Open Policy Agent Gatekeeper vs Tirith

Open Policy Agent Gatekeeper is a free, open-source Kubernetes policy controller for Linux and self-hosted deployments. It runs as a validating and mutating webhook, enforcing CRD-based policies evaluated by Open Policy Agent. Validation constraints can deny requests, warn, run in dry-run mode or report violations without changing resources. Mutation policies can alter matching resources during admission before validation checks the final object. An audit function periodically evaluates existing cluster resources against constraints to identify pre-existing misconfigurations. The project includes an extensible policy library with parameterized examples, and provider connections to external data such as image registries and directory services. Named integrations or backed services include Azure Policy for Kubernetes, Google Kubernetes Engine Policy Controller, Rancher, AWS EKS Blueprints and Alibaba Cloud. Installation options include prebuilt, development or locally built images and Helm. Gatekeeper requires Kubernetes resources introduced in version 1.16 and cluster-admin permissions for installation. The gator CLI is documented as beta from version 3.11.

Tirith is a free, open-source Linux command-line tool that evaluates infrastructure documents against declarative JSON policies. It reports passed and failed rules together with the affected resource and value. Its pipeline gate checks Terraform plan output and can return a nonzero exit code when a policy violation appears, before infrastructure changes are applied. Documented inputs include Terraform plans and state, Kubernetes manifests, Infracost breakdowns, and other JSON documents. Policies can use providers and evaluators such as ContainedIn, Equals, and RegexMatch, while the provider architecture can be extended for other formats. Local evaluation runs on the user’s runner, and the project says policies and documents are not sent elsewhere. Optional organization mode checks plans against StackGuardian policies and uploads a masked document. Tirith documents GitHub Actions, GitLab CLI use, Bitbucket Pipelines, other CI, and local execution. OpenTofu plans work like Terraform plans for the documented gate.