Chrome extension. Install Now.
Cairn is a Chrome extension for people working through real university lecture series — Stanford CS, MIT OCW, and the rest. It tracks your progress across devices, and surfaces traces left by other learners who came before.
How it works
Track your progress automatically.Cairn watches what you watch on YouTube and tracks it against your enrolled courses. There is no "mark complete" button to click. Sign in and your progress follows you to the next computer, picking up where you left off — at the second you left off.
Find traces from other learners.When you reach a hard moment in a lecture, see notes left by people who were there before you. Each trace is anchored to the second it was posted. It is not a conversation — it is a marker on a trail. You can leave one of your own and keep walking.
Pick up on any computer.Sign in with Google and your courses, progress, and notes follow you across devices. Watch a lecture on your laptop, finish it on your work computer. No account needed if you'd rather keep things local — Cairn works fully offline as a guest.
What's different
Cairn is built around the idea that other people walked this trail before you, and what they left behind can help. Most YouTube tools are built for the solo note-taker.
A trace is a short, timestamped note pinned to a moment in a lecture. You see traces only at the seconds they were posted, and only when they are useful — at the kinds of moments where someone reached out a hand. There are no follower counts, no DMs, no replies threading off into the woods. People appear under anonymous nicknames they choose.
The course catalog grows from the people walking the trail. Anyone can add more.
The one conversation Cairn does have is the optional study chat, and it is private to you — you are talking to an assistant about the lecture, not to other learners. Between people, Cairn stays asynchronous on purpose.
Study chat
Cairn has an optional AI study chat in the sidebar. It already knows which course you are in, which lecture is playing, and the second you are paused at — so you can point at something with "this" and it knows what you mean.
A general chatbot is vague about a specific course. So courses on Cairn can carry a knowledge base: a public GitHub repository built from that course's own materials — every lecture transcript, the slide decks and handouts from the course website, and topic pages compiled from them and linked together.
When a course has one, the chat reads it before answering and treats it as more reliable than its own memory, citing the file it used so you can go and read the source yourself.
Look at one. The knowledge base for Stanford's CS224N — Natural Language Processing with Deep Learning — is public: chaimantec/cairn-kb-cs224n. All 23 lectures, with timestamped transcripts, the full text of every slide deck, and 86 interlinked topic pages.
You don't have to use Cairn to read it. A knowledge
base is ordinary markdown in a public Git repository, with an
AGENTS.md at the root explaining how it is laid out. So
you can browse it on GitHub, clone it, or point Claude Code, Codex,
or any other agent at it and work through the course there instead.
Nothing in it is locked to the extension — which also means you can
check what the assistant is working from, and correct it.
Not every course has one yet. They are built course by course, and whoever added a course to the catalog can attach a knowledge base to it. Courses that have one link to it from the catalog.
Bring your own key
The chat is off until you add your own API key from DeepSeek or OpenRouter, so you pay the model provider directly at cost — Cairn takes no cut and resells nothing. Your key is stored encrypted and never sent back to your browser, and your conversations are kept in your browser rather than on Cairn's servers. The privacy policy explains exactly where each message goes.
Honest about what this is
Cairn is a free side project. It exists because the person who made it kept abandoning Stanford CS and MIT OCW courses around lecture four, and wanted a tracker tool that is accessible. If that's you too — welcome.