Firecracker vs gVisor isolation, overhead and compatibility

Which should you use, Firecracker or gVisor?

It depends on what the workload expects from Linux and what your hosts allow. For agents whose code needs a full kernel, with package managers, compilers and Docker, Runtime gives every sandbox a Firecracker microVM. Pick Firecracker yourself if you run hosts with KVM. Pick gVisor if you already run containers on Kubernetes or Docker and want a stronger boundary by switching to runsc, with no VMs to manage.

How Firecracker works

Firecracker is a virtual machine monitor written in Rust. It uses the Linux kernel's KVM to create a virtual machine per workload and boots a real guest Linux kernel inside it. The project puts compute-only guest performance at over 95% of bare metal. The guest has little hardware: Firecracker's site counts "only 5 emulated devices": virtio-net, virtio-block, virtio-vsock, a serial console and a minimal keyboard controller.

The monitor process is itself confined. Seccomp filters limit Firecracker to "the bare minimum set of system calls and parameters" it needs, and a jailer drops its privileges inside cgroups and namespaces, which the project calls "a second line of defense in case the virtualization barrier is ever compromised."

How gVisor works

gVisor puts an application kernel, the Sentry, between the program and the host. It "intercepts application system calls and acts as the guest kernel, without the need for translation through virtualized hardware." The Sentry is written in Go and runs in user space; a separate Gofer process mediates file access, and the runsc OCI runtime plugs it into Docker and Kubernetes.

On the default platform, systrap, the host kernel traps each system call with seccomp and hands it to the Sentry. The Sentry then uses "a minimal set of host system calls", which exclude opening files and creating sockets in the default modes. There is no guest Linux kernel and, in gVisor's words, no "device emulation" at all.

Side by side

Firecracker's figures are from its specification, measured on AWS metal hosts with a minimal guest. They describe the monitor alone, not a full sandbox.

Is gVisor safer than Firecracker?

They make different bets, and each project argues its own. Firecracker relies on the processor's virtualization boundary plus a tiny device model and a jailed, seccomp-filtered monitor. gVisor relies on a memory-safe kernel that implements Linux itself and touches the host through a short, enumerated list of calls, and it warns that "one should not assume that the mere use of virtualization hardware makes a system more or less secure."

What differs for the code you run: under Firecracker, code that breaks the guest kernel has broken only its own throwaway machine. Under gVisor, code that breaks the Sentry lands in a user-space process whose host surface is, in the project's words, "explicitly enumerated and controlled".

When each fits

• Pick Firecracker when you run untrusted or model-written code and want a full Linux kernel per workload: package managers, compilers, Docker, system tools and anything that expects real Linux behavior.
• Pick gVisor when you already run containers on Kubernetes or Docker and want a stronger boundary by switching the runtime to runsc, without managing virtual machines or needing KVM on the host.

Why Runtime chose Firecracker

An agent sandbox runs whatever a model writes, so each one gets its own kernel behind a hardware boundary. The rest follows from that:

• Real Linux inside. Ubuntu 24.04, Python 3.12, Node.js 24, Bun, git and gcc, with sudo working. Docker runs in a sandbox after sudo enable-docker (Docker).
• Root inside changes nothing outside. Network rules, CPU, memory and cost are enforced on the host, outside the microVM (security).
• Whole-machine state. A paused sandbox keeps its files, memory and running processes while you have credit, and a fork copies a running sandbox, memory included.

TypeScriptimport { Sandbox } from "withruntime";await using sbx = await Sandbox.create();const run = await sbx.exec("uname -r"); // the sandbox's own kernelconsole.log(run.stdout);

Pythonfrom withruntime import Sandboxwith Sandbox.create() as sbx:    run = sbx.exec("uname -r")  # the sandbox's own kernel    print(run.stdout)

Try it free: 100 free hours of a 1 GB machine in the first 30 days, no card:

Terminalnpx withruntime sandboxes run -- uname -r

More: microVM vs container, gVisor vs Docker, Runtime vs Modal, run untrusted LLM code.

Sources

Checked 26 October 2026.

• Firecracker, README, specification, design
• gVisor documentation, security model, platforms, performance, users
• Provider isolation from Runtime's comparisons, checked 25 October 2026: E2B, Vercel Sandbox, Modal

Facts on this page were checked on 26 October 2026.