Git Blame Investigator

git blame tells you who last touched a line, which is almost never the question you actually have. The real question — "why is this here, and what breaks if I remove it?" — lives in the commit message, the surrounding diff, and the PR that shipped it. This skill does code archaeology: it walks from a suspicious line back to the commit that introduced the logic (not the one that reindented it), reads the intent, and returns a verdict on whether the code is a dead artifact or a Chesterton's fence guarding a bug you can't see.

When to use this skill

• A line looks redundant, wrong, or pointless and you're about to delete or "simplify" it.
• You're tracing a regression and need the exact commit that changed the behavior.
• You're onboarding to unfamiliar code and need to reconstruct why it was written this way.
• A workaround, magic constant, or odd conditional has no comment explaining it.
• blame keeps pointing at a formatting, rename, or merge commit that obviously isn't the real author.

Instructions

1. Locate the line precisely, then blame with context. Run git blame -L <start>,<end> <path> on the suspicious range (not the whole file) and note the commit SHA, not the author name. Add -w to ignore whitespace-only changes and -C -C -M to follow lines that were moved or copied in from other files — without these, blame stops at the refactor that relocated the code and you lose its true origin.
2. Distrust the first SHA — it's usually noise. If the blamed commit is a Prettier run, a lint autofix, a mass rename, or a "merge branch" commit, it did not author the logic. Re-blame ignoring it: git blame --ignore-rev <sha> -L <start>,<end> <path>. If the repo has recurring reformatting commits, list them in a .git-blame-ignore-revs file and set git config blame.ignoreRevsFile .git-blame-ignore-revs so every blame sees through them automatically.
3. Read the intent, not just the patch. Once you have the real commit, run git show <sha> to read the full commit message and the entire diff — not only the line you care about. Then find the PR with git log --merges --ancestry-path <sha>..HEAD -- <path> or gh pr list --search <sha> and read the PR description and review discussion. The "why" is in prose far more often than in code.
4. Track the exact line or string through time with line-history and the pickaxe. For a moving target use git log -L <start>,<end>:<path> to see every commit that changed that line range, in order, with diffs. To find when a specific string, identifier, or value entered or left the codebase, use the pickaxe: git log -S '<exact-string>' -- <path> (changes in the count of that string) or git log -G '<regex>' -- <path> (any diff line matching the regex). -S answers "when did this magic number / flag / call site appear or disappear?" in seconds.
5. Follow the code across moves and renames. A file rename or extraction silently truncates history. Use git log --follow -- <path> to span renames, and when logic was hoisted into a new file, use blame's -C -C -C (copy detection across the whole tree, even unmodified files) to find where it was lifted from. Confirm the trail is unbroken before drawing conclusions — a gap means the real origin is in a pre-rename path.
6. Trace a regression to its commit, by bisection if needed. First try git log --oneline -- <path> plus git log -L to spot an obvious culprit. If the offending change isn't obvious, run git bisect: git bisect start, git bisect bad (current), git bisect good <known-good-sha>, then test each checkout (script it with git bisect run <test-cmd> for an exact, automated answer). Bisect finds the precise breaking commit even across hundreds of revisions.
7. Reconstruct the decision from the neighborhood. Read the commits immediately before and after the originating one (git log --oneline <sha>~3..<sha> -- <path> plus the linked issue) to see what problem the change was solving. A line that looks pointless in isolation often makes sense as one half of a fix — the other half being the bug it prevents.
8. Render a verdict tied to evidence. Conclude with one of: safe to remove (origin found, the problem it solved no longer exists — cite the commit/issue), do not touch (it guards a known bug or invariant — cite the commit), or needs a test first (intent is plausible but unverified — name the behavior to lock down before changing). Never conclude "safe to remove" without having found and read the originating intent.

Output

A short investigation report containing: (1) the originating commit(s) — SHA, message, and the intent reconstructed from the diff and PR; (2) the line/string history — the ordered list of commits that introduced, moved, or altered the code (from log -L / -S), with the rename or refactor boundaries it crossed; and (3) a verdict — safe to change/remove, do not touch, or needs a test first — each justified by the cited commit or issue. All claims trace to a SHA the reader can re-run.