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Diag

Category: General  ·  Sub-category: operations  ·  Last updated:
type:debug
Read-only root-cause diagnosis for a symptom, bug, failure, or unexpected behavior. Establishes the mechanism at file:line and reads up the facts instead of guessing, before any fix is attempted. Produces a diagnosis report; the fix is always a separate step gated by an explicit GO. Use when something is broken, failing, throwing, or behaving unexpectedly and the cause is not yet proven, or when asked to investigate or find the root cause. Not for judging whether finished work meets its requirement (use audit) and not for confirming rendered UI behavior (use verify).
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From the source SKILL.md

A read-only root-cause diagnosis. diag explains WHY something is broken and proves the mechanism, before anyone changes code. It is a separate skill from audit and must never be conflated with it.

What this skill does

Diag is a community-contributed Claude Code skill in the operations sub-category. It ships as a SKILL.md file that Claude Code auto-discovers under ~/.claude/skills/diag/ and loads when your prompt matches the skill's trigger.

When to invoke it: Use when something is broken, failing, throwing, or behaving unexpectedly and the cause is not yet proven, or when asked to investigate or find the root cause. Not for judging whether finished work meets its requirement (use audit) and not for confirming rendered UI behavior (use verify).

Who uses this skill

The Diag Claude Code skill is built for Claude Code users and developers across all disciplines looking for general-purpose AI assistance. It's part of ClaudSkills (also referred to as Claude Skills or Claude Code Skills) — the open community-curated registry of 157,000+ SKILL.md files for Anthropic's Claude Code agent and the wider Claude ecosystem (Claude API, Claude Agent SDK).

How to install

Free

Manual install (2 steps)

mkdir -p ~/.claude/skills/diag
curl -L https://claudskills.com/skills/diag/SKILL.md \
  -o ~/.claude/skills/diag/SKILL.md

Or just download SKILL.md directly and drop it into ~/.claude/skills/diag/. Claude Code auto-discovers it on next session.

Skills live at ~/.claude/skills/diag/SKILL.md on macOS/Linux, or %USERPROFILE%\.claude\skills\diag\SKILL.md on Windows. See the full install guide for step-by-step instructions.

Telegram

📱 Install from your phone or desktop Telegram

Open @claudskills_bot on Telegram, tap Open Desktop App, and the desktop app installs this skill for you. Or share the bot link with a colleague — they get the same one-tap install. Learn more →

Pro

One-click install via the desktop app

The ClaudSkills desktop app installs any skill directly into ~/.claude/skills/ with one click — no terminal required. Pro starts at $9/mo or $149 lifetime.

Pro

For the full experience including quality scoring and one-click install features for each skill — upgrade to Pro.

See pricing → Download desktop app

Frequently asked questions

How do I install the Diag Claude Code skill?
Install via the ClaudSkills desktop app (one click) or copy SKILL.md from the source repository to ~/.claude/skills/diag/SKILL.md and restart Claude Code. Both flows are detailed at claudskills.com/install/.
What does the Diag skill do?
Read-only root-cause diagnosis for a symptom, bug, failure, or unexpected behavior. Establishes the mechanism at file:line and reads up the facts instead of guessing, before any fix is attempted. Produces a diagnosis report; the fix is always a separate step gated by an explicit GO. Use when something is broken, failing, throwing, or behaving unexpectedly and the cause is not yet proven, or when asked to investigate or find the root cause. Not for judging whether finished work meets its requirement (use audit) and not for confirming rendered UI behavior (use verify).
Is this skill free to install?
Yes. ClaudSkills is an open registry — every skill keeps its source repository's license, and manual install via copy is free. ClaudSkills Pro ($9/mo, $79/yr, or $149 one-time) adds one-click install via the desktop app and a multi-signal Quality Score.
When should I use the Diag skill?
Use Diag when your Claude Code task falls under the General category — specifically in the operations area. Claude Code auto-discovers installed skills and invokes the right one based on the task description, so you can also ask Claude directly (e.g. "use Diag" or describe the task and let Claude pick). Browse related skills at /category/general/.
What is a Claude Code skill and how does the Diag skill fit in?
A Claude Code skill is a SKILL.md file that lives under ~/.claude/skills/<name>/ and tells the Claude Code CLI agent how to perform a specific task (instructions, prompts, allowed tools). Skills are auto-discovered at session start. Diag is one of 67,000+ skills indexed in the open ClaudSkills catalog, classified under the General category. Learn more at /learn/what-is-a-claude-skill/.

Attribution & license

Cite this skill

If you reference this skill in a blog post, paper, or documentation, you can cite it as:

APA
Marcel-Bich. (2026). Diag [Claude Code skill]. ClaudSkills. https://claudskills.com/skills/diag/
BibTeX
@misc{diag-2026,
  author    = {Marcel-Bich},
  title     = {Diag [Claude Code skill]},
  year      = {2026},
  publisher = {ClaudSkills},
  url       = {https://claudskills.com/skills/diag/}
}

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Security scan

Grade A · scanned 2026-07-22 — free static scan against the OWASP Agentic Skills Top 10.

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Browse all General skills in the ClaudSkills registry, or explore these other picks from the same category:

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Part of ClaudSkills — the open registry for Claude Skills & Claude Code Skills.  ·  What's New  ·  Install guide  ·  About  ·  llms.txt

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