How to Use GitHub Copilot to Explain Legacy Code

Legacy code often lacks comments, uses outdated patterns, or was written by developers who left the team. Understanding what a block of code does can take hours of tracing logic and reading documentation. GitHub Copilot can generate plain-English explanations for selected code, reducing the time needed to understand unfamiliar functions or modules. This article explains how to use Copilot’s chat and inline features to explain legacy code, what prerequisites are needed, and what to watch out for when relying on AI-generated explanations.

What Copilot Can Do with Legacy Code

GitHub Copilot is an AI pair programmer that runs inside editors like VS Code, JetBrains, and Visual Studio. Its core feature is code completion, but it also includes a chat interface and inline commands for explaining code. When you ask Copilot to explain a block of legacy code, it analyzes the syntax, variable names, control flow, and function calls to produce a summary in plain English. Copilot does not execute the code. It uses a large language model trained on public code repositories to infer the code’s purpose.

The explanation quality depends on several factors. Clean variable names and consistent formatting help Copilot produce accurate summaries. Obscure one-letter variable names or deeply nested logic can lead to vague or incorrect explanations. You should treat Copilot’s output as a starting point, not a definitive documentation replacement.

Before using Copilot for code explanation, you need these prerequisites:

• A GitHub Copilot subscription. Copilot Individual costs $10 per month. Copilot for Business costs $19 per user per month. Copilot Enterprise costs $39 per user per month.
• VS Code, JetBrains IDE, or Visual Studio 2022 with the Copilot extension installed and activated.
• An active internet connection. Copilot sends code snippets to GitHub’s servers for processing.
• The legacy code file must be open in the editor. Copilot works on any language it supports, including C, C++, Java, Python, JavaScript, COBOL, and more.

Steps to Explain Legacy Code with GitHub Copilot

There are three main methods to get code explanations from Copilot. Each method works best in different scenarios.

Method 1: Using Copilot Chat with the Explain Command

1. Open Copilot Chat
   In VS Code, press Ctrl+Shift+I or click the Copilot icon in the activity bar. In JetBrains, press Ctrl+Shift+I or use the Copilot Chat tool window. In Visual Studio 2022, open View > Other Windows > Copilot Chat.
2. Select the legacy code block
   Highlight the function, class, or code section you want explained. Copilot uses the selection as context for its response.
3. Type the explain command
   In the chat input box, type /explain and press Enter. Copilot generates a structured explanation that includes the code’s purpose, parameters, return values, and logic flow.
4. Review and refine the explanation
   Read the output. If the explanation is too generic, ask a follow-up question like “What does the processData function do with the input array?” Copilot remembers the conversation context within the same session.

Method 2: Using Inline Explain in the Editor

1. Select the code block
   Highlight the legacy code you want explained in the editor.
2. Right-click and choose Copilot > Explain
   In VS Code, right-click the selected code, then select Copilot > Explain from the context menu. In JetBrains, right-click and select Copilot > Explain Code. Copilot opens a hover panel or a new chat tab with the explanation.
3. Copy the explanation or insert as a comment
   You can copy the text to a separate document or use the “Insert as Comment” option if available. This method does not keep a conversation history.

Method 3: Using a Natural Language Prompt in Copilot Chat

1. Open Copilot Chat
   Use the same shortcut as Method 1.
2. Paste the code snippet
   Copy the legacy code and paste it into the chat input box. Surround the code with triple backticks and specify the language for better formatting.
3. Write a specific prompt
   Type a question like “Explain what this C++ function does step by step” or “Describe the algorithm used in this Java method.” Copilot responds with a detailed explanation.
4. Ask follow-up questions
   If the explanation mentions a concept you do not understand, ask “What is a mutex in this context?” or “Why does this loop use a sentinel value?” Copilot can provide deeper context.

Common Mistakes and Limitations When Using Copilot to Explain Legacy Code

Copilot Returns an Incorrect Explanation for Obscure Code

Legacy code often uses non-standard patterns, custom macros, or domain-specific abbreviations. Copilot may guess the wrong intent. For example, a variable named tmp might be interpreted as a temporary storage when it actually holds a persistent state. Always cross-check the explanation with the actual behavior by running the code or reading related documentation.

Copilot Ignores the Full Context of a Large Codebase

Copilot only sees the code you select plus the active file. It does not scan your entire repository. If a legacy function relies on a global variable defined in another file, Copilot’s explanation will miss that dependency. To get better context, include relevant imports or call sites in your selection or prompt.

Copilot Produces a Generic Explanation for Very Short Code Snippets

If you select only a single line or a small expression, Copilot may return a trivial description like “This line assigns a value to a variable.” Select at least a full function or a block of 10-20 lines to get a meaningful explanation. For very short snippets, ask a specific question about the snippet’s purpose within the larger function.

Copilot Cannot Explain Non-Deterministic or Runtime Behavior

Copilot analyzes static code only. It cannot explain what happens during runtime, such as race conditions, memory leaks, or external API responses. If the legacy code has a bug that only appears under certain conditions, Copilot will not detect it. Use debugging tools and logs to complement Copilot’s static analysis.

Copilot Individual vs Copilot Business vs Copilot Enterprise: Key Differences

Item	Copilot Individual	Copilot Business
Price	$10 per month	$19 per user per month
Code explanation feature	Available in chat and inline	Available in chat and inline
IDE support	VS Code, JetBrains, Visual Studio, Neovim, Azure Data Studio	Same plus GitHub.com code view
Privacy and IP indemnity	No indemnity	IP indemnity included
Admin controls	None	Policy management, audit logs
Telemetry exclusion	Not available	Can opt out of code telemetry

Copilot Enterprise includes all Business features plus the ability to explain code in GitHub.com pull requests and a knowledge base for your organization’s codebase. For most developers explaining legacy code locally, Copilot Individual or Business is sufficient.

You can now use GitHub Copilot to generate explanations for legacy code in your editor. Start with the /explain slash command in Copilot Chat for the most structured output. For quick inline summaries, use the right-click Explain option. Always verify the AI’s explanation against the actual code behavior, especially for complex logic or domain-specific terms. Copilot’s explanations are a productivity tool, not a substitute for thorough code review and testing.