Vercel Sandbox vs Fly Machines: pricing, isolation and limits

Both are Firecracker microVMs. Vercel Sandbox bills active CPU plus memory; a Fly Machine bills its whole size by the hour while started.

A Fly Machine undercuts Vercel on this job, and Runtime undercuts both: by 55% against the Machine and 70% against Vercel. 1,000 one-minute runs of a 2 vCPU, 4 GiB sandbox cost $2.12 on Vercel, $1.44 on a Fly Machine and $0.64 on Runtime. Runtime meters CPU as Vercel does, but its CPU rate is about a fifth of Vercel's and its memory rate less than half. Rates checked 23 September 2026.

At a glance

Both rivals are priced in Ashburn: Vercel's default iad1 region and Fly's iad.

	Vercel Sandbox	Fly Machines	Runtime
Isolation	Firecracker microVM, own kernel	Firecracker microVM	Firecracker microVM, own kernel
What it is	A sandbox API for untrusted code	General-purpose VMs, run through the Machines API and flyctl	A sandbox API for agent code
CPU	$0.128 per active CPU-hour	By size while started: performance-2x with 4 GB, $0.0861 an hour	$0.025 per vCPU-hour of measured CPU, with a small floor
Memory	$0.0212 per provisioned GB-hour	Included in the size; more at $5 per GB a month	$0.0075 per reserved GiB-hour
Largest size	8 vCPUs and 16 GB on Pro; 32 and 64 GB on Enterprise	16 performance vCPUs and 128 GB	vCPUs and memory chosen per sandbox
Plan fee	Hobby allowance free; usage beyond it needs Pro	None; pay as you go	None; prepaid credit from $10
When idle	Stop keeps the filesystem; processes start again	Root disk is temporary; volumes keep files, suspend keeps memory	Pause keeps files, memory and processes, 1 to 365 days
Regions	19	18	One US region

Is a Fly Machine cheaper than Vercel Sandbox?

At this size, yes, for almost any job. A performance-2x Machine costs $0.0861 an hour however busy it is. Vercel's memory alone, 4 GB at $0.0212 a GB-hour, is $0.0848 an hour, so by the time a Vercel sandbox has paid for its memory it has nearly paid for the whole Machine. Vercel costs more as soon as a one-minute run uses a single CPU-second, and with both CPUs busy it costs almost four times as much.

A Machine's performance vCPUs are whole cores kept for it. A shared-cpu-2x Machine with 4 GB costs about $0.50 for the same 1,000 runs, but its shared vCPUs are guaranteed 6.25% of a core each, with a small burst allowance, so a new one would not fit 20 CPU-seconds into the minute.

What the Machine does not give you is a sandbox API. You create and destroy Machines yourself through the Machines API, and add a volume if files must outlive the Machine's temporary root disk. Runtime and Vercel create a sandbox in one call. Runtime's default CPU is shared and billed as measured; cpu: "reserved" keeps every vCPU for the sandbox, as a performance Machine does.

TypeScriptimport { Runtime } from "withruntime";const runtime = new Runtime();const box = await runtime.sandboxes.create({  funding: "trial",  vcpu: 2,  memoryMiB: 4096,  timeoutSeconds: 120,  onLeaseEnd: "stop", // gone when the job ends, like a destroyed Machine});try {  console.log((await box.exec(["python3", "-c", "print(6 * 7)"], { check: true })).stdout);} finally {  await box.stop();}

Cost for the same job

The job behind the headline: 1,000 runs, 60 seconds each, 20 CPU-seconds of work, on 2 vCPU with 4 GiB (4 GB on Vercel), against a performance-2x Machine with 4 GB.

TextVercel    CPU    1,000 × 20 s / 3,600 × $0.128      = $0.71          Memory 1,000 × 60 s / 3,600 × 4 × $0.0212 = $1.41          Total                                        $2.12Machines  Size   1,000 × 60 s / 3,600 × $0.0861     = $1.44Runtime   CPU    1,000 × 20 s / 3,600 × $0.025      = $0.14          Memory 1,000 × 60 s / 3,600 × 4 × $0.0075 = $0.50          Total                                        $0.64

For a month of 100,000 runs, the totals are $212.44 on Vercel, $143.50 on Machines and $63.89 on Runtime. With both CPUs busy for the whole minute, it is $1.33 on Runtime, $5.68 on Vercel and $1.44 on Machines. The Machine is priced as destroyed at the end of each run. Plan fees, creation charges, storage, network and free allowances do not appear in the totals.

Which one to pick

• Vercel Sandbox, when the application already lives on Vercel and the sandboxes should share its account; Pro allows up to 10,000 at once.
• Fly Machines, when you want general-purpose VMs and are happy to manage them, need sizes up to 128 GB, or want volumes and Managed Postgres next to them.
• Runtime, when you want a sandbox API rather than raw VMs, priced 55% under a Machine and 70% under Vercel, with pause and forks that carry memory and running processes.

Moving to Runtime

Vercel Sandbox code keeps its shape with a new import, and no longer needs a Vercel project or token:

TypeScriptimport { Sandbox } from "withruntime/vercel"; // was: from "@vercel/sandbox"

Pythonfrom withruntime.vercel import sandbox  # was: from vercel import sandbox

On Fly, the usual pattern is one Machine created per job through the Machines API. Swap each of those for a Runtime sandbox created for the job and stopped at the end, as the sample above does; commands, files and teardown follow map the calls.

npx withruntime switch --from vercel or --from fly before the first top-up gets it matched with up to $100 of credit. A coding agent can run the whole port from the single instruction in migration, on a branch and on the free trial. Detail: Runtime vs Vercel Sandbox, Runtime vs Fly.io.

Sources

Vercel Sandbox pricing rechecked 25 September 2026; the rest checked 23 September 2026.

• Vercel Sandbox pricing, Understanding Vercel Sandboxes
• Fly.io pricing, Machine CPU performance, Machine suspend and resume, Fly Volumes, Fly.io regions
• Runtime pricing

Facts on this page were checked on 25 September 2026.