debugging

A guide to finding why software behaves incorrectly by reading error messages and tracing problems back to their source.

In plain words
What is it for?
It helps interpret common errors, locate the relevant file and line, and choose troubleshooting steps while fixing code.
Why use it?
It helps turn vague failures into specific clues about what happened, where it happened, and why. This makes bugs easier to investigate instead of guessing at fixes.

Skill for Claude CodeCodex

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.

agentmods
npx agentmods add skills/wewpellex21/code-sensei/debugging
Any agent
npx skills add wewpellex21/code-sensei --skill debugging
Clone the repo
git clone --depth 1 https://github.com/wewpellex21/code-sensei

Made for: Claude Code, Codex.

Per session 36 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 710 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin 100% copy Near-identical to another mod 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 $0.00036 $0.00710
Opus 5 $0.00018 $0.00355
Sonnet 5 $0.00007 $0.00142
Haiku 4.5 $0.00004 $0.00071

Measured yesterday against content hash 7812de715f7c, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

debugging 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 yesterday.

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.

Origin

This is a copy

100% identical to debugging — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

skills/debugging/SKILL.md · 63 lines

How it starts

The opening of the file, as written. The whole thing — 63 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Debugging — CodeSensei Teaching Module

What is Debugging?

  • Analogy: Debugging is detective work. Something isn't working, and you need to find out why. Every error message is a clue.
  • Key insight: Errors are NOT failures. They're the computer telling you exactly what went wrong and where. Learning to read errors is the single most valuable programming skill.

How to Read Error Messages

Teach users to look for these three things in EVERY error:

  1. What happened — the error type (TypeError, SyntaxError, 404, etc.)
  2. Where it happened — the file name and line number
  3. Why it happened — the description message

Example:

TypeError: Cannot read property 'name' of undefined
    at UserCard (src/components/UserCard.jsx:12)

Translation: "On line 12 of UserCard.jsx, you tried to get .name from something that doesn't exist yet."

Common Error Types (teach as encountered)

Frontend Errors

  • "Cannot read property X of undefined" — you're trying to use data that hasn't loaded yet. Like trying to read a book that hasn't been delivered.
  • "X is not defined" — you're using a variable/function that doesn't exist. Likely a typo or missing import.
  • "X is not a function" — you're trying to call something as a function, but it's not one.
  • White screen of death — usually a component crashed during rendering. Check the browser console.

Backend Errors

  • 404 Not Found — the URL/route doesn't exist. Like going to a room number that doesn't exist in the building.
  • 500 Internal Server Error — something broke on the server. Check the server logs.
  • 401 Unauthorized — you're not logged in (or your token expired).
  • 403 Forbidden — you're logged in but don't have permission.
  • CORS error — your frontend is trying to talk to a backend that doesn't trust it. Like calling a secure building and not being on the approved list.

Database Errors

  • "relation X does not exist" — the table hasn't been created yet. Run your migrations.
  • "unique constraint violation" — you're trying to add a duplicate entry (like two users with the same email).
  • "null value in column X violates not-null constraint" — you forgot to include a required field.

Read the full file on GitHub · 63 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. yesterday First seen · 63 lines · 36 tokens per session scan A 7812de715f7c

Subscribe to this mod's changes

debugging is a skill published in the GitHub repository wewpellex21/code-sensei (3 stars, last pushed 3d ago), licensed MIT. It adds 36 tokens to every session and 710 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to debugging, differing in 0 lines, and is treated as a copy.

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