The Tweet That Named a Movement

On a Sunday night in February 2025, Andrej Karpathy posted 997 characters about how he had been writing code lately. He was describing a habit, not launching a movement. Nineteen months later the phrase he used is in the dictionary.

You need this story for one reason, and it is not nostalgia. Almost every argument you will have about AI and code — with a teammate, with a client, with a lecturer, with yourself at two in the morning — comes from people using one phrase to mean three different things. By the end of this page you will be able to tell those three apart and say out loud which one is safe where.

What he actually posted, and the line everybody skips

He asked for changes as small as “decrease the padding on the sidebar by half”, because finding the line himself was more effort than asking. He accepted every diff without reading it. When something broke, he pasted the error message back with no comment, and usually that was enough. The code grew past what he could hold in his head. Bugs he could not fix, he worked around, or attacked with random changes until they went away.

Then the sentence almost nobody quotes: “It’s not too bad for throwaway weekend projects”.

The original definition had three parts. One: you do not read the code. Two: the project is disposable. Three: you talk instead of typing. Later usage kept the first part as a vague vibe, dropped the second, and forgot the third. Hold on to the second one. It is the part that keeps you out of trouble.

Nine months from a tweet to the Word of the Year

Look closely at what Collins actually printed. Its definition is “the use of artificial intelligence prompted by natural language to write computer code” — and that is the whole thing. No mention of reviewing. No mention of scope. The press copy that summarised Merriam-Webster’s new entry lands in the same place: vibe coding “describes using an AI system to generate computer code” (CP24, with Associated Press files, 19 Sep 2026).

So the dictionary meaning is now: any coding with AI. The original meaning was: coding you do not look at. Those are not the same thing, and one of them is dangerous in places the other is not. That gap is why a second word had to be invented.

Simon Willison drew the line the dictionaries erased

In March 2025, Simon Willison published the post that most professionals still cite. His definition is narrow on purpose: vibe coding is “building software with an LLM without reviewing the code it writes”. His rule for himself is the useful part — he does not commit code to his repository if he could not explain exactly what it does to another person. Review it, test it, understand it, and you are not vibe coding any more. You are doing software development.

He was not sneering at it. The same post argues vibe coding is genuinely good: it is how beginners build intuition, and how anyone automates a tedious task. His conditions are the ones to memorise — keep it low-stakes, and watch what you expose: secrets, private data, and APIs that bill you per call.

Then the man who named it renamed the serious half

Something changed at the end of 2025. Karpathy’s own account of it is unusually specific: in November he was at roughly 80% manual and autocomplete coding with 20% agents; by December it had flipped to 80% agent coding, 20% edits and touch-ups. He also wrote, on 26 December 2025, that he had never felt this much behind as a programmer — a sentence worth remembering the next time you feel lost in this course.

On 4 February 2026, a year and two days after the original, he posted a retrospective. Vibe coding, he said, made sense when models were weak enough that the whole thing was for fun. By 2026, programming through agents was becoming the default professional workflow, with real oversight and no trade against software quality. He needed a different name for that, and his current favourite was “agentic engineering”. His reasoning is two lines long and worth copying down: “agentic” because “you are not writing the code directly 99% of the time”, and “engineering” because there is “an art & science and expertise to it” — meaning it can be taught, which is the premise of this entire masterclass.

In April 2026, at Sequoia’s AI Ascent, he gave the framing this course is built on. In his own written summary of that conversation: “Vibe coding raises the floor.” and “Agentic engineering raises the ceiling.” The floor is that anyone can now make software exist. The ceiling is how good that software can get without a proportional drop in quality, security and maintainability. Agentic engineers, in his description, design the specs, supervise the plans and inspect the diffs.

And his sentence to take out of the room with you: “You can outsource your thinking, but you can’t outsource your understanding.”

Where your work sits on the spectrum

Here is the payload. Three modes, and the question that decides which one you are in.

The tool does not decide your row. Claude Code will happily run all three. You decide, usually without noticing — which is exactly why it is worth naming out loud before you start.

Two questions settle almost every case. First: did anyone read this? Second: what happens if it is wrong — does a weekend project break, or does a stranger read somebody’s phone number? Those two answers put you in a row. Note that the boundary is genuinely blurring at the top: in May 2026 Willison admitted that as agents got more reliable he stopped reviewing every line even for production work, and called that drift closer than he would like. If it happens to him, it will happen to you. Name your row on purpose.

Before you write any code, answer these four questions in plain text, then stop.

1. In one sentence, what exactly am I asking you to build?
2. If this code is wrong, what breaks and who is affected? Be concrete.
3. Which mode does this task call for:
   vibe coding (nobody reviews the output),
   AI-assisted (I read every diff before it lands),
   or agentic engineering (spec + success criteria + tests before you start)?
4. What do you need from me before you begin?

Do not write code yet. Wait for my answer.

Paste that in before your next build. It takes thirty seconds, it costs almost nothing, and it is the difference between choosing a mode and drifting into one.