Modal Sandboxes vs Fly.io Sprites
Answer in brief
Choose Modal Sandboxes for high-concurrency and GPU-backed sandboxes — RL rollouts, evals and coding agents at scale; choose Fly.io Sprites for long-running, stateful agent computers that should cost nothing while idle. Modal Sandboxes uses gVisor or VM isolation; Fly.io Sprites uses MicroVM isolation.
Modal Sandboxes vs Fly.io Sprites at a glance
| Attribute | Modal Sandboxes | Fly.io Sprites |
|---|---|---|
| What it is | Sandboxes on Modal’s serverless compute platform for running untrusted code at very high scale, with GPUs, snapshots and tunnels. | Persistent, hardware-isolated Linux computers for AI agents with a durable 100 GB disk, checkpoints, and no compute charges while idle. |
| Isolation | gVisor by default, or VMs with their own Linux kernel; GPU sandboxes use gVisor | Dedicated microVM with hardware-level isolation (Fly docs); reported to be Firecracker-based |
| Open source | No (Client SDK Apache-2.0) | No (SDKs MIT) |
| Self-host / BYOC | No / Not documented | No / No |
| SDKs | Python, JavaScript/TypeScript, Go | JavaScript/TypeScript, Go, Python, Elixir |
| Cold start (vendor claim) | Sub-second, including for large custom images (vendor claim) | Warm wake 100–500 ms with processes preserved; cold wake 1–2 s (vendor docs) |
| Max session | 5 minutes default, configurable up to 24 hours; snapshots carry state beyond that | No lifetime limit — a Sprite persists until destroyed and sleeps after ~30 s idle |
| Persistence | Filesystem snapshots, directory snapshots, memory snapshots (alpha) and Modal Volumes | 100 GB durable disk per Sprite that survives sleep; copy-on-write checkpoints (~1 s) with restore; memory preserved on warm pause |
| GPU | Yes — Modal’s full GPU range (T4 to B300) at standard per-second rates, e.g. H100 $0.001097/s | No GPU option documented |
| Networking | Outbound open by default; block all, CIDR allowlists or domain allowlists (beta), updatable at runtime; encrypted tunnels with public URLs and connect tokens | HTTPS URL per Sprite (org-private by default); outbound open by default with optional DNS-based domain allow/deny policy; connectors broker API credentials |
| Pricing | Per second at sandbox (non-preemptible) rates: $0.00003942 per physical core-second (1 core = 2 vCPU) + $0.00000667 per GiB-second; GPUs at standard rates. Team plan $250/month | Per second, compute only while active: $0.0385 per CPU-hour + $0.021875 per GB-hour memory; storage billed while idle |
| Compliance | SOC 2 on all plans; HIPAA on Enterprise | Fly.io is SOC 2 Type 2; HIPAA BAA and DPA available (company-wide) |
| Best for | High-concurrency and GPU-backed sandboxes — RL rollouts, evals and coding agents at scale | Long-running, stateful agent computers that should cost nothing while idle |
Verified November 8, 2026 from each provider’s documentation. Cold-start figures are vendor claims, not our measurements.
Which should you choose?
Choose Modal Sandboxes if…
- You need very high concurrency (RL, evals, batch agents)
- You need GPUs inside the sandbox
- You already use Modal for inference or training
- You want Go as well as Python and TypeScript SDKs
Choose Fly.io Sprites if…
- You want a persistent computer per agent or per user
- Your agents are idle most of the time and you want to pay only for active compute
- You want fast checkpoints and rollback
- You want Elixir or Go SDKs
Both are worth piloting with your own image and region. See the full Modal Sandboxes profile and Fly.io Sprites profile, or the complete comparison.
Frequently asked questions
Is Modal Sandboxes or Fly.io Sprites better for AI agents?
Neither is better for every workload. Modal Sandboxes is best for high-concurrency and GPU-backed sandboxes — RL rollouts, evals and coding agents at scale. Fly.io Sprites is best for long-running, stateful agent computers that should cost nothing while idle.
Which is open source, Modal Sandboxes or Fly.io Sprites?
Neither is open source.
How do Modal Sandboxes and Fly.io Sprites isolate code?
Modal Sandboxes: gVisor by default, or VMs with their own Linux kernel; GPU sandboxes use gVisor. Fly.io Sprites: Dedicated microVM with hardware-level isolation (Fly docs); reported to be Firecracker-based.
Sources
Cite this page
Agent Sandboxes. (2026). Modal Sandboxes vs Fly.io Sprites. agentsandboxes.org. Last reviewed November 8, 2026. https://agentsandboxes.org/compare/modal-vs-fly-sprites/
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