Ruby Upgrade

Ruby Upgrade

Overview

This skill upgrades a Ruby application from one major/minor Ruby version to another — currently targeting the Ruby 3.x → 4.0.x jump. It runs a breaking-change risk audit against the codebase, writes a stakeholder-ready analysis, applies the mechanical toolchain edits (.ruby-version, Dockerfile, CI, RuboCop TargetRubyVersion, Gemfile pins), and verifies via bundle, RuboCop, type checks, and the test suite.

This is the Ruby-interpreter upgrade. For Rails framework version bumps (6.x → 7.x → 8.x, config.load_defaults, railsdiff), use the superpowers-ruby:rails-upgrade skill instead. They are often done together — run the Rails upgrade's Ruby-compatibility table first to confirm the target Ruby is supported by the current Rails version.

Announce at start: "I'm using the ruby-upgrade skill to plan this Ruby upgrade."

Breaking-change inventory: references/ruby-4-0-changes.md (bundled with this skill). It is the source of truth for detection patterns and remediations.

Modes

The user may ask for one of:

• audit-only — stop after writing the analysis report (Phases 0–3).
• full (default) — audit, then apply mechanical updates, then verify.

If the target patch isn't specified, default to the latest stable in the target line and say which you picked.

Phase 0: Verify repo + anchor target

Confirm this is a Ruby/Rails repo before doing anything:

git rev-parse --show-toplevel
test -f Gemfile && test -f Gemfile.lock

Bail if Gemfile/Gemfile.lock are missing — this skill operates on a Bundler project. Read references/ruby-4-0-changes.md now and keep it open as you audit.

Phase 1: Capture current toolchain state

Read in one parallel batch, recording each into a "Current state" table:

• .ruby-version (current pin)
• Gemfile (note any ruby "..." / ruby file: directive)
• Gemfile.lock (RUBY VERSION, BUNDLED WITH, and key versions: rails, sorbet/tapioca if present, openssl, logger, ostruct, benchmark, irb, reline, pry, debug, nokogiri, pg, grpc, ffi, bcrypt, msgpack)
• Dockerfile / *.Dockerfile (the Ruby base-image line)
• CI config: any of .github/workflows/*.yml, .gitlab-ci.yml, .gitlab/ci/**/*.yml, .circleci/config.yml mentioning a Ruby version
• .rubocop.yml (AllCops: TargetRubyVersion)
• sorbet/config (only if the project uses Sorbet)

Phase 2: Risk inventory — parallel grep sweep

Dispatch ONE Agent subagent (subagent_type: Explore) to sweep the codebase for every change in references/ruby-4-0-changes.md. Hand it that file and ask for counts + up to 5 file:line examples per change-NN entry. Cover all entries (change-01 … change-14), and additionally list any HTTP-client gems from the lockfile that wrap Net::HTTP.

Wait for the structured findings report before proceeding.

Phase 3: Write the analysis report

Write to a project-relative folder so it works in any repo:

docs/ruby-upgrade/<YYYY-MM-DD>/analysis.md

and if docs folder is not present write to:

tmp/ruby-upgrade/<YYYY-MM-DD>/analysis.md

(Use date +%Y-%m-%d. If the project already has a conventional analysis/report location, prefer that and say so.)

Sections:

1. TL;DR — verdict (LOW / MEDIUM / HIGH risk), estimated effort, top 3 blockers.
2. Current state — the Phase 1 table.
3. Risk matrix — one row per references/ruby-4-0-changes.md entry: # | change | exposure (counts + file:line) | risk | action, each linking back to the inventory anchor (e.g. (./../../skills/ruby-upgrade/references/ruby-4-0-changes.md#change-01) or by change number).
4. Phased checklist — pre-filled boxes for Phase 4.
5. Surprises — repo-specific findings (vendored gems, monkey-patches in config/initializers/, lock already on a newer Bundler, etc.).
6. Open questions for the team — type-checker (Sorbet/Steep) Ruby-4 support, internal Docker-registry mirroring of the target image, monitoring owner for fingerprint drift, gems known to lag Ruby majors (e.g. grpc).

If the user asked for audit-only, STOP HERE and print the report path.

Phase 4: Apply mechanical toolchain updates

Branch hygiene first: if the current branch doesn't already encode this work, suggest a name like upgrade-ruby-4-0-5 (prefix with the user's ticket id if they gave one).

Make each edit only if the file actually pinned the old version — never invent files:

• .ruby-version: write the target (e.g. 4.0.5\n).
• Dockerfile(s): change FROM ...ruby:3.X.X → ruby:<target>. Preserve any private-registry prefix on the image (don't rewrite the registry host).
• CI YAML: bump every literal Ruby version string across all CI files found in Phase 1.
• .rubocop.yml: set AllCops: TargetRubyVersion: <major.minor>.
• Gemfile: add gem "cgi" if any removed CGI API is used (change-02). Add gem "ostruct" / other demoted gems ONLY if app code requires them directly and they aren't already resolved (change-03).

Then re-resolve:

bundle update --ruby

Capture the resolution diff. If it fails on a specific gem (commonly grpc, a Sorbet package, or a hard pre-Ruby-4 pin), STOP applying and record the failing gem under a "Blockers" section in the report.

Phase 5: Verify

Run the relevant checks (in parallel where possible):

1. Rubocop

bundle exec rubocop         # changed files or full repo

2. Full test suite:

If the project uses RSpec execute:

bundle exec rspec --fail-fast 

else execute:

bundle exec rails test

3. If the project is using sorbet:

bundle exec srb tc          # only if sorbet/config exists
bundle exec tapioca gem     # only if the project uses Tapioca — regenerate RBIs

On failure: append a "Verification failures" section with the output, then either fix-forward the obvious cases (RBI staleness, safe RuboCop autocorrects) or stop and surface to the user. Do not claim success without showing passing output — see superpowers-ruby:verification-before-completion.

Native-extension hint: if bundle install succeeded but tests crash on require of a C-extension gem, a cached vendor/bundle was built against the old ABI — rm -rf vendor/bundle && bundle install (change-09).

Phase 6: Post-merge watch (recommend, don't auto-run)

Suggest 24-hour monitoring on whatever error tracker the project uses for:

• new ArgumentError/backtrace fingerprints (change-10),
• a spike in Net::HTTP 400/415 responses from internal clients (change-01),
• new parse errors in any log scrapers that read Ruby internal frames.

Hard rules

• Never bump .ruby-version without bumping every Dockerfile/CI image in the same commit. Drift between them is the most common deploy-time failure for a Ruby-major upgrade.
• Never auto-add gem "ostruct" (or other demoted gems) unless app code directly requires them. Rails 7.1+ pins several transitively; a duplicate fights the lock.
• Never enable --zjit in production on 4.0.x — experimental. Keep YJIT.
• Do not skip the Phase 2 grep sweep. "Just bump the version and run tests" misses CGI.parse, openssl pins, and monitoring fingerprint drift — none of which the test suite catches.

Common mistakes