pool/session API. Advanced Node-only helpers are available from @pydantic/monty/node, and wasm-specific factories from @pydantic/monty/wasm.

Installation

npm install @pydantic/monty

Basic Usage

import { Monty } from '@pydantic/monty'

await using pool = await Monty.create()
await using session = await pool.checkout()

const result = await session.feedRun('1 + 2') // 3

A session is a REPL in a dedicated worker — state persists across feeds:

await session.feedRun('x = 21')
await session.feedRun('x * 2') // 42

Without await using, call session.close() (returns the worker to the pool) and pool.close() explicitly.

Inputs

Pass values as globals for a feed:

await session.feedRun('x + y', { inputs: { x: 10, y: 20 } }) // 30

External Lookup

externalLookup resolves names a snippet leaves undefined, lazily and on demand. A function entry becomes a host function the sandbox can call by name — sync or async (async functions are awaited while other sandbox tasks keep running). Any other value is converted and returned directly when the name is read. An absent name raises NameError.

await session.feedRun('add(2, 3)', {
  externalLookup: { add: (a: number, b: number) => a + b },
}) // 5

await session.feedRun('await fetch_data(url)', {
  inputs: { url: 'https://example.com' },
  externalLookup: {
    fetch_data: async (url: string) => {
      const response = await fetch(url)
      return response.text()
    },
  },
})

await session.feedRun('greeting + name', {
  inputs: { name: 'Ada' },
  externalLookup: { greeting: 'hello ' },
}) // 'hello Ada'

externalLookup is the lazy counterpart to inputs, which eagerly binds every entry as a global whether or not it is referenced; a name in both is served by the eager inputs binding.

For function entries, keyword arguments arrive as a trailing object; thrown errors cross into the sandbox as Python exceptions (the error's name is used when it matches a Python exception type, e.g. TypeError, otherwise RuntimeError).

Snapshots: pausing and resuming

feedStart is the suspendable counterpart of feedRun: instead of driving a snippet to completion, it returns a snapshot at each external call, OS call, or name lookup. Answer it with snapshot.resume(...), which resolves to the next snapshot or a MontyComplete.

import { FunctionSnapshot, MontyComplete } from '@pydantic/monty'

const snap = await session.feedStart('greet(name) + "!"', { inputs: { name: 'Ada' } })
if (snap instanceof FunctionSnapshot) {
  // snap.functionName === 'greet', snap.args === ['Ada']
  const done = await snap.resume('hello Ada')
  if (done instanceof MontyComplete) console.log(done.output) // 'hello Ada!'
}

To iterate a snippet to completion without answering each suspension by hand, pass an externalLookup (and/or os) to feedStart and drive with snapshot.resumeAuto(), which resolves each external call and name lookup from them automatically — the same resolution feedRun performs, but one step at a time so you can inspect or dump() each snapshot along the way. A promise-returning external is awaited concurrently (surfacing as an intermediate FutureSnapshot), exactly as under feedRun:

let snap = await session.feedStart('greet(name) + "!"', {
  inputs: { name: 'Ada' },
  externalLookup: { greet: (n: string) => `hello ${n}` },
})
while (!(snap instanceof MontyComplete)) {
  snap = await snap.resumeAuto()
}
console.log(snap.output) // 'hello Ada!'

snapshot.dump() serializes the paused worker to bytes; a fresh session's loadSnapshot restores it and returns the snapshot to resume. Re-supply the same mounts the paused feed used — their host paths are not stored in the dump.

const blob = await snap.dump()
// ...later, in a fresh session:
const restored = await session.loadSnapshot(blob)
if (restored instanceof FunctionSnapshot) await restored.resume('value')

session.dump() between feeds serializes an idle session instead; restore it with await session.loadSession(blob) (which resolves to void) and keep feeding. Both loadSession and loadSnapshot are valid only on a fresh session, before any feed; using the wrong one for a dump's kind throws.

Print Output

printCallback accepts a function or a host collector (PrintTargetInput in TypeScript). Output is line-buffered; without a callback it goes to the host process stdout/stderr.

// Function form
await session.feedRun('print("hello")', {
  printCallback: (stream, text) => console.log(`[${stream}] ${text}`),
})

// Collectors — accumulate on the host (not covered by ResourceLimits.maxMemory)
import { CollectString, CollectStreams, DEFAULT_MAX_PRINT_COLLECT_BYTES } from '@pydantic/monty'

const text = new CollectString()
await session.feedRun('print("hello")', { printCallback: text })
text.output // 'hello\n'

const streams = new CollectStreams()
await session.feedRun('print("hello")', { printCallback: streams })
streams.output // [{ stream: 'stdout', text: 'hello\n' }]

Both collectors default to a 10 MiB cap (DEFAULT_MAX_PRINT_COLLECT_BYTES). Pass maxBytes: null to disable (trusted hosts only). maxBytes must be a finite non-negative number or null (constructors throw TypeError otherwise). Exceeding the cap rejects the feed with MontyRuntimeError / MemoryError (memory limit exceeded: …).

Filesystem Mounts

Mount host directories into the sandbox at virtual POSIX paths:

import { MountDir } from '@pydantic/monty'

const mount = new MountDir({ hostPath: '/path/on/host', virtualPath: '/mnt/data', mode: 'read-only' })
await session.feedRun("open('/mnt/data/file.txt').read()", { mount })

Each mount has a 100 MB aggregate memory budget by default. Configure it with memoryUsageLimit; retained overlay data and filesystem results share it, and operations that exceed it raise a MontyRuntimeError wrapping MemoryError.

Modes: 'read-only', 'read-write', and 'overlay' (default — writes are kept in memory and discarded at the end of the feed). Mount I/O is serviced on the host side of the pool, so mounts work even for remote workers.

feedRun answers every OS call automatically: mounts get first refusal, then the os callback. feedStart answers none — a mounted read surfaces as a FunctionSnapshot with isOsFunction set, and resumeAuto() is what consults the mounts and os. OS calls mounts don't cover reach the os callback:

import { NOT_HANDLED } from '@pydantic/monty'

await session.feedRun('import os\nos.getenv("HOME")', {
  os: (name, args) => (name === 'os.getenv' && args[0] === 'HOME' ? '/home/user' : NOT_HANDLED),
})

Resource Limits

Enforced inside the worker, configured per session:

const limited = await pool.checkout({
  limits: { maxMemory: 100 * 1024 * 1024, maxDurationSecs: 5, maxRecursionDepth: 100 },
})

requestTimeout on the pool is the backstop for code that wedges the interpreter itself: the worker is killed and the session fails with MontyCrashedError (timedOut: true).

maxDurationSecs limits cumulative execution time: the sandbox clock runs only while the interpreter executes, never while suspended waiting on an external function or between feeds. Sessions with the limit also get an automatic backstop: the worker reports its execution time on every protocol turn and the host kills it durationLimitGrace (default 1s) after the remaining budget expires, covering cases where the in-sandbox limit cannot fire (its check only runs at interpreter checkpoints). Set durationLimitGrace: null to disable it.

Assert message annotations

Failed assert statements carry a pytest-style introspected message by default (AssertionError: assert 2 == 5) — a deliberate divergence from CPython's empty AssertionError. Each operand's repr is truncated to 120 characters by default. Disable the messages per session to restore CPython's behavior, or pass an integer to customize the truncation length:

const session = await pool.checkout({ assertMessageAnnotations: false })
const verbose = await pool.checkout({ assertMessageAnnotations: 1000 })

Type Checking

import { MontyTypingError } from '@pydantic/monty'

const session = await pool.checkout({ typeCheck: true, typeCheckStubs: 'def fetch(url: str) -> str: ...' })
try {
  await session.feedRun('fetch(123)')
} catch (err) {
  if (err instanceof MontyTypingError) {
    console.log(err.display()) // rendered diagnostics, one per line
  }
}

A snippet that fails type checking does not run; the session survives.

Error Handling

import { MontyError, MontySyntaxError, MontyRuntimeError, MontyCrashedError } from '@pydantic/monty'

try {
  await session.feedRun('1 / 0')
} catch (err) {
  if (err instanceof MontyRuntimeError) {
    console.log(err.exception.typeName) // 'ZeroDivisionError'
    console.log(err.display('traceback')) // full Python-style traceback
  }
}

MontyError is the base class; MontyCrashedError means the worker process died (the session is lost, the pool recovers).

Pool Configuration

const pool = await Monty.create({
  minProcesses: 1, // prewarmed workers
  maxProcesses: 8, // cap; checkouts beyond it wait (default: CPU count)
  checkoutTimeout: 10, // seconds to wait for a free worker
  requestTimeout: 30, // hard per-turn deadline (seconds)
  durationLimitGrace: 1, // maxDurationSecs backstop grace (seconds, null disables)
  maxCheckoutsPerWorker: 100, // recycle workers after this many sessions
  binaryPath: '/path/to/monty', // explicit binary (default: auto-resolved)
})

The monty binary resolves from: explicit binaryPath → the MONTY_BIN environment variable → the installed platform package → PATH → a cargo workspace target/ build (development).

Value Conversion

| Python | JavaScript | | ----------------- | ------------------------------------------------------- | | None | null | | bool | boolean | | int | number (±2^53) or BigInt | | float | number | | str | string | | bytes | Buffer | | list | Array | | tuple | Array with non-enumerable __tuple__: true | | dict | Map (preserves key types and order) | | set/frozenset | Set | | datetime types | marker objects ({ __monty_type__: 'DateTime', ... }) | | dataclasses | marker objects ({ __monty_type__: 'Dataclass', ... }) |

Plain objects are accepted as dict inputs (string keys).