Modal Sandboxes vs Fly Machines for AI code sandboxes

Modal is a serverless compute platform used to run highly parallel workloads at scale. Sandboxes are a specific product in Modal that can be used to safely run untrusted, LLM-generated code. Fly.io is a distributed cloud VM platform used to deploy applications near end users.
Get Started
Ramp
Quora
Substack
Cartesia
Cursor
Suno
Mistral AI
Contextual AI

What are Modal Sandboxes?

Modal is a serverless compute platform for running compute-intensive or highly parallel AI workloads in the cloud. Modal's proprietary container stack enables blazing fast cold starts for rapid developer iteration and production-grade scalability. Sandboxes are built on top of this container stack and are specifically used to define containers at runtime and safely run LLM-generated code. Like other Modal products, Sandboxes autoscale up and down from 0 to 10,000s of containers and are easily managed via Modal's simple SDK.

What are Fly Machines?

Fly.io is a distributed cloud VM platform for application deployments. Fly specializes in running applications close to where end users are. Unlike Modal's function-based serverless approach, Fly is primarily used for the deployment of application backends on stateful VMs. Fly Machines are VMs that can be managed via Fly's CLI or REST API. They are the compute primitive underlying Fly's app deployment product, but they can also be managed directly.

Performance and scalability

Modal LogoFly Logo
Enterprise-level scaling to 10,000+ sandboxesCheckmarkCross
Proven network reliability at scaleCheckmarkCross
Sub-second container cold startsCheckmarkCheckmark
Filesystem snapshotting for fast custom container creationCheckmarkCross
Memory snapshotting for fast restoresCheckmarkCross
Multi-region supportCheckmarkCheckmark
White-glove engineering support to optimize your deploymentsCheckmarkCross
Lovable Logo

Lovable uses Modal to power millions of AI-generated apps

Lovable is the fastest-growing platform for creating AI-generated apps. They use Modal Sandboxes at massive scale to run LLM-generated code during user sessions.

We've previously managed to break services like GitHub because of our load, so when Modal was able to handle the massive scale of our AI weekend event so smoothly, that meant a lot. We trust Modal to keep up with our growth in the long term.
— Anton Osika, CEO and Founder at Lovable
Profile image

In a single weekend, Modal powered 250,000 user app creations for Lovable with no downtime. Not only that, Lovable reduced their sandbox orchestration code from 15,000 lines to 700 lines, ensuring long-term maintainability of this infrastructure.

Features

Modal LogoFly Logo
Run arbitrary LLM-generated code in an isolated environmentCheckmarkCheckmark
Support for custom imagesCheckmarkCheckmark
Support for dynamic, runtime image builds per sandboxCheckmarkCross
Data storageCheckmarkCheckmark
Built-in sandbox state managementCheckmarkCross
Built-in tunnel feature for direct external connections to sandboxesCheckmarkCross
Built-in distributed file system that can be attached to sandboxesCheckmarkCross
First-party log collection and telemetry integrationsCheckmarkCross
Multi-region supportCheckmarkCheckmark
Persistent VMsCrossCheckmark

Developer experience

Modal LogoFly Logo
Infrastructure defined in application codeCheckmarkCross
Python, JavaScript, Go SDKsCheckmarkCross
Create sandboxes with one line of codeCheckmarkCross
No YAML or Dockerfiles neededCheckmarkCross
Easy-to-use network control featuresCheckmarkCross
Granular and intuitive observability featuresCheckmarkCross
EnvironmentsCheckmarkCross
Build a proof of concept in days, not monthsCheckmarkCross

Security and enterprise readiness

Modal LogoFly Logo
SOC 2 Type IICheckmarkCheckmark
HIPAACheckmarkCheckmark
Static IP proxiesCheckmarkCross
Granular control over outbound networkingCheckmarkCross

Choose Modal Sandboxes for:

Robust, reliable sandboxes at enterprise scale
Rich feature set specific for code sandboxes, including tunnels and state management
Streamlined developer experience that is code-first and low on infrastructure configuration

Choose Fly Machines for:

Stateful VMs for long-running processes in your application backend