monty-plugin

A hyper-mcp plugin that executes Python code in a sandboxed Monty interpreter, compiled to WebAssembly.

Overview

monty-plugin exposes a single MCP tool called run that accepts Python source code (and optional input variables), executes it inside the Monty interpreter, and returns the captured stdout output along with the result value.

Because the plugin compiles to a .wasm module (wasm32-wasip1), it runs in a fully sandboxed environment — there is no access to the host system beyond what the plugin explicitly provides.

Tool: run

Parameters

Name	Type	Required	Description
code	string	✅	The Python code to execute.
inputs	map<string, MontyObject>	No	Input variables passed into the Python runtime.
resource_limits	ResourceLimits	No	Execution resource limits for the interpreter.

ResourceLimits

All fields are optional. When omitted, the corresponding limit is not enforced (except max_recursion_depth, which defaults to 1000).

Field	Type	Description
max_allocations	integer	Maximum number of heap allocations allowed.
max_duration	{secs: integer, nanos: integer}	Maximum execution time.
max_memory	integer	Maximum heap memory in bytes (approximate).
gc_interval	integer	Run garbage collection every N allocations.
max_recursion_depth	integer	Maximum recursion depth (function call stack depth).

Response

Field	Type	Description
output	string	Captured print/stdout output produced by the Python code.
result	MontyObject	The return value of the Python code.

Built-in Functions

The following functions are available to Python code running inside the plugin:

http_request

http_request(
    url: str,
    method: str | None = None,
    headers: dict[str, str] | None = None,
    body: str | bytes | None = None,
) -> tuple[int, dict[str, str], str | bytes]

Make HTTP requests from within the sandbox. Returns a tuple of (status_code, response_headers, response_body).

notify_progress

notify_progress(
    message: str | None,
    progress: int | float,
    total: int | float | None = None,
) -> None

Report progress back to the MCP client. Requires a progressToken in the request context.

Supported pathlib.Path Operations

The plugin implements a subset of Python's pathlib.Path API:

• Path.exists(*, follow_symlinks=True) -> bool
• Path.is_file(*, follow_symlinks=True) -> bool
• Path.is_dir(*, follow_symlinks=True) -> bool
• Path.is_symlink() -> bool
• Path.read_text(encoding=None, errors=None, newline=None) -> str
• Path.read_bytes() -> bytes
• Path.write_text(data, encoding=None, errors=None, newline=None) -> None
• Path.write_bytes(data) -> None
• Path.mkdir(mode=0o777, parents=False, exist_ok=False) -> None
• Path.unlink(missing_ok=False) -> None
• Path.rmdir() -> None
• Path.iterdir() -> list[str]
• Path.stat(*, follow_symlinks=True) -> os.stat_result
• Path.rename(target) -> None
• Path.resolve(strict=False) -> str
• Path.absolute() -> str

Supported os Operations

• os.getenv(key, default=None) -> str | None

Supported datetime Operations

• datetime.datetime.now(tz=None) -> datetime.datetime
• datetime.date.today() -> datetime.date

Building

Prerequisites

• Rust 1.94+ (pinned via rust-toolchain.toml)
• The wasm32-wasip1 target

Install the WASM target

rustup target add wasm32-wasip1

Build the plugin

cargo build --release --target wasm32-wasip1

The compiled plugin will be at target/wasm32-wasip1/release/plugin.wasm.

Usage with hyper-mcp

Add the plugin to your hyper-mcp configuration. You can reference the OCI artifact published to GitHub Container Registry:

{
  "name": "monty",
  "oci": "ghcr.io/hyper-mcp-rs/monty-plugin:latest"
}

Or pin to a specific version tag:

{
  "name": "monty",
  "oci": "ghcr.io/hyper-mcp-rs/monty-plugin:v0.1.0"
}

You can pull the artifact directly with ORAS:

oras pull ghcr.io/hyper-mcp-rs/monty-plugin:latest

All release artifacts are signed with Cosign. Verify with:

cosign verify \
  --certificate-identity-regexp "https://github.com/hyper-mcp-rs/monty-plugin/.github/workflows/" \
  --certificate-oidc-issuer-regexp "https://token.actions.githubusercontent.com" \
  ghcr.io/hyper-mcp-rs/monty-plugin:latest

Docker

A minimal scratch-based Docker image is provided for packaging the .wasm artifact:

# First build the plugin
cargo build --release --target wasm32-wasip1
cp target/wasm32-wasip1/release/plugin.wasm .

# Then build the Docker image
docker build -t monty-plugin .

Project Structure

monty-plugin/
├── src/
│   ├── lib.rs                   # MCP entry-points and Monty execution loop
│   ├── types.rs                 # RunArguments, RunResponse, MontyObject wrapper
│   ├── function_calls.rs        # http_request, notify_progress implementations
│   ├── os_calls.rs              # pathlib.Path and os.* operation handlers
│   ├── python_args.rs           # Positional/keyword argument resolution helpers
│   ├── monty_object.schema.json # JSON Schema for MontyObject
│   └── pdk/                     # Plugin Development Kit glue
│       ├── mod.rs
│       ├── exports.rs           # Extism-exported MCP handler functions
│       ├── imports.rs           # Host-imported functions
│       └── types.rs             # MCP protocol types
├── Cargo.toml
├── Dockerfile
├── rust-toolchain.toml
└── LICENSE

License

This project is licensed under the Apache License 2.0.