Ready-to-use AI prompts for explain legacy code — written for Software Engineer and easy to paste into ChatGPT, Claude or Gemini.
When you need explain legacy code done, these software engineer prompts give you a fill-in-the-blanks starting point covering legacy, documentation, onboarding. Fill in fields like languages I know, framework or domain, file or module and paste the result into ChatGPT, Claude, or Gemini.
Explain this file to a new team member
You are a staff engineer onboarding me to an unfamiliar codebase. I am fluent in [languages I know] but have never seen [framework or domain].
<code>
[file or module]
</code>
Explain it in this order:
1. One paragraph: what business problem this code solves and who calls it.
2. A walkthrough of the main flow, step by step, in plain language, referencing function names.
3. The non-obvious parts — anything that looks wrong but probably exists for a reason, plus your best guess at that reason.
4. Inputs, outputs, side effects, and external systems touched.
5. Where I would most likely break something if I edited this.
Mark anything you are inferring rather than reading directly as 'assumption'. Keep it under 600 words.
Document an undocumented module
Act as a technical writer who also codes. Produce documentation for this [language] module so the next engineer does not have to read the source.
<code>
[code]
</code>
Write it as markdown with: a one-sentence purpose, a usage example that would actually run, a table of public functions (signature, what it does, gotchas), a short 'how it fits in' section describing upstream callers and downstream dependencies, and a 'known limitations' section based only on what the code shows.
Use the vocabulary already present in the code rather than inventing new terms. Where the code's intent is genuinely ambiguous, add a 'TODO: confirm with the team' note instead of guessing confidently.
Plan a safe refactor of legacy code
You are a senior engineer who has been paged at 3am for changes like this one. Here is a legacy [language] file we need to change to support [new requirement].
<code>
[code]
</code>
Constraints: [constraints].
Give me: (1) the smallest change that satisfies the requirement without restructuring anything, (2) the characterization tests I should write first to lock in current behavior, (3) a step-by-step refactor sequence where each step is independently shippable and reversible, and (4) the parts of this file I should leave alone and why.
Be explicit about which risks are real and which are theoretical. Prefer the boring option.
How to use this prompt
Fill in the blanks. Type your details into each highlighted field of this explain legacy code prompt — fields like languages I know, framework or domain, file or module — and it rewrites itself live.
Copy the prompt. Press Copy prompt to put the finished explain legacy code prompt on your clipboard.
Paste into your AI model. Paste it into ChatGPT, Claude, Gemini, or any chat LLM to get your explain legacy code result.
Review and iterate. Read the response as a software engineer would, then adjust fields like languages I know, framework or domain, file or module and re-run to sharpen it.
Fill-in fields explained
languages I know
Your languages i know.
framework or domain
Your framework or domain.
file or module
paste file or module
language
Your language.
code
paste code
new requirement
Your new requirement.
constraints
test coverage, deploy cadence, risk tolerance
Frequently asked
Which AI models does the Explain Legacy Code prompt work with?
The Explain Legacy Code prompts for Software Engineer are plain-text and model-agnostic — paste them into ChatGPT (GPT-4o), Claude, Gemini, Llama, or any other chat model.
How do I customize the Explain Legacy Code prompt for my own use?
Fill in the 7 labelled fields (languages I know, framework or domain, file or module, …) before copying, or edit the [bracketed] text after you paste.
Are the Explain Legacy Code prompts for Software Engineer free to use?
Yes. Every Explain Legacy Code prompt here is free to copy and reuse, including for commercial and client work, with no sign-up.