Fly.io Machines vs Modal VM Sandboxes

Fly.io Machines are microVMs for hosting your own apps near users, with no time limit but a root disk that resets. Modal VM Sandboxes are full Linux VMs with Docker and systemd built for agents, but each one ends within 24 hours.

The companies and products

Fly.io Machines

Fly.io is an app hosting platform, and Machines are the microVMs underneath it that you can also start and stop through a REST API.

A Fly Machine boots your Docker image as a Firecracker microVM on Fly's own servers, in one of 17 regions. You pick a shared or performance CPU preset, attach at most one Volume for data that must last, and Fly Proxy can stop, suspend and wake Machines as traffic comes and goes. It is built for teams that host web apps and services worldwide; it is not a sandbox product, so this page compares the parts that overlap with agent work.

Kind of product: App hosting platform; Machines are its VM building block with a REST API
Isolation: Firecracker microVMs with hardware virtualization, on Fly's own servers
Regions: 17 regions in North America, Europe, Asia, Oceania, South America and Africa
CPU classes: Shared vCPUs guarantee 6.25% of each core with a burst balance; performance vCPUs get the full core
Storage: Root file system is ephemeral; one Volume of up to 500GB per Machine for lasting data
Free trial: 2 Machine hours or 7 days, up to 10 Machines of 2 vCPU / 4GB that stop after 5 minutes
Compliance: Lists SOC 2 Type II; a HIPAA package with a pre-signed BAA is sold as an add-on

Modal VM Sandboxes

Modal is a serverless cloud for AI work, and VM Sandboxes are the version of its sandboxes that run in a virtual machine with their own Linux kernel.

You create a VM Sandbox from Python, TypeScript or Go code with runtime="vm", run commands in it, and terminate it when the job is done. Modal built the runtime on the Rust-based Cloud Hypervisor, so Docker, FUSE, systemd and nested cgroups work, with the same API, images and prices as its gVisor sandboxes. It became generally available on 1 October 2026 and suits teams that want many short-lived agent machines next to Modal's inference and training.

Founders: Erik Bernhardsson and Akshat Bubna
Offices: New York, Stockholm and San Francisco
Funding: Over $466M raised
Isolation: Virtual machine per sandbox on Cloud Hypervisor, with its own Linux 7.2 kernel
Generally available: 1 October 2026; early customers launched over 20 million VMs before that
Session length: Default 5 minutes, maximum 24 hours
Compliance: SOC 2 on every plan; HIPAA on Enterprise

Which use cases each one fits

For each use case, Battleships prices every sandbox provider on the same workload and sorts them from the cheapest monthly bill up. “92nd cheapest of 367” means 91 of the other 366 providers run that workload, with every feature it needs, for less. “With limits” means the provider runs it but misses something, so it is listed after every full match. Open a use case for its whole ranking and what each provider misses.

Use case and its workload	Fly.io Machines: price rank	Modal VM Sandboxes: price rank	boat: price rank
Software factory 100 at once · 4 vCPU / 8 GB · 2,200 sessions of 8 h a month Modal gives the agent Docker and systemd in a real VM but cuts it off at 24 hours, Fly runs as long as you like but resets the root disk; boat fits fully with a kept disk and a desktop.	92nd of 367, with limits $836 a month	84th of 367, with limits $759 a month	3rd cheapest of 367 $634 a month
Always-on AI employees 50 machines 24/7 · 2 vCPU / 4 GB An assistant that lives on its machine all month cannot run on Modal VM Sandboxes, which stop at 24 hours, and on Fly it keeps only what sits on its one Volume.	125th of 366, with limits $816 a month	Can't run it	5th cheapest of 366 $657 a month
Code interpreter bursts 100 at once · 1 vCPU / 2 GB · 200,000 sessions of 1 min a month Fly fits short code runs better than boat here: per-second billing with no plan fee and cheap shared CPU for code that mostly waits.	75th of 345, with limits $94 a month	Can't run it	186th of 345, with limits $2,000 a month
Sandboxes for your users 300 at once · 2 vCPU / 4 GB · 30,000 sessions of 23 min a month Users who come back to the same sandbox fit boat, because a stopped boat sandbox keeps its disk and costs nothing while it waits.	69th of 361, with limits $600 a month	Can't run it	Cheapest of 361 $209 a month
RL rollouts 1,000 at once · 2 vCPU / 4 GB · 1,000,000 sessions of 3 min a month Fly (rank 48) and boat (rank 34) both rank well above Modal (rank 171) for RL rollouts, though none of the three is a full fit.	48th of 360, with limits $2,838 a month	171st of 360, with limits $16,322 a month	34th of 360, with limits $2,000 a month
Agent evals 250 at once · 4 vCPU / 8 GB · 15,000 sessions of 20 min a month	117th of 364, with limits $457 a month	Can't run it	10th cheapest of 364 $200 a month
CI runners 40 at once · 8 vCPU / 16 GB · 6,000 sessions of 12 min a month CI keeps the CPU busy, which drains a shared Fly vCPU's burst balance; Modal runs Docker builds in a real VM; boat adds /dev/kvm for emulators on every plan.	152nd of 377, with limits $288 a month	188th of 377, with limits $1,064 a month	4th cheapest of 377 $86 a month
24/7 dev boxes 1 machine 24/7 · 4 vCPU / 8 GB	118th of 368, with limits $53 a month	Can't run it	18th cheapest of 368 $26 a month
Computer-use agents 50 at once · 4 vCPU / 8 GB · 20,000 sessions of 15 min a month Neither Fly nor Modal ships a desktop, so you build and stream one yourself; boat has a 60fps desktop with Chromium and computer use.	95th of 369, with limits $173 a month	Can't run it	6th cheapest of 369 $200 a month
Browser-use agents 50 at once · 2 vCPU / 4 GB · 30,000 sessions of 10 min a month	160th of 367, with limits $90 a month	Can't run it	7th cheapest of 367 $200 a month
Scraping & bots (startup scale) 50 at once · 1 vCPU / 2 GB · 100,000 sessions of 3 min a month	161st of 346, with limits $67 a month	Can't run it	6th cheapest of 346 $90 a month
Mass scraping & bots (undetectable) 1,000 at once · 1 vCPU / 2 GB · 3,000,000 sessions of 2 min a month Fly ranks higher than boat here: it can spread scrapers across 17 regions and pay only for the seconds they run.	44th of 338, with limits $1,232 a month	Can't run it	71st of 338, with limits $1,800 a month
Parallel red teaming 200 at once · 2 vCPU / 4 GB · 5,000 sessions of 30 min a month	118th of 360, with limits $63 a month	294th of 360, with limits $642 a month	2nd cheapest of 360 $200 a month
GPU eval jobs 10 at once · 8 vCPU / 32 GB · 500 sessions of 30 min a month	Can't run it	Can't run it	Can't run it
macOS agents 5 at once · 4 vCPU / 8 GB · 1,000 sessions of 20 min a month	Can't run it	Can't run it	Can't run it

Key features

Feature	Fly.io Machines	Modal VM Sandboxes	boat
Kind of product	App hosting platform; you boot your own image as a microVM	Sandbox runtime inside Modal's serverless AI cloud	Agent sandbox: a full VM per agent, ready to use
Isolation	Firecracker microVM with its own kernel	VM with its own Linux kernel on Cloud Hypervisor	Full VM with its own kernel on dedicated AMD Ryzen 9 9950X hosts
Docker and systemd	Root, and Docker you install yourself; Fly's own init, no systemd	Docker, systemd and root; you start dockerd yourself	Docker, root and systemd ready in every sandbox
Nested virtualization	/dev/kvm not offered	Team and Enterprise plans only, on request	/dev/kvm on every plan
CPU benchmark (4 vCPU / 8 GB)	Not measured; shared vCPUs guarantee 6.25% of a core plus bursts	15.54 runs/s	26.69 runs/s
Longest session	No documented cap; a Machine runs until you stop it	24 hours (default 5 minutes)	No cap on paid plans; you set the time to live
Files after a stop	Root file system is ephemeral; one Volume per Machine, billed for its full size around the clock	No paused state; only Volumes and filesystem snapshots keep files	The whole disk is kept, saved every minute and on stop; a stopped sandbox costs nothing
Memory kept on pause	Suspend keeps memory for Machines up to 2GB	Memory snapshots in alpha; snapshotting ends the sandbox, expires after 7 days	No; resume boots again from disk
Fork	Clone copies a Machine's config; a cloned Volume starts empty	Snapshot the filesystem into an image and boot many sandboxes from it	Fork a running sandbox, disk included
Wake on request	Fly Proxy stops idle Machines and wakes them when a request arrives	No	You start and stop machines through the API
Desktop and coding agents	None built in; you add them to your own image	Examples only; you set up Xvfb, VNC and agents yourself	60fps desktop, Chromium, computer use, and Claude Code, Codex and more driven from outside
Keeping keys out of the sandbox	Environment variables; deploy Fly's open-source tokenizer proxy yourself	Environment variables, or your own proxy in a sidecar	AI credentials proxy for model providers and any HTTPS API
Outbound rules	Network policies allow chosen ports and protocols	Block all, IP-range allowlist, domain allowlist (beta)	Open internet, no provider-side allowlist
Regions	17 regions on six continents	US, EU and Asia-Pacific; pinning costs 1.15x to 1.75x	EU only: Germany, Finland, France

Every feature Battleships tracks (43)

Feature	Fly.io Machines	Modal VM Sandboxes	boat
Full VM (own kernel)	Yes	Yes	Yes
Docker inside	Yes	Yes	Yes
Nested virtualization	No	Yes	Yes
Root	Yes	Yes	Yes
Systemd	No	Yes	Yes
Desktop GUI	No	No	Yes
Browser	No	No	Yes
Computer-use API	No	No	Yes
SSH	Yes	Yes	Yes
Persistent disk	Yes	Yes	Yes
Pause/resume	Yes	No	Yes
Automatic snapshots	Yes	No	Yes
Fork/clone	Yes	Yes	Yes
Live fork (no pause)	Yes	Yes	Yes
Memory fork	No	Yes	No
Custom image (Docker or snapshot)	Yes	Yes	Yes
≥24 h sessions	Yes	Yes	Yes
Idle auto-stop	Yes	Yes	No
Wake on request	Yes	No	No
HTTPS preview URLs	Yes	Yes	Yes
Raw TCP inbound	Yes	Yes	Yes
Public IPv4	Yes	No	No
Egress allowlist	Yes	Yes	No
Secret proxy	No	No	Yes
Secret proxy for any API	No	No	Yes
Preinstalled agents	No	No	Yes
Agent harness API	No	No	Yes
MCP server	Yes	No	No
GPU	No	No	No
Windows	No	No	No
Arm64	No	No	No
Self-hosting	No	No	No
BYOC	No	No	No
Open source	Yes	No	No
Scoped API keys	Not published	Not published	Yes
Spend limits	No	Not published	Yes
Usage per sandbox (API)	No	Not published	Yes
Audit logs	Not published	Yes	No
SSO	No	Yes	No
SOC 2	Yes	Yes	No
HIPAA	Yes	Yes	No
EU data residency	Yes	Yes	Yes
Zero data retention	Not published	Not published	Yes

Performance

Fly.io MachinesNot measured

Modal VM Sandboxes15.54 runs/s

boat26.69 runs/s

In the same public StarSling run at 4 vCPU / 8 GB, boat finished 26.69 Node.js build-and-test runs per second and Modal's VM runtime finished 15.54, so boat did about 1.7 times the CPU work in the same time. Fly.io Machines were not part of that run, so there is no comparable score; Fly's shared vCPUs guarantee 6.25% of a core with a burst balance, and its performance vCPUs get the full core at a higher rate.

Node.js build-and-test loop at 4 vCPU / 8 GB, higher is better. Measured with the open hpc-sandbox-benchmarks suite. Modal VM Sandboxes measured on: VM Sandbox, request = size. Fly.io Machines has no published run in this suite yet, so there is no like-for-like number to compare.

Pricing in five scenarios

Each bill is priced from the providers' public price lists by the open Battleships engine, with the cheapest plan that runs the scenario.

Scenario	Fly.io Machines, a month	Modal VM Sandboxes, a month	boat, a month
One coding agent, workdays 4 vCPU / 8 GB, 50 GB disk, 8 hours a day, 22 days a month, files kept between days	$15 · Pay As You Go · with limits	$15 · Starter · with limits	$20 · $20/mo
One agent that never stops 4 vCPU / 8 GB, 50 GB disk, running 24/7 all month	$36 · Pay As You Go · with limits	$178 · Starter · with limits	$26 · $20/mo
50 agents in parallel 50 machines of 4 vCPU / 8 GB at once, 8-hour sessions, 1,100 sessions a month, Docker inside	$487 · Pay As You Go · with limits	$2,229 · Starter · with limits	$317 · $20/mo × 3 seats
Sandboxes for your users 2 vCPU / 4 GB, 30,000 sessions of 10 minutes a month, up to 300 at once	$110 · Pay As You Go · with limits	$792 · Team	$200 · $100/mo × 2 seats
1,000 short jobs at once 2 vCPU / 4 GB, 1,000 at once, 3 minutes each, 200,000 jobs a month	$232 · Pay As You Go · with limits	$3,193 · Team · with limits	$2,000 · $2000/mo · with limits

Fly.io Machines: Fly bills each Machine per second while it runs, by CPU preset and memory, with no plan fee; Volumes are billed for their full size all month. The table uses Fly's lowest rate: shared vCPUs bought in 1-year reservation blocks for 40% off.

Modal VM Sandboxes: Modal bills by the second for the higher of the CPU and memory you request or use, until the sandbox ends. Starter is free with $30 of monthly credit, Team is $250 a month with $100 of credit, and pinning a region multiplies the price by 1.15x to 1.75x.

Modal VM Sandboxes: You pick a monthly plan from $20, and all of it becomes usage credit spent per second on running sandboxes. A stopped sandbox costs nothing and keeps its disk.

Limits in those scenarios

Fly is a compromise in every scenario: its lowest figures need a 1-year commitment and use shared vCPUs that guarantee only 6.25% of each core, so long builds get throttled. Modal VM Sandboxes end within 24 hours and keep files only in Volumes or snapshots, so the workday, always-on and fleet scenarios do not fit; its 1,000-job burst runs on shared vCPUs that other tenants can slow. On boat the limit that bites is start speed: even the $2,000 plan starts 90 sandboxes a minute, so 1,000 jobs at once get queued.

Scenario	Fly.io Machines	Modal VM Sandboxes	boat
One coding agent, workdays	different kind of product: app platform 1-year commitment burstable CPU: 6.25% of each vCPU guaranteed, throttled above that	files survive a stop only in Volumes: packages installed elsewhere reset	None that bite
One agent that never stops	different kind of product: app platform 1-year commitment burstable CPU: 6.25% of each vCPU guaranteed, throttled above that	Starter: sessions capped at 24 h files survive a stop only in Volumes: packages installed elsewhere reset	None that bite
50 agents in parallel	different kind of product: app platform 1-year commitment burstable CPU: 6.25% of each vCPU guaranteed, throttled above that	files survive a stop only in Volumes: packages installed elsewhere reset	None that bite
Sandboxes for your users	different kind of product: app platform 1-year commitment burstable CPU: 6.25% of each vCPU guaranteed, throttled above that	None that bite	None that bite
1,000 short jobs at once	different kind of product: app platform 1-year commitment burstable CPU: 6.25% of each vCPU guaranteed, throttled above that	shared vCPUs on this tier (Modal (VM Sandboxes) sells dedicated cores separately as “VM Sandbox, request = size”): heavy, sustained CPU use can be slowed by other tenants	max 2,400 starts/day (needs ~6,667) max 600 starts/hour (needs ~1,000) bursts above 90 starts/min are queued (a burst of 333 takes ~4 min to start)

Which one should you use?

Use Fly.io Machines when

you host web apps or light services that must run near users worldwide, sleep between requests and wake on traffic.

17 regions worldwide. Useful if users sit far from Europe and the US: Fly runs Machines in 17 regions, from Sydney to Johannesburg.
No time limit and wake on request. Useful for services that sleep between requests: a Machine runs as long as you want, and Fly Proxy wakes it on traffic.
A full app platform around it. Useful if you host whole apps: private networking, Anycast IPs, custom domains and port-level network policies come with each Machine.

Use Modal VM Sandboxes when

you run huge bursts of short agent jobs that need a real kernel, finish within a day, or sit next to models you already run on Modal.

A real VM made for agents. Useful when agents need Docker, FUSE and systemd: each sandbox gets its own Linux kernel, plus systemd that Fly Machines do not have.
Very large bursts. Useful for huge waves of short jobs: Modal runs 5,000 containers at once on Team and more on Enterprise.
Strict outbound rules and one AI platform. Useful if you serve models on Modal already: sandboxes can be limited to chosen domains and share one bill with inference.

Where boat fits

If what you are really running is coding agents that work for hours or days, neither option is made for it: Fly is an app host that resets its root disk, and Modal ends each VM within 24 hours. boat gives every agent a full VM that keeps its whole disk, stops for free and comes with a desktop, coding agents and a keys proxy.

A disk that keeps everything

An agent leaves tools, repos and caches all over its disk. Fly keeps only one Volume, billed even while stopped, and a Modal VM loses its root disk within 24 hours. boat saves the whole disk every minute and on stop.

Docker, systemd and /dev/kvm ready

Fly has no systemd and no /dev/kvm, and Modal gives nested virtualization only on Team after asking support. On boat, Docker builds, emulators and nested VMs work on every plan from the first boot.

Desktop, agents and keys built in

On both Fly and Modal you build the desktop, browser and agent harness yourself, and keys sit in environment variables unless you run your own proxy. boat ships a 60fps desktop, coding agents driven from outside and an AI credentials proxy.

Use boat when your agents do real work for hours or days: installs, builds, tests, Docker and emulators on a faster VM whose disk survives every stop, with a desktop, coding agents and a keys proxy built in. Compare it one to one: boat vs Fly.io Machines and boat vs Modal VM Sandboxes.

Questions people ask

Is Fly.io Machines cheaper than Modal VM Sandboxes?

At Fly's cheapest rate, shared vCPUs on a 1-year reservation, mostly yes: one always-on agent is $36 a month on Fly against $178 on Modal, and 50 parallel agents are $487 against $2,229. One agent on workdays is $15 on both. Fly's figures assume work that stays within a shared vCPU's 6.25% baseline.

Which is faster, Fly.io Machines or Modal VM Sandboxes?

There is no shared benchmark: Fly.io Machines were not in the StarSling run. In that run Modal's VM runtime did 15.54 runs per second at 4 vCPU / 8 GB and boat did 26.69. Fly's speed depends on whether you pick shared or performance vCPUs.

Can Fly.io Machines and Modal VM Sandboxes run Docker?

Yes. On Fly you install Docker yourself inside the microVM, which uses Fly's own init and no systemd. Modal's VM runtime supports Docker and systemd, and you start dockerd as the entrypoint. boat has Docker and systemd ready in every sandbox.

How long can each one run?

A Fly Machine has no documented time cap and runs until you stop it. A Modal VM Sandbox defaults to 5 minutes and lasts at most 24 hours; for longer work you snapshot the filesystem and start a new one. boat has no session cap on paid plans.

Do they keep my files when they stop?

Fly's root file system is ephemeral, so only the one attached Volume lasts. A finished Modal VM Sandbox is gone, and only Volumes and snapshots keep files. On boat the whole disk is kept, saved every minute and on stop.

Can I run Android emulators or nested VMs?

Fly does not offer /dev/kvm, so nested VMs are not available. Modal offers nested virtualization on Team and Enterprise after you ask support. boat exposes /dev/kvm on every plan.

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