`Capsule`

A secure, durable runtime for AI agents

Getting Started • Documentation • Contributing

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

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

For TypeScript and JavaScript, use the task() wrapper function with full access to the npm 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] The runtime requires a task named "main" as the entry point. Python can define the main task itself, but it's recommended to set it manually.

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.

Response Format

Every task returns a structured JSON envelope containing both the result and execution metadata:

{
  "success": true,
  "result": "Hello from Capsule!",
  "error": null,
  "execution": {
    "task_name": "data_processor",
    "duration_ms": 1523,
    "retries": 0,
    "fuel_consumed": 45000
  }
}

Response fields:

success — Boolean indicating whether the task completed successfully
result — The actual return value from your task (json, string, null on failure etc..)
error — Error details if the task failed ({ error_type: string, message: string })
execution — Performance metrics:
- task_name — Name of the executed task
- duration_ms — Execution time in milliseconds
- retries — Number of retry attempts that occurred
- fuel_consumed — CPU resources used (see Compute Levels)

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:

Parameter	Description	Type	Default	Example
`name`	Task identifier	`str`	function name (Python) / required (TS)	`"process_data"`
`compute`	CPU allocation level: `"LOW"`, `"MEDIUM"`, or `"HIGH"`	`str`	`"MEDIUM"`	`"HIGH"`
`ram`	Memory limit for the task	`str`	unlimited	`"512MB"`, `"2GB"`
`timeout`	Maximum execution time	`str`	unlimited	`"30s"`, `"5m"`, `"1h"`
`max_retries` / `maxRetries`	Number of retry attempts on failure	`int`	`0`	`3`
`allowed_files` / `allowedFiles`	Folders accessible in the sandbox	`list`	`[]`	`["./data", "./output"]`
`env_variables` / `envVariables`	Environment variables accessible in the sandbox	`list`	`[]`	`["API_KEY"]`

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.

Project Configuration (Optional)

You can create a capsule.toml file in your project root to set default options for all tasks and define workflow metadata:

# capsule.toml

[workflow]
name = "My AI Workflow"
version = "1.0.0"
entrypoint = "src/main.py"  # Default file when running `capsule run`

[tasks]
default_compute = "MEDIUM"
default_ram = "256MB"
default_timeout = "30s"
default_max_retries = 2

With an entrypoint defined, you can simply run:

capsule run

Task-level options always override these defaults when specified.

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

Standard libraries like fetch are already compatible, so no custom HTTP client is needed for TypeScript/JavaScript.

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

export const main = task({
    name: "main",
    compute: "MEDIUM"
}, async () => {
    const response = await fetch("https://api.example.com/data");
    return response.json();
});

File Access

Tasks can read and write files within directories specified in allowed_files. Any attempt to access files outside these directories is not possible.

[!NOTE] Currently, allowed_files supports directory paths, not individual files.

Python

Python's standard file operations work normally. Use open(), os, pathlib, or any file manipulation library.

from capsule import task

@task(name="restricted_writer", allowed_files=["./output"])
def restricted_writer() -> None:
    with open("./output/result.txt", "w") as f:
        f.write("result")

@task(name="main")
def main() -> str:
    restricted_writer()

TypeScript / JavaScript

Node.js built-ins like fs are not available in the WebAssembly sandbox. Instead, use the files API provided by the SDK.

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

export const restrictedWriter = task({
    name: "restricted_writer",
    allowedFiles: ["./output"]
}, async () => {
    await files.writeText("./output/result.txt", "result");
});

export const main = task({ name: "main" }, async () => {
    await restrictedWriter();
    return await files.readText("./data/input.txt");
});

Available methods:

files.readText(path) — Read file as string
files.readBytes(path) — Read file as Uint8Array
files.writeText(path, content) — Write string to file
files.writeBytes(path, data) — Write bytes to file
files.list(path) — List directory contents
files.exists(path) — Check if file exists

Environment Variables

Tasks can access environment variables to read configuration, API keys, or other runtime settings.

Python

Use Python's standard os.environ to access environment variables:

from capsule import task
import os

@task(name="main", env_variables=["API_KEY"])
def main() -> dict:
    api_key = os.environ.get("API_KEY")
    return {"api_key": api_key}

TypeScript / JavaScript

Use the env API provided by the SDK:

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

export const main = task({
    name: "main",
    envVariables: ["API_KEY"]
}, () => {
    const apiKey = env.get("API_KEY");
    return { apiKeySet: apiKey !== undefined, environment };
});

Available methods:

env.get(key) — Get a specific environment variable (returns undefined if not found)
env.has(key) — Check if an environment variable exists
env.getAll() — Get all environment variables as an object

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

Fork the repository
Create a feature branch: git checkout -b feature/amazing-feature
Run tests: cargo test
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.

capsule-core 0.4.2

Capsule

Overview

How It Works

With Python

With TypeScript / JavaScript

Response Format

Quick Start

Python

TypeScript / JavaScript

Documentation

Task Configuration Options

Compute Levels

Project Configuration (Optional)

HTTP Client API

Python

TypeScript / JavaScript

File Access

Python

TypeScript / JavaScript

Environment Variables

Python

TypeScript / JavaScript

Compatibility

Contributing

Development setup

How to contribute

Credits

License

`Capsule`