Which is better, Fly Machines or Prime Sandboxes?
It depends on how your agent spends its time and how many sandboxes you run. 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 Fly Machines and $1.52 on Prime. Its pauses keep memory and processes. Pick Fly Machines for apps spread over 17 regions, with volumes and Postgres. Pick Prime Sandboxes for reinforcement learning rollouts with room for a thousand sandboxes concurrently, though its rate is promised only through 22 December 2026.
At a glance
| Fly Machines | Prime Sandboxes | Runtime | |
|---|---|---|---|
| Isolation | Firecracker microVM | Hardware-virtualized microVM, own guest kernel | Firecracker microVM, own kernel |
| Price | performance-2x with 4 GB, $0.0916704 an hour, iad |
$0.091 an hour for 2 vCPUs, 4 GiB, 5 GiB disk | $0.03125 an hour waiting, $0.08 busy |
| Largest | 16 performance vCPUs and 128 GB | 16 vCPUs, 64 GiB, 128 GiB of disk | 16 vCPUs and 64 GiB, paid |
| State | Volumes keep files; suspend at 2 GB or less | Filesystem checkpoints and forks; processes start fresh | Pause keeps memory and processes |
| Regions | 17 | Not stated | One US region |
Nearly the same hourly price
At 2 vCPUs and 4 GB, the two differ by half a cent an hour. Fly's performance vCPUs are whole cores, and its Machines sit in 17 regions. Prime's rate is promised only through 22 December 2026 but comes with room for a thousand sandboxes concurrently. Neither discounts an idle core, which is where Runtime's measured CPU saves the most.
Cost for the same job
2 vCPU and 4 GB, 60 seconds, 20 CPU-seconds, 1,000 runs. Prime's 5 GiB disk is included:
- Size
- 1,000 × 60 s / 3,600 × $0.0916704
- $1.53
- 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
Both bill by allocation. With both CPUs busy for the whole minute, they stay at $1.53 on Machines and $1.52 on Prime, while the job costs $1.33 on Runtime.
Which one to pick
- Fly Machines for apps in many regions with Fly's volumes and Postgres.
- Prime Sandboxes for RL training at large parallelism.
- Runtime for the lowest bill at any load, with memory kept across pauses.
See Runtime vs Fly.io and Runtime vs Prime Sandboxes.
Runtime Cloud
Runtime runs the same work for 58% less than a Fly Machine and 58% less than Prime. The benchmark is a 2 vCPU, 4 GiB sandbox running for 60 seconds with 20 CPU-seconds of work, repeated 1,000 times: $1.53 on Fly Machines, $1.52 on Prime Sandboxes and $0.64 on Runtime.
Runtime bills measured CPU in a Firecracker microVM per sandbox, with no plan fee. Run npx withruntime login to start on the free usage. Read the migration guide when you move existing code.
Sources
Fly.io prices checked 25 October 2026.
Fly Machines rates checked 25 October 2026; Prime Sandboxes rates checked 25 October 2026.
Facts on this page were checked on 26 October 2026.