Modal vs Fly Sprites Sprites are 39% cheaper per agent run

Which is better, Modal or Fly Sprites?

It depends on what sits around your agents and how long they sleep. For agents that mostly wait on a model, Runtime has the lowest bill: $0.64 for 1,000 one-minute runs, against $2.40 on Sprites and $3.97 on Modal. It stays the cheapest with both CPUs busy, and each sandbox is a microVM. Pick Modal for GPU sandboxes, or for sandboxes beside Modal functions, web endpoints and scheduled jobs. Pick Sprites for a 100 GB disk that costs very little asleep, or for an Elixir SDK.

At a glance

Modal bills a physical core, which is two vCPUs.

Which is cheaper, Modal or Sprites?

It depends on how busy the sandbox is, because the two count different things. Modal bills the larger of what a sandbox requested and what it used, so a 2 vCPU sandbox pays for its whole core while it waits on a model. Sprites bill CPU only while it works, at $0.0385 a CPU-hour, and memory only while it is in use, at $0.021875 a GB-hour.

Memory decides most of the Sprites bill. 4 GB for a minute costs more there than on Modal. A Sprite that uses only 1 GB costs $0.76 for the same runs. Runtime bills measured CPU at $0.025 a vCPU-hour and reserved memory at $0.0075 a GiB-hour, so it is the cheapest of the three at both ends.

Cost for the same job

1,000 runs of a 2 vCPU, 4 GiB sandbox (one physical core on Modal; the Sprite is assumed to use all 4 GB for the whole minute). Each run lasts 60 seconds and keeps the CPU busy for 20 CPU-seconds.

At 100,000 runs a month, the cost is $396.60 on Modal, $240.14 on Sprites, 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 $3.47 on Sprites. Runtime is cheaper however busy the sandbox is. The Sprite figure includes the 30 seconds or so that a Sprite stays awake and billed after its run. Storage, network, plan fees and free credits are left out.

What happens when the agent goes quiet?

• Modal keeps the sandbox running, and bills its request, until its timeout or idle_timeout. Past 24 hours, state moves to a new sandbox through a filesystem snapshot.
• Sprites sleep by themselves. A Sprite keeps its memory at first, drops it when it goes cold at a time Fly chooses, and keeps its disk always. A sleeping Sprite's disk costs $0.02 per GB a month.
• Runtime pauses when you say, or after idlePauseSeconds, and keeps files, memory and running processes while your account has credit. The next request wakes it.

Pythonfrom withruntime import Sandboxsbx = Sandbox.create(idle_pause_seconds=600)sbx.update(idle_pause_seconds=1800)  # 0 turns it offsbx.stop()

See how to pause and resume a sandbox.

Which one to pick

• Pick Modal for GPU sandboxes, or sandboxes beside Modal functions, web endpoints and scheduled jobs.
• Pick Sprites for a 100 GB disk with very cheap sleep, or an SDK in Elixir.
• Pick Runtime for measured CPU at 35% below Sprites' rate, memory sized to the job at 66% below, and memory kept on pause for as long as you choose, in a microVM per sandbox.

Switch from either one

The calls map one to one. From Modal, modal.Sandbox.create(app=...) becomes Sandbox.create(), sb.exec(*args) becomes exec, and Sandbox.from_name becomes Sandbox.getOrCreate(name). From Sprites, client.createSprite(name) becomes Sandbox.create() (or getOrCreate to find it by name), and sprite.execFile(cmd, args) becomes an argument array:

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();}

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

• Modal pricing, Modal resources and billing, Modal Sandboxes
• Fly.io pricing, Sprites, Sprites documentation, lifecycle and persistence
• Runtime pricing

Facts on this page were checked on 26 October 2026.