VirtusLab

Deprecate CLI options, option aliases, using directives, sub-commands, or config keys in Scala CLI. Use when marking a feature as deprecated with a warning.

Add or change using directives in Scala CLI. Use when adding a new //> using directive, registering a directive handler, or editing directive preprocessing.

Add or run Scala CLI integration tests. Use when adding integration tests, debugging RunTests/CompileTests/etc., or working in modules/integration.

Reduce Scala compiler or standard library regressions found in community-build projects to minimal, self-contained scala-cli repros. Use when a Scala nightly breaks a project under community-build3, when Codex needs to auto-discover the matching `scripts/run.sh` project key and baseline Scala version, when the regression must be confirmed with a known-good build and a nightly build, or when a failing build log needs to be minimized into `test.scala` / `test-macro.scala` files.

Draft GitHub issues for Scala 3 compiler regressions from minimized repros, version boundaries, bisect results, and community-build failures. Use when Codex needs to turn a confirmed Scala 3 regression into a ready-to-file `scala/scala3` issue title and body that matches existing regression reports.

Use whenever invoking the `git machete` CLI to organize branch chains, compute fork points, run stacked rebases/merges, or manage GitHub/GitLab PR/MR chains - especially in a repo that already has a `.git/machete` file. Lists which subcommands modify `.git/machete`, run rebase/merge, run hooks, or read stdin, so the agent can pick the right one and pass the right flags. Crucially, names the commands that MUST be invoked with `-y/--yes` (and the few that have no `-y` and therefore must NOT be invoked at all without a tty) so the agent never hangs on an interactive prompt. Also warns against hand-editing the `.git/machete` layout file.

Look up the public API of any JVM dependency (Scala 3, Scala 2, Java) from the terminal — type signatures, members, docs, and source as Markdown, no JAR unpacking needed. Use this skill whenever you need to call an unfamiliar library method, explore a package's types, or check a dependency's API. Prefer cellar over Metals MCP only for looking up external dependency APIs (`cellar get-external` vs Metals `inspect`/`get-docs`) — cellar needs no project import and queries any published Maven artifact.

Scala coding style, tooling, and functional programming guidance, with dedicated sections on direct-style Scala, Ox structured concurrency, and synchronous Tapir. Auto-load for any task involving Scala code, especially when using direct-style or "plain" Scala.

Opinionated modern Java (21+) coding best practices, style guides, and anti-patterns. Curated by VirtusLab engineers. Covers code style, null safety, error handling, immutability, testing, concurrency, and tooling.