Buildkite vs GitHub Actions — Hybrid CI Compared

I've run both of these at real scale, and the thing nobody tells you up front is that they're not really the same category of product. GitHub Actions is a CI service that runs your jobs for you. Buildkite is a control plane that orchestrates jobs your own machines run. Once that clicks, most of the comparison sorts itself out. But the details are where the money and the headaches live, so let's go through them.

The two models #

GitHub Actions gives you a workflow file in .github/workflows, and GitHub schedules the work onto runners. By default those are hosted runners GitHub owns and bills by the minute. You can register self-hosted runners too, and plenty of teams do, but the default path is fully managed.

Buildkite flips it. The Buildkite platform stores your pipelines, shows you the UI, streams logs, and decides what runs next. The actual compute is an agent you install on hardware you control: an EC2 fleet, a Kubernetes cluster, a Mac mini rack, whatever. Buildkite never touches your code or your build environment. It just hands work to your agents and collects the results.

That single design decision drives the cost model, the security story, and the scaling behavior.

Cost #

This is usually the first question, and the answer flips depending on your size.

GitHub Actions bills per minute on hosted runners. Small teams love it because the free tier and low volume mean you basically pay nothing. But per-minute pricing has a nasty property: your bill grows with every test you add, every matrix leg, every retry. Big monorepos with heavy test suites can rack up genuinely alarming invoices, and larger runner sizes multiply the rate. Self-hosted runners drop the compute charge but you're back to managing infrastructure yourself.

Buildkite charges a flat price per user, not per build minute. The compute is on your cloud bill, where you already have committed-use discounts, spot instances, and autoscaling you understand. For a 20-person team running light CI, Buildkite's per-seat fee can look expensive next to Actions' free minutes. For a 300-engineer org burning millions of build minutes a month, the flat seat price plus your own cheap spot fleet is dramatically cheaper and, more importantly, predictable. You know next month's Buildkite bill to the dollar.

Scale and performance #

Buildkite was built by people running very large monorepos, and it shows. It handles high concurrency and huge pipelines without breaking a sweat, because the scheduling bottleneck is tiny (just dispatch) and the heavy lifting happens on your fleet, which you size however you want. Teams routinely run thousands of concurrent agents. If you need a build to fan out across 500 parallel jobs, you provision 500 agents and Buildkite feeds them.

Actions scales fine for most teams, and GitHub has invested heavily in larger hosted runners and better concurrency. But you're living inside their queueing and their concurrency limits, and at the extreme end you feel it. Self-hosted runners let you push past hosted limits, at which point you're doing roughly the same infra work Buildkite expects, minus Buildkite's scheduler.

Security #

For regulated shops this is often the deciding factor. With Buildkite, your source code, secrets, and build artifacts never leave your network. The agent reaches out to Buildkite over an outbound connection, pulls work, and runs it entirely inside your VPC. Buildkite's control plane sees metadata and logs (which you can redact), not your codebase. That's an easy story to bring to a security review: the compute, the network, and the data are all yours.

Actions can be locked down too, especially with self-hosted runners inside your network, and GitHub's OIDC support for cloud auth is genuinely good. But on hosted runners your code and secrets do execute on GitHub-owned machines, and some compliance teams simply won't sign off on that.

Pipeline configuration #

Actions uses static YAML workflows. It's approachable, well documented, and most engineers can read one on day one. The weakness is dynamism: expressing "generate these steps based on which files changed" gets awkward fast, and you end up with clever conditionals or matrix gymnastics.

Buildkite's standout feature is dynamic pipeline generation. A pipeline step can run a script that emits more pipeline steps as JSON or YAML, uploaded back to Buildkite at runtime. That means you can compute your build graph from the actual diff, the affected packages, or anything else you can script. For monorepos where "only test what changed" is the whole game, this is a different league from static YAML. It's more setup up front and more power once you're there.

UI and ecosystem #

Buildkite's UI is clean and fast, and its wait/block steps for manual gates and deploys are well thought out. But its plugin ecosystem is modest.

This is where Actions runs away with it. The Marketplace has thousands of prebuilt actions for nearly everything, and the tight coupling with pull requests, checks, and the rest of GitHub is seamless. For a GitHub-native team, that integration is hard to give up.

Side by side #

Dimension | Buildkite | GitHub Actions
Cost model | Flat per-user + your own infra | Per-minute (hosted) or self-hosted
Compute | Your agents, anywhere | GitHub-hosted or self-hosted runners
Predictability | High, fixed seat cost | Variable, grows with usage
Very large scale | Excellent, huge concurrency | Good, hits limits at extremes
Security | Code/secrets stay in your network | Hosted runs on GitHub machines
Pipeline config | Dynamic generation, static too | Static YAML
Ecosystem | Modest plugin set | Massive Marketplace
Onboarding | Steeper | Very easy

If you're weighing this as part of a broader CI/CD platform decision, keep both the scale curve and the ecosystem lock-in in mind, because those pull in opposite directions.

The call we'd make #

Pick Buildkite if you're a large org that wants control over compute, a security posture where code never leaves your network, and a CI bill you can predict to the dollar while your engineer count grows and your build minutes don't punish you for it. The dynamic pipeline model alone justifies it for big monorepos.

Pick GitHub Actions if you live inside GitHub, your scale is moderate, and you value getting a working pipeline in an afternoon over squeezing out cost efficiency at the extreme end. The Marketplace and the native PR integration are real advantages, and for most teams they outweigh everything else.

The honest summary: Buildkite is the better tool the bigger and more compliance-bound you get, and Actions is the better default for everyone else. Figure out which side of that line you're on and the decision mostly makes itself.