bug-blame

Imported from bonez-io/re_gent (.claude/skills/bug-blame/SKILL.md). Install upstream with npx skills add bonez-io/re_gent --skill bug-blame. Copyright stays with the author.

Answer four questions about a defect, in order: what changed, when, why, and what else moved with it.

git blame answers the first two. Only re_gent answers the third — the prompt that caused the edit — and the fourth is what turns a fix into a safe fix.

1. What changed

If given a path or path:line, start there:

rgt blame <path>[:<line>]

Each line carries the step hash that last wrote it. Take the step for the suspect line. If given only a symptom, find candidate files first with rgt log --json -n 50 and match against the described behaviour.

2. When, and in what company

rgt show <step-hash>

This gives the full step: timestamp, tool calls with their arguments and results, every file the turn touched, and the surrounding conversation.

3. Why — the prompt

Read the user message on that step. This is the instruction the agent was following. State it verbatim. Very often the bug is not a coding mistake but a faithful execution of an ambiguous or wrong instruction, and that distinction changes the fix: rewrite the code, or rewrite the expectation.

Check the assistant message too. If the agent explained a tradeoff or flagged an assumption, the defect may be a known compromise rather than an accident.

4. Blast radius

The step's files list is the immediate radius — everything that turn changed under one intention. Widen it:

• Other steps in the same session, before and after (rgt log --session <id>), since a multi-turn task spreads one intention across several steps.
• Files that habitually travel with the suspect file — invoke the file-coupling skill. A file coupled to the culprit but not touched in this step is the highest-value thing to check: it usually should have changed and did not.

Report

Symptom     <what the user described>
Cause       step <hash> · <timestamp> · <tool>
Prompt      "<the user instruction, verbatim>"
Reasoning   <what the agent said it was doing, if relevant>
Changed     <files in that step>
Also check  <coupled files that did NOT change — likely gaps>
Session     <session id>, step N of M

Finish with a recommended next action: fix forward, or rgt rewind <step> to restore the workspace to before the change. Name the rewind target explicitly and say that rewind snapshots the current state first, so it is reversible.