Which is better, Fly Sprites or Prime Sandboxes?
It depends on whether you run one lasting machine or thousands of short ones. Agents that idle between model calls cost least on Runtime: $0.64 for 1,000 one-minute runs, against $2.40 on Sprites and $1.52 on Prime. It measures CPU and forks a running sandbox with its memory. Pick Fly Sprites for a persistent machine per developer or agent, with a large disk that costs little while asleep. Pick Prime Sandboxes for training runs with thousands of short environments running concurrently.
At a glance
| Fly Sprites | Prime Sandboxes | Runtime | |
|---|---|---|---|
| Isolation | Firecracker microVM | Hardware-virtualized microVM, own guest kernel | Firecracker microVM, own kernel |
| CPU | $0.0385 per CPU-hour of measured CPU | $0.02 per vCPU-hour while running | $0.025 per vCPU-hour of measured CPU, with a small floor |
| Memory | $0.021875 per GB-hour of memory in use | $0.0125 per GiB-hour | $0.0075 per reserved GiB-hour |
| Lifetime | Persistent, pauses itself; 100 GB disk kept | Default timeout 60 minutes, up to unlimited | No time limit with persistent; pauses kept while you have credit |
| Built for | A personal computer that stays | RL rollouts in the thousands | Agents and evals |
One cheap meter each
Sprites' CPU line is cheap because it is measured. Prime's is cheap because the rate is low. Memory is where Sprites cost more, at $0.021875 per GB-hour against Prime's $0.0125. Runtime measures CPU and charges less for memory than both.
Cost for the same job
The benchmark, 60 seconds a run, with 4 GB in use on Sprites and a 5 GiB disk on Prime:
- CPU
- 1,000 × 20 s / 3,600 × $0.0385
- $0.21
- Memory
- 1,000 × 90 s / 3,600 × 4 × $0.021875
- $2.19
- CPU
- 1,000 × 60 s / 3,600 × 2 × $0.02
- $0.67
- Memory
- 1,000 × 60 s / 3,600 × 4 × $0.0125
- $0.83
- Disk
- 1,000 × 60 s / 3,600 × 5 × $0.0002
- $0.02
- CPU
- 1,000 × 20 s / 3,600 × $0.025
- $0.14
- Memory
- 1,000 × 60 s / 3,600 × 4 × $0.0075
- $0.50
If both CPUs were busy for the whole minute, the job would cost $3.47 on Sprites, $1.52 on Prime and $1.33 on Runtime.
Which one to pick
- Fly Sprites for one persistent machine per developer or agent, with a large disk that costs little while it sleeps.
- Prime Sandboxes for training runs with thousands of short environments.
- Runtime for the lowest price busy or waiting, and forks that keep memory.
See Runtime vs Fly.io and Runtime vs Prime Sandboxes.
Runtime Cloud
Runtime costs 73% less than Sprites and 58% less than Prime on that benchmark: 1,000 one-minute runs of a 2 vCPU, 4 GiB sandbox with 20 CPU-seconds of work each, which come to $2.40, $1.52 and $0.64.
Each Runtime sandbox is a Firecracker microVM, and a pause keeps its memory and processes. Start on the free usage by running npx withruntime login. See the migration guide for existing code.
Sources
Fly.io prices checked 25 October 2026.
Fly Sprites rates checked 25 October 2026; Prime Sandboxes rates checked 25 October 2026.
Facts on this page were checked on 26 October 2026.