Rails Upgrade Assistant Skill

Skill Identity

• Name: Rails Upgrade Assistant
• Purpose: Intelligent Rails application upgrades from 2.3 through 8.1
• Skill Type: Modular with external workflows and examples
• Upgrade Strategy: Sequential only (no version skipping)
• Methodology: Based on FastRuby.io upgrade best practices and "The Complete Guide to Upgrade Rails" ebook
• Attribution: Content based on "The Complete Guide to Upgrade Rails" by FastRuby.io (OmbuLabs)

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Terminology

Term	Meaning	Where
Workflow	One numbered unit of the upgrade flow, one file	workflows/<NN>-<name>-workflow.md
Step	A Step N header inside a workflow file	inside the workflow
Reference	Material loaded on demand: lookup tables, playbooks, conditional branches	references/<name>-reference.md

No sub-steps: a "Step 4.1" is either two sequential steps or a conditional branch inside one step (### A. / ### B. headers). Refer to a whole workflow as Workflow NN; refer to a step as file path plus step, e.g. workflows/01-run-test-suite-workflow.md Step 4. Not used in headings: Stage, Phase, Sub-step, dotted step numbers.

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Dependencies

• dual-boot skill (github.com/ombulabs/claude-code_dual-boot-skill) — Sets up and manages dual-boot environments using the next_rails gem. Covers setup, NextRails.next? code patterns, CI configuration, and post-upgrade cleanup. Must be installed for Workflow 04.
  • Workflow 04 file: workflows/04-setup-next-rails-workflow.md
• rails-load-defaults skill (github.com/ombulabs/claude-code_rails-load-defaults-skill) — Handles incremental load_defaults updates with tiered risk assessment (Tier 1: low-risk, Tier 2: needs codebase grep, Tier 3: requires human review). Used as the final step after the Rails version upgrade is complete.

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Core Methodology (FastRuby.io Approach)

This skill follows the proven FastRuby.io upgrade methodology:

1. Incremental Upgrades - Always upgrade one minor/major version at a time
2. Assessment First - Understand scope before making changes
3. Dual-Boot Testing - Test both versions during transition using next_rails gem
4. Test Coverage - Ensure adequate test coverage before upgrading (aim for 80%+)
5. Gem Compatibility - Check gem compatibility at each step using RailsBump
6. Deprecation Warnings - Address deprecations before upgrading
7. Backwards Compatible Changes - Deploy small changes to production before version bump

Key Resources:

• DELEGATE to the dual-boot skill for dual-boot setup with next_rails (see Dependencies)
• See references/deprecation-warnings-reference.md for managing deprecations
• See references/staying-current-reference.md for maintaining upgrades over time

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CRITICAL: Dual-Boot Code Pattern with NextRails.next?

When proposing code fixes that must work with both the current and target Rails versions (dual-boot), always use NextRails.next? from the next_rails gem — never use respond_to? or other feature-detection patterns.

DELEGATE to the dual-boot skill for:

• Setup and initialization (next_rails --init, Gemfile.next)
• NextRails.next? code patterns and examples
• CI configuration for dual-boot testing
• Post-upgrade cleanup (removing dual-boot branches)

DEPENDENCY: Requires the dual-boot skill

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Available Resources

Core Documentation

• SKILL.md - This file (entry point)

Version-Specific Guides (Load as needed)

Legacy Rails:

• version-guides/upgrade-3.2-to-4.0.md - Rails 3.2 → 4.0 (Strong Parameters)
• version-guides/upgrade-4.0-to-4.1.md - Rails 4.0 → 4.1 (Spring, secrets.yml, enums)
• version-guides/upgrade-4.1-to-4.2.md - Rails 4.1 → 4.2 (ActiveJob, Web Console)
• version-guides/upgrade-4.2-to-5.0.md - Rails 4.2 → 5.0 (ApplicationRecord)

Modern Rails:

• version-guides/upgrade-5.0-to-5.1.md - Rails 5.0 → 5.1 (Encrypted secrets)
• version-guides/upgrade-5.1-to-5.2.md - Rails 5.1 → 5.2 (Active Storage, Credentials)
• version-guides/upgrade-5.2-to-6.0.md - Rails 5.2 → 6.0 (Zeitwerk)
• version-guides/upgrade-6.0-to-6.1.md - Rails 6.0 → 6.1 (Horizontal sharding)
• version-guides/upgrade-6.1-to-7.0.md - Rails 6.1 → 7.0 (Hotwire/Turbo)
• version-guides/upgrade-7.0-to-7.1.md - Rails 7.0 → 7.1 (Composite keys)
• version-guides/upgrade-7.1-to-7.2.md - Rails 7.1 → 7.2 (Transaction jobs)
• version-guides/upgrade-7.2-to-8.0.md - Rails 7.2 → 8.0 (Propshaft)
• version-guides/upgrade-8.0-to-8.1.md - Rails 8.0 → 8.1 (bundler-audit)

Examples (Load when user needs clarification)

• examples/simple-upgrade.md - Single-hop upgrade example
• examples/multi-hop-upgrade.md - Multi-hop upgrade example

External Dependencies

• dual-boot skill - Dual-boot setup and management with next_rails (Workflow 04) (https://github.com/ombulabs/claude-code_dual-boot-skill)
• rails-load-defaults skill - Incremental load_defaults alignment (Workflow 12) (https://github.com/ombulabs/claude-code_rails-load-defaults-skill)
• upgrade-cleanup companion plugin - User-triggered. Removes dual-boot scaffolding and drops NextRails.next? / NextRails.current? branches. Deprecation triage stays with this skill for the next hop.

Reference Materials

• references/sequential-strategy-reference.md - Sequential upgrade rule (no version skipping) and the supported upgrade paths tables. Load in Workflow 03.
• references/request-patterns-reference.md - Which workflows run for each request shape (full upgrade, multi-hop, analysis only, reports only). Read at the start of a session.
• references/no-test-suite-smoke-reference.md - Load from Workflow 01 when no runnable RSpec/Minitest suite exists - Rails boot, routes, migration-status, and build smoke baseline with partial-confidence reporting
• references/deprecation-warnings-reference.md - Finding and fixing deprecations
• references/staying-current-reference.md - Keeping up with Rails releases
• references/breaking-changes-by-version-reference.md - Quick lookup
• references/multi-hop-strategy-reference.md - Multi-version planning
• references/testing-checklist-reference.md - Comprehensive testing
• references/gem-compatibility-reference.md - Gem update order and the "no compatible version" playbook (fork / vendor / replace). Load only when Workflow 06's compatibility check produced blockers.
• references/js-compressor-sprockets-mismatch-reference.md - Keeping terser / closure-compiler working when the target Rails pins Sprockets to the 2.x line. Load only when JS_COMPRESSOR_GEM_MISMATCH fires.

Detection Pattern Resources

• detection-scripts/patterns/rails-*.yml - Version-specific patterns for direct detection
• detection-scripts/scan_patterns.rb - Runs every pattern for a hop against the app and prints a findings table. Run it in Workflow 05 instead of one Grep per pattern.

Report Templates

• templates/upgrade-report-template.md - Main upgrade report structure
• templates/app-update-preview-template.md - Configuration preview

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Workflow index

When user requests an upgrade, follow this workflow. Sequential Process is Critical: run in order, read each file when you reach it. Gates block.

#	Name	Purpose	File
00	Verify latest patch	MANDATORY PRE-STEP. App on the latest patch of its current series before any minor/major hop	workflows/00-verify-latest-patch-workflow.md
01	Run test suite	MANDATORY FIRST STEP. How to run and verify test suite; blocks on any failure	workflows/01-run-test-suite-workflow.md
02	Resolve deprecation warnings	Make warnings visible, collect what the current version emits, fix them on the current version	workflows/02-resolve-deprecation-warnings-workflow.md
03	Validate upgrade path	Single-hop or multi-hop, individual hops planned	workflows/03-validate-upgrade-path-workflow.md
04	Set up next_rails	EARLY SETUP. Delegated to the skill listed under External Dependencies	workflows/04-setup-next-rails-workflow.md
05	Detect breaking changes	How to run breaking change detection directly	workflows/05-detect-breaking-changes-workflow.md
06	Check gem compatibility	Per-lockfile gem compatibility check against the target Rails version	workflows/06-check-gem-compatibility-workflow.md
07	Boot smoke test	Boot under Gemfile.next and run the models suite there, to catch what the resolver and the grep can't see	workflows/07-boot-smoke-test-workflow.md
08	Generate upgrade report	How to generate upgrade reports	workflows/08-generate-upgrade-report-workflow.md
09	Generate app:update preview	How to generate app:update previews	workflows/09-generate-app-update-preview-workflow.md
10	Implement and upgrade	Present reports, apply fix-before-bump changes, bump the Gemfile, run tests against both versions	workflows/10-implement-and-upgrade-workflow.md
11	Sync CI config	MANDATORY before opening the upgrade PR. How to verify CI config matches the upgraded Gemfile	workflows/11-sync-ci-workflow.md
12	Align load_defaults	AFTER THE UPGRADE IS COMPLETE. Delegates to the rails-load-defaults skill	workflows/12-align-load-defaults-workflow.md
13	Mention cleanup	USER-TRIGGERED. Delegates to the upgrade-cleanup plugin only when the user explicitly asks	workflows/13-mention-cleanup-workflow.md

The Purpose column is a summary. Gates live only in each workflow's ## Gates section. Which workflows run depends on the request shape (full upgrade, multi-hop, analysis only, reports only): references/request-patterns-reference.md.

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Key Principles

1. Run Detection Directly (run detection-scripts/scan_patterns.rb, then Read the affected files; never generate a detection script for the user to run)
2. Load Workflows as Needed (don't hold everything in memory)
3. Follow FastRuby.io Methodology (incremental upgrades, assessment first)

Everything else that used to be listed here lives in the workflow it governs: see each workflow's Purpose and Gates.