GitHub Copilot is great for one-off questions. But when you need to document a 50,000-line legacy repo, a CLI tool designed for batch processing wins.
GitHub Copilot has transformed how developers write code. Its chat interface can explain functions, suggest fixes, and yes—even generate docstrings for individual functions. For day-to-day development, it's indispensable.
But when your team faces a massive legacy codebase with outdated or missing documentation, Copilot's strengths become limitations. Here's why a dedicated CLI tool like Legacy Doc-AI is the better choice for repo-wide documentation work.
To document a legacy repo with Copilot, you must open each file, copy functions into the chat, and wait for responses. For a 50,000-line repo with 200 undocumented functions, that's 200 chat interactions—each taking time to frame prompts, wait for completions, and copy results back.
A CLI processes the entire repo in one command. legacy-doc-ai docstrings ./src/ finds every undocumented function and generates docstrings for all of them. What takes hours manually becomes minutes.
Legacy repos aren't just big—they're complex. Functions interconnect, modules depend on each other, and patterns repeat across files. Copilot sees one function at a time. It can't understand the full context of your codebase.
Legacy Doc-AI parses your entire repo locally, building a full picture of what exists and what needs documentation. It then generates consistent docstrings across all files, following the same style and structure throughout.
When code changes but documentation doesn't, you have stale docs—documents that mislead new team members. Copilot has no way to check existing docstrings against current signatures. It just generates new content when asked.
Legacy Doc-AI includes legacy-doc-ai check ./src/ which scans your repo and flags functions where docstrings no longer match the actual code. Missing params, wrong types, outdated descriptions—it all gets caught.
Documentation changes, like code changes, should be reviewed before merging. Copilot gives you text in a chat window. You have to manually copy it into your files and hope you got it right.
With Legacy Doc-AI, run legacy-doc-ai docstrings ./src/ --dry-run to see unified diffs of every proposed change. Review them like pull requests. Accept what works, reject what doesn't. The same workflow you already use for code applies to docs.
Copilot costs $10–$19 per user per month. For a 10-person team, that's $100–$190/month just for documentation work, even if only one person is documenting the codebase.
Legacy Doc-AI charges per repository, not per person. One team member can run the CLI, review diffs, and commit changes. The whole team benefits without paying seat licenses.
Copilot is excellent for individual developer productivity. For legacy repo documentation at scale, it's the wrong tool. A CLI designed for batch processing, stale detection, and diff review does what Copilot can't—efficiently, accurately, and with proper review workflow.
Keep Copilot for daily coding. Add Legacy Doc-AI when your repo needs a documentation overhaul.