Vercel Sandbox vs Fly.io Machines
Both run your code in Firecracker microVMs with their own kernel. Vercel Sandbox is a ready sandbox for short runs of untrusted code, with sessions of up to 24 hours; Fly.io Machines are the app-hosting VMs under Fly.io, with no session cap but a root disk that resets.
The companies and products
Vercel Sandbox
Vercel is a cloud platform for web apps and agents, and Vercel Sandbox is its product for running untrusted or agent-written code in isolated Linux microVMs.
Each sandbox is a Firecracker microVM with its own kernel, started from a Vercel image, your own OCI image, or a saved snapshot. You drive it with the JavaScript or Python SDK or the sandbox CLI: run commands, move files, open ports, and set network rules. It is aimed at code interpreters, app builders, test runs and agent tools, and Vercel's docs say it is not meant to run nonstop.
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.
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. “22nd cheapest of 345” means 21 of the other 344 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 | Vercel Sandbox: price rank | Fly.io Machines: price rank | boat: price rank |
|---|---|---|---|
| Software factory 100 at once · 4 vCPU / 8 GB · 2,200 sessions of 8 h a month Neither fits fully: Vercel stops each session at 24 hours and Fly resets its root disk on restart, while boat ranks 3rd with Docker, /dev/kvm and a desktop on every machine. | 244th of 367, with limits $3,351 a month | 92nd of 367, with limits $836 a month | 3rd cheapest of 367 $634 a month |
| Always-on AI employees 50 machines 24/7 · 2 vCPU / 4 GB An assistant that runs for weeks hits Vercel's 24-hour session cap, and on Fly it keeps only what sits on its one Volume; boat keeps the whole disk with no session cap. | 257th of 366, with limits $3,657 a month | 125th of 366, with limits $816 a month | 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 Vercel Sandbox ranks 22nd and fits short code bursts fully, since it bills CPU only while code uses it; Fly is cheap here but relies on shared vCPUs. | 22nd cheapest of 345 $216 a month | 75th of 345, with limits $94 a month | 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 Vercel Sandbox ranks 15th and fits fully, while Fly's API creates about one Machine per second per app, so sign-up waves need pre-created Machines; boat ranks first. | 15th cheapest of 361 $1,267 a month | 69th of 361, with limits $600 a month | 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 Vercel Sandbox ranks 3rd for many short parallel rollouts thanks to high concurrency and fast starts; Fly and boat both fit only as a compromise. | 3rd cheapest of 360 $15,761 a month | 48th of 360, with limits $2,838 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 | 69th cheapest of 364 $1,508 a month | 117th of 364, with limits $457 a month | 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; Vercel fits fully, and boat ranks 4th with one CPU class and no burst balance. | 107th cheapest of 377 $1,144 a month | 152nd of 377, with limits $288 a month | 4th cheapest of 377 $86 a month |
| 24/7 dev boxes 1 machine 24/7 · 4 vCPU / 8 GB | 276th of 368, with limits $145 a month | 118th of 368, with limits $53 a month | 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 Vercel nor Fly has a desktop, so you build and stream one yourself; boat ships a 60fps desktop with Chromium and computer use. | 235th of 369, with limits $952 a month | 95th of 369, with limits $173 a month | 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 | 259th of 367, with limits $486 a month | 160th of 367, with limits $90 a month | 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 | 277th of 346, with limits $468 a month | 161st of 346, with limits $67 a month | 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 44th here because it can spread scrapers across 17 regions and bill only the seconds they run. | 234th of 338, with limits $9,960 a month | 44th of 338, with limits $1,232 a month | 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 | 283rd of 360, with limits $554 a month | 118th of 360, with limits $63 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 | Vercel Sandbox | Fly.io Machines | boat |
|---|---|---|---|
| Kind of product | Sandbox product for running untrusted or agent-written code | App hosting platform; Machines are its VM building block with a REST API | Agent sandbox: a full VM per agent, ready to use |
| Isolation | Firecracker microVM with its own kernel | Firecracker microVM with its own kernel, on Fly's own servers | Full VM with its own kernel on dedicated AMD Ryzen 9 9950X hosts |
| Longest session | 24 hours on Pro and Enterprise, 45 minutes on Hobby; resume starts a new session | No documented cap; a Machine runs until you or autostop stop it | No cap on paid plans (--no-auto-stop); default TTL 1 hour, changeable |
| Files after a stop | Filesystem snapshot on every stop, billed per GB-month, kept 30 days after last use | Root file system is ephemeral; lasting data goes on one Volume, billed for its full size all month | The whole disk is kept, with snapshots every minute and on stop; a stopped sandbox costs nothing |
| Memory on pause | Not kept; a new session boots from the snapshot | Suspend keeps memory for Machines up to 2GB; dropped on deploy or host move | Not kept; resume reboots from disk |
| Docker, systemd and nested virtualization | Docker with sudo; no /dev/kvm | Root and Docker you install yourself; Fly's own init, no systemd; no /dev/kvm | Docker, root, systemd and /dev/kvm on every machine |
| Images | Vercel image, your own OCI image from a Dockerfile, or a saved snapshot | Boots your own Docker image | Named snapshots (templates), shareable by code; no Dockerfile builds |
| Desktop, browser, computer use | None; headless Linux only | None built in; you add a desktop and browser to your own image | 60fps desktop and Chromium, computer-use and browser-control APIs, embeddable stream |
| Coding agents | Claude Code, Codex, OpenCode and pi preinstalled; you start them as commands | None built in; you install and run agents yourself | Claude Code, Codex, OpenCode and more preinstalled, with an API to prompt, steer and stream events |
| Inbound access | Public URLs for up to 15 ports, no built-in auth; no raw TCP or public IP | Public HTTPS and TCP services through Fly Proxy, Anycast IPs, custom domains, private network | HTTPS URLs for up to 50 ports, token-protected by default; raw TCP; dedicated IPv6 or IPv4 |
| Outbound rules | Allow-all, deny-all or a domain and CIDR allowlist, changeable while running | Network policies allow chosen ports and protocols and deny the rest | Open internet on every plan; no provider-side allowlist, firewall inside the VM |
| Keeping keys out of the sandbox | Credentials brokering injects headers for any HTTPS domain you allow | Secrets arrive as environment variables; Fly's open-source tokenizer proxy can be deployed yourself | AI credentials proxy for model providers and any HTTPS API; switch a credential to another account when credits run out |
| CPU | CPU billed only while code uses it; 12.96 runs/s measured at 4 vCPU / 8 GB | Shared vCPUs guarantee 6.25% of a core plus bursts; performance vCPUs get the full core; not measured | One CPU class with no burst balance; 26.69 runs/s measured at 4 vCPU / 8 GB |
| Regions | 19 regions, default iad1 (US East) | 17 regions on six continents | EU only: Germany, Finland, France |
Every feature Battleships tracks (43)
| Feature | Vercel Sandbox | Fly.io Machines | boat |
|---|---|---|---|
| Full VM (own kernel) | Yes | Yes | Yes |
| Docker inside | Yes | Yes | Yes |
| Nested virtualization | No | No | Yes |
| Root | Yes | Yes | Yes |
| Systemd | Not published | No | Yes |
| Desktop GUI | No | No | Yes |
| Browser | No | No | Yes |
| Computer-use API | No | No | Yes |
| SSH | No | Yes | Yes |
| Persistent disk | Yes | Yes | Yes |
| Pause/resume | Yes | Yes | Yes |
| Automatic snapshots | Yes | Yes | Yes |
| Fork/clone | Yes | Yes | Yes |
| Live fork (no pause) | Yes | Yes | Yes |
| Memory fork | No | No | No |
| Custom image (Docker or snapshot) | Yes | Yes | Yes |
| ≥24 h sessions | Yes | Yes | Yes |
| Idle auto-stop | No | Yes | No |
| Wake on request | No | Yes | No |
| HTTPS preview URLs | Yes | Yes | Yes |
| Raw TCP inbound | No | Yes | Yes |
| Public IPv4 | No | Yes | No |
| Egress allowlist | Yes | Yes | No |
| Secret proxy | Yes | No | Yes |
| Secret proxy for any API | Yes | No | Yes |
| Preinstalled agents | Yes | No | Yes |
| Agent harness API | No | No | Yes |
| MCP server | Yes | Yes | No |
| GPU | No | No | No |
| Windows | No | No | No |
| Arm64 | No | No | No |
| Self-hosting | No | No | No |
| BYOC | No | No | No |
| Open source | No | Yes | No |
| Scoped API keys | Not published | Not published | Yes |
| Spend limits | Yes | No | Yes |
| Usage per sandbox (API) | No | No | Yes |
| Audit logs | Not published | Not published | No |
| SSO | Yes | No | 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
On the same hpc-sandbox-benchmarks Node.js build-and-test loop at 4 vCPU / 8 GB, boat did 26.69 runs per second and Vercel Sandbox did 12.96, so boat finished about twice as many runs. Fly.io Machines were not part of that run, so there is no comparable number; results would also depend on whether you pick shared vCPUs, which guarantee 6.25% of a core, or full-core performance vCPUs.
Node.js build-and-test loop at 4 vCPU / 8 GB, higher is better. Measured with the open hpc-sandbox-benchmarks suite. 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 | Vercel Sandbox, a month | Fly.io Machines, 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 | $57 · Pro | $15 · Pay As You Go · with limits | $20 · $20/mo |
| One agent that never stops 4 vCPU / 8 GB, 50 GB disk, running 24/7 all month | $236 · Pro · with limits | $36 · Pay As You Go · 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 | $2,884 · Pro | $487 · Pay As You Go · 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 | $828 · Pro | $110 · Pay As You Go · with limits | $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 | $3,152 · Pro | $232 · Pay As You Go · with limits | $2,000 · $2000/mo · with limits |
Vercel Sandbox: Vercel charges for CPU only while your code uses it, but for memory for the whole time the sandbox runs, plus a small fee per sandbox created and a monthly fee per GB of snapshots. The $20 Pro credit is shared with your other Vercel usage.
Fly.io Machines: Fly bills each Machine per second while it runs, by CPU preset and memory, with no plan fee; stopped Machines pay for their root file system and Volumes are billed for their full size. The figures here use Fly's lowest rate: shared vCPUs bought in 1-year reservation blocks.
Fly.io Machines: 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
For one agent that never stops, Vercel ends each session at 24 hours and running processes start over. Fly is a compromise in all five scenarios: its lowest figures need a 1-year reservation and use shared vCPUs that guarantee only 6.25% of each core, so long builds get throttled, and its API starts about one Machine per second per app. For 1,000 short jobs at once Vercel fits at $3,152 a month, while boat's $2,000 plan still hits its start limits and queues the burst.
| Scenario | Vercel Sandbox | Fly.io Machines | boat |
|---|---|---|---|
| One coding agent, workdays | None that bite |
| None that bite |
| One agent that never stops |
|
| None that bite |
| 50 agents in parallel | None that bite |
| None that bite |
| Sandboxes for your users | None that bite |
| None that bite |
| 1,000 short jobs at once | None that bite |
|
|
Which one should you use?
Use Vercel Sandbox when
you run many short bursts of untrusted code, want a sandbox SDK ready today, or need a firewall you can change while code runs.
- A sandbox that is ready to use. Useful if you want a sandbox today: agents come preinstalled and the SDK runs commands, moves files and opens ports.
- A firewall you can change live. A running sandbox can be switched to deny-all, or to a list of allowed domains, without a restart.
- Built for big short bursts. Useful for many short jobs at once: CPU is billed only while code runs, and it fits 1,000 parallel jobs.
Use Fly.io Machines when
you host apps or light services near users worldwide that sleep between requests and wake on traffic.
- No session cap. Useful for long-running work: a Machine runs until you or autostop stop it, with no 24-hour limit.
- Wake on request and memory suspend. Useful for services that sleep between requests: Fly Proxy wakes a Machine on traffic, and small Machines resume with memory kept.
- A full app platform around each Machine. Useful if you host whole apps: raw TCP, Anycast IPs, custom domains and a private network come with every Machine.
Where boat fits
If what you are running is coding agents that work for hours, neither option was built for that: Vercel stops sessions at 24 hours and Fly is an app host whose root disk resets. boat gives each agent a full VM that keeps its whole disk, with Docker, /dev/kvm, a desktop and the agents ready to drive from outside.
Long work that keeps its disk
On Vercel a session ends at 24 hours and every process starts over. On Fly the root disk resets on restart, so tools and caches must live on one Volume billed for its full size all month.
A desktop, a browser and /dev/kvm
Neither Vercel nor Fly has a desktop or offers /dev/kvm, so an agent cannot open the app it built, and emulators or nested VMs will not run. You would build and maintain that image yourself.
Agents you drive from outside
Vercel installs agents but you run them as plain commands, and Fly ships none, so you write the code that prompts, steers and watches each one. On Fly, keeping keys out also means running your own tokenizer proxy.
Use boat when your agents work for hours on one machine: builds, tests, Docker and emulators on a full VM whose disk survives every stop, with a desktop and agents you drive from outside. Compare it one to one: boat vs Vercel Sandbox and boat vs Fly.io Machines.
Questions people ask
Is Vercel Sandbox cheaper than Fly.io Machines?
On paper Fly is lower in all five of our scenarios, using its cheapest rate: shared vCPUs on a 1-year reservation. One workday agent is $57 a month on Vercel and $15 on Fly, and 50 parallel agents are $2,884 on Vercel and $487 on Fly. Those Fly figures assume work that stays near a shared vCPU's 6.25% baseline; boat is $20 and $317 for the same two.
Which is faster, Vercel Sandbox or Fly.io Machines?
There is no comparable number for Fly. Vercel Sandbox did 12.96 Node.js build-and-test runs per second at 4 vCPU / 8 GB, and boat did 26.69 in the same run. Fly's speed depends on whether you buy shared or performance vCPUs.
Can Vercel Sandbox or Fly.io Machines run 24/7?
Fly Machines have no documented session cap and run until you stop them. Vercel sessions last up to 24 hours on Pro and Enterprise, then resume from a snapshot with processes started over. boat has no session cap on paid plans.
Can they run Docker?
Yes, both can. Vercel Sandbox runs Docker with sudo, and on Fly you install Docker yourself inside the microVM. Neither offers /dev/kvm, so nested VMs and Android emulators do not run; boat has Docker, systemd and /dev/kvm on every machine.
Do they keep my files when they stop?
Vercel takes a filesystem snapshot on every stop and keeps it 30 days after last use by default. On Fly the root file system is ephemeral, so only what is on the Machine's one Volume lasts. boat keeps the whole disk, with snapshots every minute and on stop.
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