The Buildkite Agent

The Buildkite agent is a small, reliable and cross-platform build runner that makes it easy to run automated builds on your own infrastructure. Its main responsibilities are polling for work, running build jobs, reporting back the status code and output log of the job, and uploading the job's artifacts.

This page contains reference information for Buildkite organization administrators. It covers agent installation and configuration details and how agents communicate with Buildkite. If you're working with a team that already uses Buildkite and you want to write code that agents will run, read Pipelines. If you're setting up a Buildkite organization and you don't already have agents running, read Getting started.

You (or your organization) need one or more running agents to run builds, but once you've installed the agent and got it running on your own infrastructure, you don't need to interact with it directly. Whether you're starting builds automatically with every commit, or running them manually by clicking a button, Buildkite handles everything from telling the agent what version control references to use, where to get the changes from, and what code to run; as well as reporting the outcome back to

How it works

The agent works by polling Buildkite's agent API over HTTPS. There is no need to forward ports or provide incoming firewall access, and the agents can be run across any number of machines and networks.


The agent starts by registering itself with Buildkite, and once registered it's placed into your organization's agents pool. The agent periodically polls Buildkite looking for new work, waiting to accept an available job.

After accepting a build job the agent will execute the command, streaming back the build script's output and then posting the final exit status.

Whilst the job is running you can use the buildkite-agent meta-data command to set and get build-wide meta-data, and buildkite-agent artifact for fetching and retrieving binary build-wide artifacts. These two commands allow you to have completely isolated build jobs (similar to a 12 factor web application) but have access to shared state and data storage across any number of machines and networks.


You can install the agent on a wide variety of platforms, see the installation instructions for a full list and for information on how to get started.


$ buildkite-agent --help

  buildkite-agent [command] [arguments...]

Available commands are:

  start      Starts a Buildkite agent
  annotate   Annotate the build page within the Buildkite UI with text from within a Buildkite job
  artifact   Upload/download artifacts from Buildkite jobs
  env        Process environment subcommands
  meta-data  Get/set data from Buildkite jobs
  pipeline   Make changes to the pipeline of the currently running build
  bootstrap  Run a Buildkite job locally
  step       Retrieve and update the attributes of steps
  help       Shows a list of commands or help for one command

Use "buildkite-agent [command] --help" for more information about a command.

To start an agent you'll need your organization's agent token from the Agents page of your Buildkite dashboard. You pass the token to the agent using an environment variable or command line flag, and it will register itself with Buildkite and wait to accept jobs.


The agent has a standard configuration file format on all systems to set meta-data, priority, etc. See the configuration documentation for more details.

Experimental features

We frequently introduce new experimental features to the agent. Use the --experiment flag to opt-in to them and test them out:

buildkite-agent start --experiment experiment1 --experiment experiment2

Or you can set them in your agent configuration file:


If an experiment doesn't exist, no error will be raised.


Please note that there is every chance we will remove or change these experiments, so using them should be at your own risk and without the expectation that they will work in future!

Redacted variables

The Buildkite agent can redact strings that match the value of environment variables whose names match common patterns for passwords and other secure information before the build log is uploaded to Buildkite.

See redacted-vars for more information.

Git mirrors

Maintain a single bare git mirror for each repository on a host that is shared amongst multiple agents and pipelines. Checkouts reference the git mirror using git clone --reference, as do submodules.

🛠 Experimental feature

To use it, set experiment="git-mirrors" in your agent configuration.

See the following agent configuration options for more information:

ANSI timestamps

Outputs inline ANSI timestamps for each line of log output which enables toggle-able timestamps in the Buildkite UI.

🛠 Experimental feature

To use it, set experiment="ansi-timestamps" in your agent configuration.

Normalised upload paths

Artifacts uploaded by buildkite-agent artifact upload will be uploaded using URI/Unix-style paths, even on Windows. This makes sure that artifacts uploaded from Windows agents are stored in a URI-compatible URL.

🛠 Experimental feature

To use it, set experiment="normalised-upload-paths" in your agent configuration.

Artifact names displayed in Buildkite's web UI, as well as in the API, are changed by this.

For example, when using this experimental feature buildkite-agent artifact upload coverage\report.xml uploads to s3://example/coverage/report.xml instead of to s3://example/coverage\report.xml.

Resolve commit after checkout

After repository checkout, resolve BUILDKITE_COMMIT to a commit hash. This makes BUILDKITE_COMMIT useful for builds triggered against non-commit-hash refs such as HEAD.

🛠 Experimental feature

To use it, set experiment="resolve-commit-after-checkout" in your agent configuration.

Flock file locks

Changes the file lock implementation from to to address an issue where file locks are never released by agents that don't shut down cleanly. The new file locks are implemented at the kernel level, and are aware of when their parent process dies.

When the experiment is enabled, the agent will use different lock files from agents where the experiment is disabled, so the agents with this experiment enabled should not be run on the same host as the agents where the experiment is disabled.

🛠 Experimental feature

To use flock file locks, set experiment="flock-file-locks" in your agent configuration.

Customizing with hooks

The agent's behavior can be customized using hooks, which are shell scripts that exist on your build machines or in each pipeline's code repository. Hooks can be used to set up secrets as well as overriding default behavior. See the hooks documentation for full details.

Signal handling

When a build job is canceled the agent will send the build job process a SIGTERM signal to allow it to gracefully exit.

If the process does not exit within the 10s grace period it will be forcefully terminated with a SIGKILL signal. If you require a longer grace period, it can be customized using the cancel-grace-period agent configuration option.

The agent also accepts the following two signals directly:

  • SIGTERM - Instructs the agent to gracefully disconnect, after completing any job that it may be running.
  • SIGQUIT - Instructs the agent to forcefully disconnect, cancelling any job that it may be running.

Exit codes

The agent reports its activity to Buildkite using exit codes. The most common exit codes and their descriptions can be found in the table below.

0 The job exited with a status of 0 (success)
1 The job exited with a status of 1 (most common error status)
128 + signal number The job was terminated by a signal (see note below)
255 The agent was gracefully terminated
-1 Buildkite lost contact with the agent or it stopped reporting to us

📘 Jobs terminated by signals

When a job is terminated by a signal, the exit code will be set to 128 + the signal number. For more information about how shells manage commands terminated by signals, see the Wiki page on Exit Signals.

Exit codes for common signals:

Exit code Signal Name Description
130 2 SIGINT Terminal interrupt signal
137 9 SIGKILL Kill (cannot be caught or ignored)
139 11 SIGSEGV Segmentation fault; Invalid memory reference
141 13 SIGPIPE Write on a pipe with no one to read it
143 15 SIGTERM Termination signal (graceful)