why-this-change

name	why-this-change
description	Use when the user asks "why was this cop added/removed", "find upstream PR for a cop", "trace cop change in Rails", "what's the reason for this cop", or wants to find the rails/rails PR or commit that introduced or removed a specific RuboCop cop.
allowed-tools	Bash, WebSearch, WebFetch
args	copName

Why This Change

Research the upstream rails/rails repository to find the PR or commit that introduced or removed the given RuboCop cop, and summarize why the change was made.

Steps

1. Search rails/rails with gh CLI

Search for PRs and commits mentioning the cop name:

gh search prs "{copName}" --repo rails/rails --limit 5
gh search commits "{copName}" --repo rails/rails --limit 5

2. Verify the match

For the most relevant result, check the .rubocop.yml diff to confirm it actually adds or removes the cop:

gh pr diff <PR_NUMBER> --repo rails/rails -- .rubocop.yml
# or for commits:
gh api repos/rails/rails/commits/<SHA> --jq '.files[] | select(.filename == ".rubocop.yml") | .patch'

3. Fallback: Web search

If gh CLI doesn't return useful results, use WebSearch:

• Query: site:github.com/rails/rails "{copName}" .rubocop.yml

4. Report findings

Present:

• PR/Commit URL: The link to the upstream change
• Summary: A brief explanation of why the cop was added or removed
• Relevant quote (if available): Key excerpt from the PR description or commit message