Modal Sandboxes vs Cloudflare Sandbox SDK
Answer in brief
Choose Modal Sandboxes for high-concurrency and GPU-backed sandboxes — RL rollouts, evals and coding agents at scale; choose Cloudflare Sandbox SDK for teams building on Cloudflare Workers that want per-user sandboxes with egress and credentials controlled in code. Modal Sandboxes uses gVisor or VM isolation; Cloudflare Sandbox SDK uses MicroVM (Containers) isolation and is open source.
Modal Sandboxes vs Cloudflare Sandbox SDK at a glance
| Attribute | Modal Sandboxes | Cloudflare Sandbox SDK |
|---|---|---|
| What it is | Sandboxes on Modal’s serverless compute platform for running untrusted code at very high scale, with GPUs, snapshots and tunnels. | A TypeScript SDK for Cloudflare Workers that runs untrusted code in Linux sandboxes backed by Cloudflare Containers and Durable Objects. |
| Isolation | gVisor by default, or VMs with their own Linux kernel; GPU sandboxes use gVisor | Each container instance is a microVM with its own kernel and network (per Cloudflare docs); Dynamic Workers (V8 isolates) available for JavaScript-only code |
| Open source | No (Client SDK Apache-2.0) | Yes (SDK Apache-2.0 (runs only on Cloudflare)) |
| Self-host / BYOC | No / Not documented | No / No |
| SDKs | Python, JavaScript/TypeScript, Go | TypeScript (Workers) |
| Cold start (vendor claim) | Sub-second, including for large custom images (vendor claim) | Container cold starts “often in the 1–3 second range” depending on image (vendor docs) |
| Max session | 5 minutes default, configurable up to 24 hours; snapshots carry state beyond that | No fixed cap; runs while its Durable Object is active, then up to a 6-hour inactivity timeout |
| Persistence | Filesystem snapshots, directory snapshots, memory snapshots (alpha) and Modal Volumes | Ephemeral disk by default; container snapshots (up to 20 GB, 30 days), directory backups to R2, S3/R2 bucket mounts and Durable Object storage |
| GPU | Yes — Modal’s full GPU range (T4 to B300) at standard per-second rates, e.g. H100 $0.001097/s | Not 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 | Egress controlled from Worker code: disable internet and intercept outbound HTTP to allow specific hosts and inject credentials; preview URLs routed through your Worker |
| 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 | Cloudflare Containers rates in 10 ms increments: $0.000020 per vCPU-second (active) + $0.0000025 per GiB-second + disk; requires the $5/month Workers Paid plan |
| Compliance | SOC 2 on all plans; HIPAA on Enterprise | Not documented |
| Best for | High-concurrency and GPU-backed sandboxes — RL rollouts, evals and coding agents at scale | Teams building on Cloudflare Workers that want per-user sandboxes with egress and credentials controlled in code |
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 Cloudflare Sandbox SDK if…
- You build on Cloudflare Workers and Durable Objects
- You want egress and credentials controlled in your own code
- You want global placement on Cloudflare’s network
- You want a lighter isolate option for JS-only code
Both are worth piloting with your own image and region. See the full Modal Sandboxes profile and Cloudflare Sandbox SDK profile, or the complete comparison.
Frequently asked questions
Is Modal Sandboxes or Cloudflare Sandbox SDK 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. Cloudflare Sandbox SDK is best for teams building on Cloudflare Workers that want per-user sandboxes with egress and credentials controlled in code.
Which is open source, Modal Sandboxes or Cloudflare Sandbox SDK?
Cloudflare Sandbox SDK is open source (SDK Apache-2.0 (runs only on Cloudflare)); Modal Sandboxes is not.
How do Modal Sandboxes and Cloudflare Sandbox SDK isolate code?
Modal Sandboxes: gVisor by default, or VMs with their own Linux kernel; GPU sandboxes use gVisor. Cloudflare Sandbox SDK: Each container instance is a microVM with its own kernel and network (per Cloudflare docs); Dynamic Workers (V8 isolates) available for JavaScript-only code.
Sources
Cite this page
Agent Sandboxes. (2026). Modal Sandboxes vs Cloudflare Sandbox SDK. agentsandboxes.org. Last reviewed November 8, 2026. https://agentsandboxes.org/compare/modal-vs-cloudflare-sandbox/
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