rca

rca is a skill for Claude Code from noordeen123/culprit. It costs 180 tokens per session (934 once invoked), scanned A, original, MIT.

A read-only Git investigation tool for finding what caused a bug, understanding a change, and checking whether a proposed fix is complete.

In plain words
What is it for?
Use it to trace regressions, identify the commit behind a crash, assess a change’s affected areas, score merge risk, or verify an uncommitted fix.
Why use it?
It replaces guesswork with evidence from commits, line history, differences, and—when available—GitHub data.

Skill for Claude Code

Written for Claude Code: shipped in a Claude Code plugin. Also seen: mentions subagents.

Part of the culprit plugin — 1 skill shipped together

Good fit Use it to trace regressions, identify the commit behind a crash, assess a change’s affected areas, score merge risk, or verify an uncommitted fix.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/noordeen123/culprit/rca
Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent
npx skills add noordeen123/culprit --skill rca
Clone the repo
git clone --depth 1 https://github.com/noordeen123/culprit

Made for: Claude Code.

Or install culprit, the plugin that ships this one along with the rest of its 1 skill.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for rca

README.md
[![agentmods](https://agentmods.dev/badge/skills/noordeen123/culprit/rca/github.svg)](https://agentmods.dev/skills/noordeen123/culprit/rca)
Your own site
<a href="https://agentmods.dev/skills/noordeen123/culprit/rca"><img src="https://agentmods.dev/badge/skills/noordeen123/culprit/rca/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for rca

Your own site · 80×15
<a href="https://agentmods.dev/skills/noordeen123/culprit/rca"><img src="https://agentmods.dev/badge/skills/noordeen123/culprit/rca.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 180 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 934 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00180 $0.00934
Opus 5 $0.00090 $0.00467
Sonnet 5 $0.00036 $0.00187
Haiku 4.5 $0.00018 $0.00093

Measured 10d ago against content hash cb4e59a0dab9, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade A, and why

rca scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

plugin/skills/rca/SKILL.md · 64 lines

How it starts

The opening of the file, as written. The whole thing — 64 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Root-cause analysis + fix verification (culprit)

culprit is a deterministic, offline, read-only engine that does the git forensics so you can reason about the result. This plugin installs its MCP server, so prefer the culprit MCP tools below — they return structured JSON. If they are not available, fall back to the rca / culprit CLI (same analysis, --json output).

The engine only runs git (diff/blame/log) and, when present, read-only gh. It never modifies the repo or the PR.

When to use

  • "Why did this break / what introduced this regression" -> bugfix RCA (analyze).
  • "Is this fix complete / verify my fix before committing" -> verify_fix on the diff.
  • "What does this change affect / blast radius / what should I test" -> feature analysis.
  • "How risky is this / safe to merge" -> get_risk_score.
  • "Find the culprit from this crash" -> from_trace on the stack trace.

MCP tools

  • analyze(repo, base?, head?, pr?) — one-call analysis: bugfix vs feature, suspect set, intent, lifecycle, completeness, risk. Start here.
  • verify_fix(repo, proposed_diff, base?) — check a not-yet-committed diff. Returns verdict (complete | partial | risky), untouched_references, skipped_symbols, risk_level, adds_test, notes.
  • find_suspects, classify_change, get_evolution, get_intent, check_completeness, get_test_impact, get_blast_radius, get_risk_score, from_trace — drill into one signal.

Workflow

Bugfix RCA

  1. analyze (or find_suspects) to rank the introducing commits.
  2. Read the suspect commits' actual diffs and write the "why it broke" narrative. The engine gives you structure; you supply the reasoning.
  3. get_intent / get_evolution when you need the author's original goal or the line-by-line history of the buggy range.

Verify a fix before committing (yours or a subagent's)

  1. verify_fix on the staged/proposed diff.
  2. If verdict is partial, patch the untouched_references it names, then re-run.
  3. If risk_level is not low: add the missing test (see notes), or review the skipped_symbols by hand if their contract changed.
  4. Iterate until verdict == complete. Treat this as the gate before you commit.

Read the full file on GitHub · 64 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 10d ago First seen · 64 lines · 180 tokens per session scan A cb4e59a0dab9

Subscribe to this mod's changes

rca is a skill published in the GitHub repository noordeen123/culprit (6 stars, last pushed 14d ago), licensed MIT. It adds 180 tokens to every session and 934 once invoked, about $0.0009 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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