Modal vs Fly Machines Fly Machines are 61% cheaper per agent run

Which is better, Modal or Fly Machines?

It depends on whether you want a ready sandbox API or raw VMs to build on. If your agent mostly waits on model replies, Runtime is the cheapest here: $0.64 for 1,000 one-minute runs, against $1.53 on a Machine and $3.97 on Modal. Runtime is a sandbox API on Firecracker, with nothing to build. Pick Modal for GPU sandboxes, or for sandboxes beside Modal functions and scheduled jobs in one SDK. Pick Fly Machines to build your own layer on raw VMs in any of 17 regions, beside volumes and Managed Postgres.

At a glance

Fly's figures are at the rates of its Ashburn (iad) region. Modal bills a physical core, which is two vCPUs.

Is a Fly Machine a sandbox?

A Machine is a general-purpose Firecracker VM, up to 16 performance vCPUs and 128 GB. You choose a size and start it through the Machines API. It bills that size for every second it is started, busy or not. It is a good base to build sandboxes on, and the lifecycle is yours to write: create a Machine per job, add a volume if files must survive, and destroy it at the end. Its root disk is temporary.

Modal Sandboxes are a sandbox API. Sandbox.create and exec start a gVisor sandbox inside a Modal App, with a 5-minute lifetime by default and up to 24 hours. Filesystem snapshots carry state past that.

Runtime is a sandbox API on microVMs. Sandbox.create() returns a running sandbox, and exec runs a command. Pause keeps files, memory and running processes, so there is no volume or snapshot to manage between an agent's turns:

TypeScriptimport { Sandbox } from "withruntime";const box = await Sandbox.create();try {  await box.files.write("/workspace/invoice.py", "print(sum([125, 250, 375]))\n");  console.log((await box.exec("python3 /workspace/invoice.py", { check: true })).stdout);} finally {  await box.stop();}

Cost for the same job

1,000 runs of a 2 vCPU, 4 GiB sandbox (one physical core on Modal, and a performance-2x Machine with 4 GB on Fly). Each run lasts 60 seconds and keeps the CPU busy for 20 CPU-seconds, because the agent mostly waits for a model.

At 100,000 runs a month, the cost is $396.60 on Modal, $152.78 on Machines, and $63.89 on Runtime. With both CPUs busy for the whole minute, the cost is $1.33 on Runtime, against $3.97 on Modal and $1.53 on Machines. Runtime is cheaper however busy the sandbox is. Storage, network, plan fees and free credits are left out.

A Machine's performance vCPUs are whole cores kept for it. Runtime's default CPU is shared. cpu: "reserved" keeps every vCPU for you too. A shared-cpu-2x Machine with 4 GB costs about $0.59 for the same runs, but its vCPUs are guaranteed 6.25% of a core each, so a new one would not fit 20 CPU-seconds into the minute.

Which one to pick

• Pick Modal for GPU sandboxes, or sandboxes beside Modal functions and scheduled jobs in one SDK.
• Pick Fly Machines to build your own sandbox layer on raw VMs, in any of 17 regions, beside volumes and Managed Postgres in one account.
• Pick Runtime for a sandbox API on microVMs at 84% below Modal's price and 58% below a Machine's. CPU is billed as used, and pause keeps memory and processes with nothing to build.

Switch from either one

From Modal, the calls map one to one: modal.Sandbox.create(app=...) becomes Sandbox.create(), sb.exec(*args) becomes exec, and idle_timeout becomes idlePauseSeconds, with no App needed. From Machines, if you create a Machine for each job through the Machines API, create a Runtime sandbox instead and stop it when the job ends. The migration table lists the rest.

Or give your coding agent the instruction in migration. It switches the project on a branch, tests it on the free usage, and reports the monthly saving. npx withruntime compare --from fly-machines prices your runs at the Machine rates. Credit you add within 30 days of signing up is matched, up to $1,000 in total. The full side-by-sides are Runtime vs Modal and Runtime vs Fly.io.

Sources

Fly.io prices were checked 25 October 2026.

Modal pricing and Modal Sandboxes pages were rechecked 26 October 2026. The rest were checked 25 October 2026.

• Modal pricing, Modal resources and billing, Modal Sandboxes, Modal security
• Fly.io pricing, Machine CPU performance, Machine suspend and resume, Fly Volumes, Fly.io regions
• Runtime pricing

Facts on this page were checked on 26 October 2026.