Overview

Capsule is a runtime for coordinating AI agent tasks in isolated environments. It is designed to handle, long-running workflows, large-scale processing, autonomous decision-making securely, or even multi-agent systems.

Each task runs inside its own WebAssembly sandbox, providing:

• Isolated execution: Each task runs isolated from your host system
• Resource limits: Set CPU, memory, and timeout limits per task
• Automatic retries: Handle failures without manual intervention
• Lifecycle tracking: Monitor which tasks are running, completed, or failed

This enables safe task-level execution of untrusted code within AI agent systems.

How It Works

Capsule leverages Wasm to create secure, isolated execution environments.

With Python

Simply annotate your Python functions with the @task decorator:

from capsule import task

@task(name="analyze_data", compute="MEDIUM", ram="512MB", timeout="30s", max_retries=1)
def analyze_data(dataset: list) -> dict:
    """Process data in an isolated, resource-controlled environment."""
    # Your code runs safely in a Wasm sandbox
    return {"processed": len(dataset), "status": "complete"}

With TypeScript / JavaScript

Capsule now supports TypeScript and JavaScript with the task() wrapper function. This offers compatibility with the entire JavaScript ecosystem.

import { task } from "@capsule-run/sdk";

export const analyzeData = task({
  name: "analyze_data",
  compute: "MEDIUM",
  ram: "512MB",
  timeout: "30s",
  maxRetries: 1
}, (dataset: number[]): object => {
  // Your code runs safely in a Wasm sandbox
  return { processed: dataset.length, status: "complete" };
});

// The "main" task is required as the entrypoint
export const main = task({
    name: "main",
    compute: "HIGH"
}, () => {
  return analyzeData([1, 2, 3, 4, 5]);
});

[!NOTE] TypeScript/JavaScript projects require a task named "main" as the entrypoint.

When you run capsule run main.py (or main.ts), your code is compiled into a WebAssembly module and executed in a dedicated sandbox to isolate tasks.

Each task operates within its own sandbox with configurable resource limits, ensuring that failures are contained and don't cascade to other parts of your workflow. The host system controls every aspect of execution, from CPU allocation via Wasm fuel metering to memory constraints and timeout enforcement.

Quick Start

Python

pip install capsule-run

Create hello.py:

from capsule import task

@task(name="main", compute="LOW", ram="64MB")
def main() -> str:
    return "Hello from Capsule!"

Run it:

capsule run hello.py

TypeScript / JavaScript

npm install -g @capsule-run/cli
npm install @capsule-run/sdk

Create hello.ts:

import { task } from "@capsule-run/sdk";

export const main = task({
  name: "main",
  compute: "LOW",
  ram: "64MB"
}, (): string => {
  return "Hello from Capsule!";
});

Run it:

capsule run hello.ts

[!TIP] Use --verbose to display real-time task execution details.

Documentation

Task Configuration Options

Configure your tasks with these parameters:

Compute Levels

Capsule controls CPU usage through WebAssembly's fuel mechanism, which meters instruction execution. The compute level determines how much fuel your task receives.

• LOW provides minimal allocation for lightweight tasks
• MEDIUM offers balanced resources for typical workloads
• HIGH grants maximum fuel for compute-intensive operations
• CUSTOM to specify an exact fuel value (e.g., compute="1000000") for precise control over execution limits.

HTTP Client API

Python

The standard Python requests library and socket-based networking aren't natively compatible with WebAssembly's sandboxed I/O model. Capsule provides its own HTTP client that works within the Wasm environment:

from capsule import task
from capsule.http import get, post, put, delete

@task(name="http_example", compute="MEDIUM", timeout="30s")
def main() -> dict:
    """Example demonstrating HTTP client usage within a task."""

    # GET request
    response = get("https://api.example.com/data")

    # POST with JSON body
    response = post("https://api.example.com/submit", json={"key": "value"})

    # Response methods
    is_ok = response.ok()           # Returns True if status code is 2xx
    status = response.status_code    # Get the HTTP status code
    data = response.json()           # Parse response as JSON
    text = response.text()           # Get response as text

    return {"status": status, "success": is_ok}

TypeScript / JavaScript

Capsule also provides an HTTP client for TypeScript/JavaScript via @capsule-run/sdk. However, standard libraries like fetch already compatible, so you can use whichever approach you prefer.

Compatibility

[!NOTE] TypeScript/JavaScript has broader compatibility than Python since it doesn't rely on native bindings.

Python: Pure Python packages and standard library modules work. Packages with C extensions (numpy, pandas) are not yet supported.

TypeScript/JavaScript: npm packages and ES modules work. Node.js built-ins (fs, path, os) are not available in the sandbox.

Contributing

Contributions are welcome!

Development setup

Prerequisites: Rust (latest stable), Python 3.13+, Node.js 22+

git clone https://github.com/mavdol/capsule.git
cd capsule

# Build and install CLI
cargo install --path crates/capsule-cli

# Python SDK (editable install)
pip install -e crates/capsule-sdk/python

# TypeScript SDK (link for local dev)
cd crates/capsule-sdk/javascript
npm install && npm run build && npm link

# Then in your project: npm link @capsule-run/sdk

How to contribute

1. Fork the repository
2. Create a feature branch: git checkout -b feature/amazing-feature
3. Run tests: cargo test
4. Open a Pull Request

Need help? Open an issue

Credits

Capsule builds on these open source projects:

• componentize-py – Python to WebAssembly Component compilation
• jco – JavaScript toolchain for WebAssembly Components
• wasmtime – WebAssembly runtime
• WASI – WebAssembly System Interface

License

This project is licensed under the Apache License 2.0 - see the LICENSE file for details.