CodeTrain / Learn / Understand a codebase
Learn any codebase in 30 minutes
Reading a summary of a repository does not teach you the repository. Paste a public repo URL into CodeTrain and it builds a lesson out of the code that is actually in it, then makes you write against that code one small step at a time. Free, no card, nothing to install.
Why a summary does not work
Ask a chat tool to explain a codebase and you get a confident architecture overview. It is usually right, it is occasionally invented, and either way you cannot tell which by reading it. Worse, you retain almost none of it, because you did not do anything. Half an hour later you can quote the summary and still not know where to make the change.
The thirty minute path
Point it at the repo
Paste any public repository URL. On Pro you point it at your own checkout instead and the edits land in your real branch, on 127.0.0.1.
Say what you actually want
Not "explain this repo". Say "how does the auth flow work" or "I need to add a field to the API". The intent decides which files get read and what the first step is, and a narrow intent produces a far better lesson than a broad one.
Write the first small step
It opens a real editor on a real file with a Run button. The first step is deliberately tiny. You type it, it runs, the tutor reviews what you wrote rather than what it expected.
Get stuck on purpose
Stuck is the useful part. Ask, and it shrinks the step or gives the next hint. It will not paste the solution, so the thing you end up with is yours.
Thirty minutes in you will not know the whole system. You will know one path through it end to end, which is the thing that makes the second hour fast.
Explore or change: pick one
Repo mode has two intents and they behave differently. Explore is read only: you learn the code and nothing on disk moves. Guided means you are making a change, and the work comes back as a single reviewable patch at the end of the lesson rather than a trail of edits you have to untangle. Pick explore when you are onboarding, guided when you have a real task.
What it will not read
This matters more on a public repo than people expect, because public repos
contain committed secrets more often than anyone would like. CodeTrain refuses a fixed denylist
before anything reaches the model or your browser: .env and its variants, private
keys and certificates, .netrc, .npmrc, .pypirc,
.git-credentials, service account JSON, keystores, and VPN configs. Gitignored files
are skipped too. The list is enforced in the agent and on the server, and the two are kept in
sync deliberately.
When this will not help
- The repository is enormous and your intent is vague. Narrow the question first.
- You need an answer in five minutes for an incident. Ask an assistant, learn it later.
- The code is generated or vendored. There is nothing there worth learning by hand.
- You want documentation you can hand to someone else. This makes you understand it, not a written artifact about it.