E2B vs Fly Machines: pricing, isolation and limits

E2B is a sandbox API billed on allocated vCPUs and memory; a Fly Machine is a general-purpose VM billed by size. Both use Firecracker.

Runtime runs the same job for 77% less than E2B and 55% less than a Fly Machine. 1,000 one-minute runs of a 2 vCPU, 4 GiB sandbox cost $2.76 on E2B, $1.44 on a performance-2x Machine and $0.64 on Runtime. All three run Firecracker microVMs; Runtime is the one that bills the CPU your code uses rather than the size it holds. Rates checked 23 September 2026.

At a glance

Fly Machines are at the rates of Fly.io's Ashburn (iad) region.

	E2B	Fly Machines	Runtime
What it is	A sandbox API for agents	General-purpose VMs, driven by the Machines API and flyctl	A sandbox API for agents
Isolation	Firecracker microVM, own kernel	Firecracker microVM	Firecracker microVM, own kernel
Price	$0.0504 per vCPU-hour, $0.0162 per GiB-hour	By size while started: performance-2x, 4 GB, $0.0861 an hour	$0.025 per vCPU-hour of measured CPU, $0.0075 per GiB-hour
Billed	Per second	By the second while a Machine runs	By the second
Largest size	8 vCPUs and 8 GiB	16 performance vCPUs and 128 GB	vCPUs and memory chosen per sandbox
Plan fee	Hobby $0; Pro $150 a month	None; pay as you go	None; prepaid credit from $10
Free start	$100 of usage credit, once	Not stated on the pricing page	50 sandbox hours, no card
Between jobs	Pause keeps files, memory and processes	Root disk is temporary; volumes keep files, suspend memory	Pause keeps files, memory and processes, 1 to 365 days

Is Fly Machines cheaper than E2B for agents?

At list rates, yes, by about half. A performance-2x Machine with 4 GB costs $0.0861 an hour. The same 2 vCPUs and 4 GiB on E2B cost 2 × $0.0504 plus 4 × $0.0162, which is $0.1656 an hour. Both bill for as long as the machine runs, whether the agent is working or waiting on a model.

What the lower price buys is a bare VM. E2B hands you a sandbox SDK in JavaScript and Python, a code interpreter and a pause that keeps memory. A Machine is a VM you drive through the Machines API: you create one per job, decide what image it boots, and stop it yourself. Its root disk is temporary, so files you want to keep go on a Fly Volume. A Machine's performance vCPUs are whole cores kept for it; on Runtime, cpu: "reserved" does the same.

Runtime is priced below both for this kind of agent because it bills the 20 CPU-seconds a run uses, not two vCPUs for all 60 seconds. While the agent waits, only a floor of a twentieth of a vCPU is billed.

Cost for the same job

1,000 runs of a 2 vCPU, 4 GiB sandbox. Each run lasts 60 seconds and keeps the CPU busy for 20 CPU-seconds: an agent that mostly waits for a model. The Machine is a performance-2x with 4 GB.

TextE2B       CPU    1,000 × 60 s / 3,600 × 2 × $0.0504 = $1.68          Memory 1,000 × 60 s / 3,600 × 4 × $0.0162 = $1.08          Total                                        $2.76Machines  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

At 100,000 runs a month that is $276.00 on E2B, $143.50 on Machines and $63.89 on Runtime. With both CPUs busy for the whole minute, it is $1.33 on Runtime against $2.76 on E2B and $1.44 on Machines, so Runtime stays the cheapest of the three even for work that never waits. A Machine's price does not change with load: it is the size, by the second. Plan fees, volumes, network and free credits are left out.

What happens between jobs?

A paused E2B sandbox keeps its files, memory and running processes, with no expiry. A stopped Fly Machine keeps its root filesystem for $0.15 per GB a month, but that disk is temporary: data you need across restarts belongs on a volume, and only suspend keeps memory. A paused Runtime sandbox keeps files, memory and processes for the retention you set, 1 to 365 days, and wakes by itself on the next request. See how to pause and resume a sandbox.

Which one to pick

• Pick E2B for a ready-made agent sandbox with a code interpreter, paused sandboxes kept with no expiry, or a runtime you can self-host under Apache-2.0. Sessions past an hour need the $150-a-month Pro plan.
• Pick Fly Machines if you want general VMs in 18 regions, sizes up to 16 performance vCPUs and 128 GB, and volumes and Managed Postgres in the same account, and you are happy to build the sandbox layer yourself. Runtime runs in one US region.
• Pick Runtime for a sandbox API priced below a raw Machine: measured CPU, memory sized to the job, pause that keeps memory and processes, and an agent that connects without an API key.

Switch from either one

From E2B, change one import:

TypeScriptimport { Sandbox } from "withruntime/e2b"; // was: from "e2b"

Pythonfrom withruntime.e2b import Sandbox  # was: from e2b import Sandbox

From Fly Machines, where you create a Machine for each job through the Machines API, create a Runtime sandbox instead and stop it when the job ends. There is no app or image to set up first:

TypeScriptimport { Sandbox } from "withruntime";const box = await Sandbox.create({ funding: "trial" });try {  console.log((await box.exec(["python3", "-c", "print(6 * 7)"], { check: true })).stdout);} finally {  await box.stop();}

Pythonfrom withruntime import Sandboxwith Sandbox.create(funding="trial") as box:    print(box.exec("python3 -c 'print(6 * 7)'", check=True).stdout)

Or give your coding agent the one instruction in migration: it switches the project on a branch, tests it on the free trial and reports the monthly saving. npx withruntime compare --from fly-machines (or --from e2b) prices what you ran at the old provider's rates. The full side-by-sides are Runtime vs E2B and Runtime vs Fly.io; microVM vs container explains the isolation all three share.

Sources

E2B and Fly.io pricing pages rechecked 25 September 2026; the rest checked 23 September 2026.

• E2B pricing, E2B sandbox persistence
• 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.