Modal VM Sandboxes GA: full Linux VMs behind `runtime="vm"`

Modal’s blog (2026-10-01, Amit Prasad) made VM Sandboxes generally available—a one-flag flip to a full Linux VM that keeps the familiar Sandbox surface (Modal blog).

Enable with modal.Sandbox.create(..., runtime="vm"). Modal says you get the same APIs, modal.Images, sub-second cold-starts, and CPU/memory bursting as gVisor Sandboxes, including the concurrent scale users already run (Modal blog).

What it is

Modal describes a fully capable Linux VM built on a custom runtime that starts from the Rust-based Cloud Hypervisor project, with host filesystem, lazy image loading, memory bursting, and snapshotting work layered on. Engineering deep dives are framed as coming in the weeks ahead—promised write-ups, not additional features shipping in this announce (Modal blog).

Reach for runtime="vm" when workloads hit userspace walls: Docker, FUSE filesystems, or niche Linux kernel features. Modal’s Docker-in-Docker sample is illustrative of that shape (Modal blog).

gVisor stays default

Happy Sandbox workloads can stay put. Modal states gVisor remains the default runtime; both runtimes share the same APIs, Images, and usage-based pricing framing. The GA post publishes no dollar rates—this brief invents none (Modal blog).

Modal’s docs companion notes GPU Sandboxes are only supported with runtime="gvisor"—the blog primary is silent on GPUs for runtime="vm", so do not read this GA as GPU-on-VM (VM Sandboxes guide).

Early customers (Modal-attributed)

Modal says early customers have already launched over 20 million VMs. Short named color from the same post: Linear (Coding Sessions / Docker via a single-flag switch), Legora (Docker/FUSE workarounds removed for long-horizon legal evals), and Snorkel (Harbor multi-container simulations). Quotes and product vignettes are vendor marketing—not independent case studies or SLAs (Modal blog).

Not Modal Clusters

Modal Clusters is a separate multi-GPU / multi-node story (@modal.clustered). This post is a Sandbox runtime flip for agent computers—do not merge the two Oct 1 Modal GA lanes (Modal blog).

Who should care

Teams whose agents need a real-machine shape—Docker stacks, local databases, FUSE, or kernel-flavored work—while keeping Modal Sandbox APIs and Images should start at the VM Sandboxes GA post and the VM Sandboxes guide. Flip runtime="vm" when gVisor userspace walls bite; leave the default alone when they do not.