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Sandboxer vs Blacksmith

Blacksmith runs GitHub Actions jobs on its own bare-metal fleet in ephemeral Firecracker microVMs, with colocated caching and Docker layer caching. It is limited to GitHub organizations. Sandboxer is a general sandbox service. Pick Blacksmith to speed up and cheapen existing GitHub Actions; pick Sandboxer for long-lived agent and dev environments.

Checked against Blacksmith's public docs on October 24, 2026. Sources are linked in the table and listed below.

At a glance

Sandboxer and Blacksmith compared
CapabilitySandboxerBlacksmith
IsolationmicroVM
Cloud Hypervisor, its own kernel
Firecracker microVM
Ephemeral microVMs that boot in under 3 seconds.
Full Linux with root
Ubuntu 24.04, sudo, systemd
Docker inside
Preinstalled
Nested virtualization (/dev/kvm)partial
Supported on x64 Linux runners only.
Pause and resume with memory
Running processes come back
Not documented for CI runners; microVMs are ephemeral.
Fork a sandbox
Copy-on-write disk copy that boots fresh
Not documented for CI runners; microVMs are ephemeral.
HTTPS URL per port
Private with a token, or public
Not offered for CI runners.
Longest sessionNo limit on paid plans
Auto-stop is up to 30 days or off
BillingPer second, flat rate per size
Stopped sandboxes are free
per minute by runner size
Ubuntu x64 $0.004/min, Arm $0.0025, macOS M4 $0.08, Windows $0.008; 3,000 free minutes a month.
GPUs
Self-host or open source
RegionsUS East (Virginia) and EU Central (Finland)

= not documented publicly.

Where Blacksmith is strong

  • Firecracker microVM isolation for every job, with KVM on x64 Linux.
  • Works with existing workflows by changing the runs-on label.
  • Colocated cache and Docker layer caching, and SSH into running jobs.
  • Published per-minute prices (which Blacksmith says are below GitHub-hosted rates) for Linux x64/Arm, Windows and macOS M4.

Where Sandboxer differs

  • Blacksmith runners are ephemeral microVMs for CI jobs; Sandboxer sandboxes persist and can resume with processes running.
  • Blacksmith works only with GitHub organizations, not personal repositories.
  • Blacksmith has macOS and Windows runners; Sandboxer does not.

Pick Blacksmith if

  • Your CI is GitHub Actions and you want faster, cheaper runners with minimal change.
  • You need macOS or Windows runners, or Docker layer caching.

Pick Sandboxer if

  • Need a long-lived machine you can stop, resume and reach over SSH or an HTTPS URL, not a machine that exists for one job.
  • Want coding agents preinstalled and a prompt API, MCP server, SDKs and CLI for driving them.
  • Need Docker, nested KVM and full root in a VM that can stop with memory saved and resume with processes running.

Sources

Products change. If something here is out of date, email hello@sandboxer.com and we'll fix it. Blacksmith is a trademark of its owner. Sandboxer is an independent product and is not affiliated with Blacksmith.

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