Legacy Code Explainer
Understand old, undocumented code in plain English
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Who wrote this, and did they leave the company in 2014? Why does a file nobody understands still run the nightly invoicing job? Legacy Code Explainer reads that inherited code and tells you what it is doing, without needing the person who wrote it.
Old code is rarely bad code. It is undocumented code, written for a system that changed around it.
Short answer: Legacy Code Explainer is a free AI tool on EizTools that explains inherited, undocumented code. Paste an old file or query, set the language and depth, and get a written account of what it does and which parts still matter.
What is Legacy Code Explainer?
Legacy Code Explainer is aimed at code you did not write and cannot ask about. You paste the old file into the prompt box, which asks for the code you want explained, and the answer describes its behaviour in current language, including patterns that have gone out of fashion.
What it can tell you
- What the code does now, statement by statement
- Which old patterns have modern equivalents
- Which inputs send it down a strange path
What it cannot know
- Why a rule was added, which is company history
- What else calls the file, unless you paste that too
- Whether a branch is dead or just rarely used
The People Who Inherit Code Like This
- A backend developer handed a service whose last commit predates their hire date
- A contractor assessing a client system before quoting for a rewrite
- A maintainer keeping an old billing script alive until the replacement ships
- A team writing a spec for the system they are retiring
What To Paste In When The File Is Nine Hundred Lines Old
Legacy Code Explainer only knows what is in the box, so what you paste decides the answer. Give it a section rather than the whole file, plus the surrounding facts.
- ✅ The function or query you actually care about, in full
- ✅ Any constants or config values it reads
- ✅ A line in Custom Instructions saying what the system does and what still calls it
Custom Instructions:
This is a 2011 PHP invoicing script that still runs nightly on cron.
Explain what the query selects and why the status flags matter.That one note turns a generic reading into an answer about your system.
Caution Do not refactor straight from the explanation. Odd looking conditions in old code are often a business rule added after a bad week. Confirm it with whoever owns the process before you delete it.
How Does Legacy Code Explainer Work?
Paste the code, pick a model from the selector, open the advanced options accordion, then generate. The output card holds the explanation with a word count and a reading time, with copy, share, listen and downloads as TXT, Word or HTML. Recent generations sit in the activity history panel, useful when you work through a file in sections.
Which Settings Matter On Old Code
| Option | What it controls | Where to start |
|---|---|---|
| Language | Which syntax the reading assumes | Name it, since Auto-Detect can misread old dialects |
| Explanation Depth | How far past the surface the answer goes | Detailed, because the value here is in the corners |
| Note Edge Cases | Flags inputs that behave oddly | On, this is where legacy bugs hide |
| Custom Instructions | Free text context up to a thousand characters | The system's age, its job and what still calls it |
Audience decides the vocabulary, Output Format gives you prose, comments, steps, a summary or a doc comment block, and the Depth slider sets length. Line-by-Line is worth turning on for a query or a tight loop, and off for a long file.
When you do replace an old hand written query, the SQL Generator drafts the new statement, and the Coding Tools category holds the rest of the set.
EizTools is a free AI platform where each job gets a purpose built workspace, and Legacy Code Explainer is the one for inherited code. No account, no quota, no card. Choose a model family from the selector before each run, and if the first reading misses the point, add context in Custom Instructions and generate again.
Frequently Asked Questions
How old can the code be?
Age is not the limit, the language is. The dropdown covers Auto-Detect, Python, JavaScript, TypeScript, Java, C#, C++, Go, PHP and Ruby, so most inherited systems are covered.
Can it tell me if the code is safe to delete?
No, and no tool that reads one file can. It describes what the code does. Whether anything still calls it is a question for your repository search and logs.
Should I paste the whole legacy file?
Work in sections. A few hundred lines at a time gives a sharper reading than a whole file, and the activity history panel keeps each one so you can assemble the notes.
Does it suggest modern replacements?
It names the old pattern and the current equivalent when there is one. Treat that as a starting point for a migration plan, not a finished refactor.
Inheriting a system means reading code alone, with nobody left to ask. Legacy Code Explainer turns that reading into notes for a ticket or a handover document. Paste the section that worries you most into the box above and add one line of context.
Sabir Bepari
Founder of EizTools and a passionate AI enthusiast dedicated to building practical, user-friendly AI tools that simplify everyday tasks.
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