Software engineer joining a codebase

Legacy code explainer

Immediate result: A plain-English walkthrough of what the code does, its hidden side effects and where it's safe to change.

Problem it solvesInherited code has no docs and the author left, so every change feels risky.

Download SKILL.md
Add this skill to
ChatGPTClaudeGeminiPerplexity

You paste

A 200-line PHP function processOrder() with nested ifs and three SQL queries

You get

What it does: validates a cart, charges the card, then writes the order — in that order, with no transaction.

Side effect: line 88 updates stock before payment succeeds.

Risky: a failed charge leaves stock reduced. Wrap lines 80–140 in a DB transaction.

About this skill

About Legacy code explainer

Inherited code with no documentation makes every change feel dangerous. This skill turns an unfamiliar file or function into a plain-English walkthrough: what it is for, what happens step by step, and — most importantly — its side effects, such as database writes, network calls and global state. It marks the risky spots and the safest place to make your change, and lists questions worth asking the team. Where the code calls something it can't see, it says 'unclear' rather than inventing behaviour. It's useful for onboarding, refactors and incident investigations alike.

Legacy code explainer AI skill for Software engineer joining a codebase: you paste "A 200-line PHP function `processOrder()` with nested ifs and three SQL queries" and get "What it does: validates a cart, charges the card, then writes the order — in that order, with no transaction."
Legacy code explainer — example input and the first lines it returns. Works in ChatGPT, Claude and Gemini.
Job profile
Software engineering · Software engineer joining a codebase
Works in
ChatGPT, Claude, Gemini, Copilot, Claude Code
Format
Agent Skills (SKILL.md)
Price
Free, no sign-up
Install instructions

Copy, paste, send

  1. Cold email writerCopy skillCopied ✓1 Copy the skillPress Copy skill.
  2. ANY AI ASSISTANTSKILLPaste here, then press Enter2 Paste it into a new chatChatGPT, Claude, Gemini or Copilot.
  3. ANY AI ASSISTANT3 Send your inputSend a file or function you need to understand.
The skill — paste this into your AI assistant
# Skill: Legacy code explainer

You are running the "Legacy code explainer" skill for me. I work as: Software engineer joining a codebase. What I need from you: a plain-English walkthrough of what the code does, its hidden side effects and where it's safe to change.

## When I send you a file or function I need to understand, plus the language
1. Summarise the purpose in two sentences.
2. Walk through the flow step by step in plain English.
3. List side effects: I/O, global state, database writes, network calls.
4. Mark risky spots and the safest place to make a change.

## Reply in this format
- What it does
- Step-by-step flow
- Side effects
- Risky spots
- Safe change points
- Questions for the team

## Rules
- Say 'unclear' instead of guessing what an external function does.
- Use only the facts I give you; mark anything you had to assume with [ASSUMPTION].
- If something essential is missing, ask me one short question, then continue.
- No preamble or filler: start with the result.

Confirm you're ready in one line, then wait for me to send a file or function I need to understand, plus the language.

Then sendA file or function you need to understand, plus the language. For example: A 200-line PHP function `processOrder()` with nested ifs and three SQL queries

ChatGPT

  1. Fastest: paste the skill as your first message in a new chat, then send your input.
  2. To keep it: open a Project, choose Instructions (or Settings → Personalization → Custom instructions for every chat) and paste it there.
  3. To share it with a team: create a custom GPT and paste the skill into its Instructions.
  4. In the Codex app: copy the skill's folder into ~/.codex/skills/ (or Customize → Skills → Add → Create skill).

Claude

  1. Fastest: paste the skill as your first message in a new chat, then send your input.
  2. To keep it: create a Project, open Project instructions and paste the skill.
  3. As a Claude Skill: download the skill's ZIP, then in Claude open Settings → Skills → Add → Upload skill and choose the file.

Gemini

  1. Fastest: paste the skill as your first message in a new chat, then send your input.
  2. To keep it: open Gems → New Gem, paste the skill into Instructions and save.

Microsoft Copilot

  1. Fastest: paste the skill as your first message, then send your input.
  2. In Microsoft 365 Copilot, save it as a prompt or paste it into an agent's instructions to reuse it.

Any other assistant

  1. Paste the skill as the system prompt or first message in any chat assistant (Perplexity, Meta AI, DeepSeek, Mistral, local models).
  2. Coding agents that read the Agent Skills format (SKILL.md) can load the downloaded file from their skills folder.

Install it permanently

Upload it to Claude, add it to ChatGPT's Codex app, clone it into Claude Code, or download the SKILL.md for any agent. Full guide with screenshots →

Download the ZIP and upload it in Claude. Claude then uses the skill on its own whenever a request matches.

Download ZIP

Quick setup

  1. In Claude, open Settings → Skills (under Customize).
  2. Click Add → Upload skill.
  3. Choose the ZIP — don't unzip it — and make sure the skill is switched on.
Claude Settings with Skills selected under Customize, and the Add menu open showing Upload skill, Create a skill, Create with Claude and Record your screen

Repositoryscienceofapps.com/skills/ai-skills.git · all 200 skills, one folder each, plain git over HTTPS

What it does

The steps it follows

  1. Summarise the purpose in two sentences.
  2. Walk through the flow step by step in plain English.
  3. List side effects: I/O, global state, database writes, network calls.
  4. Mark risky spots and the safest place to make a change.
What you get back

The reply format

  • What it does
  • Step-by-step flow
  • Side effects
  • Risky spots
  • Safe change points
  • Questions for the team

Rules it never breaks

  • Say 'unclear' instead of guessing what an external function does.
  • Uses only the facts you give it and marks assumptions.
FAQs

Questions about Legacy code explainer

What does the Legacy code explainer skill do?

Inherited code has no docs and the author left, so every change feels risky. The skill fixes that: a plain-English walkthrough of what the code does, its hidden side effects and where it's safe to change.

How do I install Legacy code explainer in ChatGPT, Claude or Gemini?

Copy the skill and paste it as the first message of a new chat, then send your input. To keep it, paste it into a ChatGPT Project or custom GPT, a Claude Project or a Gemini Gem, or upload the SKILL.md file as a Claude Skill.

What do I send Legacy code explainer after installing it?

Send a file or function you need to understand, plus the language. If something essential is missing, the skill asks you one short question and then continues.

Is the Legacy code explainer skill free?

Yes. It's free to copy, use and adapt, with no sign-up, and works in the free versions of ChatGPT, Claude, Gemini and Copilot.

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You paste

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You get

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You get

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You paste

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You get

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