Rules skill

Bazel rules and validation for Codex skills
Categories:
Languages:
Statuses:
Tags:

rules_skill packages all files belonging to a Codex skill and validates its instructions and optional OpenAI metadata with a hermetic Bazel aspect.

Getting started

Add the module and register the validation aspect:

bazel_dep(name = "rules_skill", version = "<VERSION>")

build --aspects @rules_skill//skill:defs.bzl%skill_validation_aspect
build --output_groups=+skill_validation

Declare one library in the skill’s named, non-root package:

load("@rules_skill//skill:defs.bzl", "skill_library")

skill_library(
    name = "skill",
    srcs = glob(
        ["**"],
        exclude = [
            "BUILD.bazel",
            "BUILD",
            "evals/**",
        ],
    ),
)

The package’s final path segment is the skill name, so a skill_library cannot be declared in a repository root package. Move a root-level skill into a named subpackage before declaring it.

Building the library materializes the skill_validation output group. The aspect checks SKILL.md, verifies that the frontmatter name matches its package directory, and validates agents/openai.yaml when it is present.

Consuming a skill

Rules that install or evaluate skills can require the public SkillInfo provider:

load("@rules_skill//skill:defs.bzl", "SkillInfo")

attrs = {
  "skill": attr.label(providers = [SkillInfo]),
}

SkillInfo exposes these fields:

  • name is the logical skill name derived from the final segment of root. The validation aspect verifies that the SKILL.md frontmatter uses the same name.
  • root is the owning Bazel package path within the skill’s repository, such as projects/agents/skills/answer-question. It has no repository, execution-path, or runfiles prefix. It is always non-empty because repository root packages are unsupported.
  • files_by_path maps slash-separated paths relative to root to Bazel File values. It includes SKILL.md, preserves nested paths such as agents/openai.yaml, and has the same shape for source and generated files.
  • files remains the depset of all skill files. skill is the distinguished SKILL.md file, and openai_yaml is the optional distinguished agents/openai.yaml file.

Consumers should use files_by_path when staging a bundle instead of parsing File.path or File.short_path. Every source must belong to the skill’s Bazel package, and duplicate logical paths are rejected during analysis.

Use skill_discovery_links when a repository exposes canonical skill directories through a local discovery index such as .agents/skills. Pass the skill_library labels directly; the BUILD declaration is the complete source of truth, so there is no separate link manifest or configuration file.

load(
    "@rules_skill//skill:defs.bzl",
    "skill_discovery_links",
)

skill_discovery_links(
    name = "write_skill_links",
    skills = [
        "//projects/agents/skills/answer-question:skill",
        "//projects/example/skills/example:skill",
    ],
)

Reconcile the checkout with the runnable target:

bazel run //:write_skill_links

The macro also declares //:write_skill_links_test. That local, uncached test reads the source checkout and requires the discovery directory to contain exactly the skill names passed in BUILD, each as a direct relative symlink to its canonical SkillInfo.root. It rejects missing, stale, extra, absolute, or indirect links. Run the updater before the test after changing the label list:

bazel test //:write_skill_links_test

Both targets intentionally accept only source SKILL.md files from the same repository. The exact-state test is a pinned-local checkout check rather than a hermetic or remotely executable test: it resolves a source runfile back to the workspace and then inspects the local symlinks. The generated programs require Bash and POSIX symlink support and work on ordinary Linux and macOS checkouts; native Windows checkouts are not supported.

The updater holds a sibling <discovery_dir>.lock directory while it changes links, so concurrent declarations cannot produce a mixed set. A process killed without running its exit trap can leave that lock behind; after confirming no updater is active, remove the empty lock directory and rerun the updater. The exact-state test detects any interrupted partial reconciliation.