Add ruby-version skill: verify Ruby releases before claiming versions exist

[OutlawAndy]

Why

Claude routinely makes confident but wrong claims about Ruby versions — most commonly insisting that a freshly-released Ruby "doesn't exist yet" because the version postdates training. This skill gives the agent a way to check the truth before asserting it, and gives developers a one-liner to install a specific Ruby.

Live demo of the problem this solves: running check.sh on my machine reports the latest stable Ruby is 4.0.3 — a version I would otherwise have denied existed.

What

New skill at skills/ruby-version/ with two bash scripts:

• scripts/check.sh — prints the latest stable MRI Ruby known to local ruby-build, the versions currently installed via rbenv, and the ruby-build version itself (so staleness is visible).
• scripts/install.sh <version> — validates the version against rbenv install -l first, then runs rbenv install --skip-existing. Does not auto-set global or local — that stays a user decision.

The SKILL.md description is written to trigger aggressively on phrases like "latest Ruby", "is Ruby X out", "does Ruby X.Y exist", "install Ruby", and any agent assertion about Ruby version existence. It explicitly instructs the agent to run check.sh before making such claims.

Design choices worth flagging

• Bash, not Ruby. A .rb script would fail on machines where Ruby is missing or broken — exactly when you'd want to run it. Bash works either way.
• Source of truth is rbenv install -l, not a network call. It's local, fast (<1s), already authoritative on a dev machine, and avoids new external dependencies. The trade-off is that ruby-build can be stale, so the script prints the ruby-build version alongside its answer — the same staleness problem as training-data, but visible.
• Targets rbenv only. Matches the maintainer's setup. Other version managers (mise, asdf, chruby) are intentionally out of scope.
• Install script never sets global/local. It prints next-step hints instead. Avoids surprising side effects on the user's environment.

Test plan

• Ran bash skills/ruby-version/scripts/check.sh on macOS + rbenv + ruby-build — output shows latest stable, ruby-build version, and the installed list.
• Verified the installed-versions filter excludes stray entries (e.g., a literal ~ directory in ~/.rbenv/versions/).
• Ran bash skills/ruby-version/scripts/install.sh 9.9.9 — fails fast with an actionable hint about brew upgrade ruby-build instead of attempting an install.
• Reviewer: sanity-check the SKILL.md trigger description — does it fire on the phrases you'd expect without being so loud it becomes noise?

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new ruby-version skill intended to prevent incorrect assertions about Ruby version availability by providing local rbenv/ruby-build based verification and a guarded install helper.

Changes:

• Added skills/ruby-version/SKILL.md describing when to invoke the skill and how to use the scripts.
• Added scripts/check.sh to report latest stable Ruby known to local ruby-build, ruby-build version, and installed rbenv versions.
• Added scripts/install.sh to validate a requested version against rbenv install -l and install via rbenv install --skip-existing.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
skills/ruby-version/SKILL.md	Documents the new skill, triggers, and usage for verifying/installing Ruby via rbenv.
skills/ruby-version/scripts/check.sh	Implements local Ruby version verification and reporting via rbenv/ruby-build.
skills/ruby-version/scripts/install.sh	Implements validated Ruby installation via rbenv with fast-fail messaging.

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