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StarSling vs Blacksmith runners for GitHub Actions

StarSling is AI-native CI for GitHub Actions: drop-in Ubuntu runners plus agents that open optimization PRs for your workflows.

Blacksmith provides fast, drop-in GitHub Actions runners on better hardware, with runner families beyond Linux and build-infrastructure features such as caching. StarSling is also a drop-in runner replacement, focuses on Ubuntu Linux runners up to 64 vCPU, and adds the AI-native layer: agents that analyze your workflows, logs, and telemetry and open pull requests that optimize caching, installs, parallelization, and test sharding against the best practices StarSling agents check for. The category difference is fast runners plus build infrastructure vs fast runners plus agent-native optimization.

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Drop-in runner replacementAI optimization PRsGitHub Actions support
How StarSling works

Last updated 2026-09-16

Head to head

Drop-in runner replacement
StarSlingYes, swap ubuntu-latest for starsling-ubuntu-24.04.
BlacksmithYes, a drop-in GitHub Actions runner replacement via the runs-on label.
AI optimization PRs
StarSlingYes. AI agents analyze logs/telemetry and open optimization PRs. For new accounts, AI-powered optimization PRs are only available to customers on paid plans and are not enabled by default.
BlacksmithNo. Blacksmith's published product surface is runners and caching; its docs list no agent that opens optimization PRs.
GitHub Actions support
StarSlingRuns your existing GitHub Actions workflows unchanged.
BlacksmithYes. Runs existing GitHub Actions workflows.
Runner OS coverage
StarSlingUbuntu 24.04 Linux runners.
BlacksmithLinux x64 and ARM and macOS runner families in the published instance-type list, plus Windows Server 2025 in public beta.
Linux runner ceiling
StarSling64 vCPU (starsling-ubuntu-24.04-64).
Blacksmith32 vCPU in the published Blacksmith Linux runner table.
Caching
StarSlingGitHub Actions caching works unchanged; the agents open PRs that fix cache keys, cache misses, and unnecessary installs.
BlacksmithDocker layer caches are kept on per-Dockerfile sticky disks that persist between runs, billed per GB-month as an add-on.
Primary focus
StarSlingFast runners plus agent-native optimization of the pipeline.
BlacksmithFast drop-in runners, caching, and build infrastructure.
Pricing model
StarSlingUsage-based per-minute, from $0.004/min (2 vCPU) to $0.128/min (64 vCPU); queue time never billed; no seat fees.
BlacksmithUsage-based per-minute, with the rate set by runner OS and architecture, plus per-GB add-on charges for cache storage.
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What Blacksmith is, and how its GitHub Actions runners work

Blacksmith is a drop-in GitHub Actions runner replacement: you keep GitHub Actions and change the runs-on label to a Blacksmith runner label. Its published instance list spans Linux x64, Linux ARM, macOS, and Windows Server 2025 beta families, with Linux instance types up to 32 vCPU.

For Docker builds, Blacksmith's docs describe per-Dockerfile sticky disks that persist Docker layer caches between runs, billed per GB-month as an add-on. That changes the cold-cache problem: repeat builds can reuse layers instead of rebuilding everything from zero.

That is runner and cache acceleration. It can make existing work start and run faster, but it does not remove work the workflow should not be doing. StarSling's axis is the agent layer: find cache misses, redundant installs, unsafe serial work, unsharded suites, and oversized runners, then open reviewable PRs that change the workflow itself.

Real StarSling optimization PRs on existing GitHub Actions workflows

The difference between fast runners and agent-native CI is visible in the customer PRs. StarSling agents do not just run jobs on different hardware; they edit the workflow and hand the team a PR to review. These are published case-study windows, not head-to-head Blacksmith benchmarks; follow each case study for source method and caveats.

  • Better Auth: agents fixed Turborepo cache configuration and cached Playwright browser installs. The published case study measures E2E at 2m 22s to 1m 04s and CI at 1m 40s to 1m 02s.
  • Mastra: agents sharded E2E kitchen-sink across parallel jobs (PR #15888) as part of the workflow speedup; E2E measured 14m 16s to 10m 50s.
  • Partcl: on synthesis-heavy CI, the agents right-sized runner classes and cut heaviest-job compute cost 13x per run while queue p95 fell from 9.5 minutes to 35 seconds.

Best fit

Choose StarSling if

Teams that want fast runners AND an AI layer that continuously improves the pipeline through reviewable PRs.

Choose Blacksmith if

Teams that primarily want faster drop-in runners, non-Linux runner families, or caching/build infrastructure without an optimization-agent layer.

FAQ

How is StarSling different from Blacksmith?

Both are fast, drop-in GitHub Actions runners. Blacksmith also covers non-Linux runner families and caching/build-infrastructure features. StarSling focuses on Ubuntu Linux runners up to 64 vCPU and adds AI agents that analyze your workflows and open optimization PRs, so the pipeline keeps getting faster, not just the hardware.

Is StarSling a drop-in replacement like Blacksmith?

Yes. Swap your runs-on label to starsling-ubuntu-24.04; your workflows, actions, and secrets are unchanged.

Blacksmith vs GitHub Actions runners: what is the difference?

Neither one changes GitHub Actions: same workflows, same YAML, same triggers. The difference is whose machines run them, and what those machines keep between runs. GitHub-hosted runners are shared hardware, and their only layer caching is BuildKit's GitHub Actions cache backend (docs.docker.com/build/cache/backends/gha) with no remote builder. Blacksmith's runners are dedicated instances that hold a Docker layer cache on disk across runs. So choosing between them is a hardware and caching decision, not a CI platform decision: the edit is one label, not a migration. That is also the ceiling. Changing which machine runs a job does not remove work the job should not be doing. StarSling sits on the same swap and adds agents that open reviewable pull requests against the workflow itself.

Other comparisons

Compare StarSling with the other runner and CI options teams evaluate.

References

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