The Harness, As Feynman Might Have Taught It
An imagined Feynman lecture on the dumbest, most important program in AI: the loop around the model. Play the harness by hand, take one apart, then build a real one on a single page of Python.
How It All Connects: The Whole Machine on One Page
Fifteen parts, one machine. Follow one customer question from Enter to cited answer, then watch where the eight billion weights came from.
Agents: An LLM in a Loop
An agent is an LLM in a loop: call a tool, read the result, decide, repeat. Watch one fix a bug, then see why long runs break, overspend and need guardrails.
MCP: One Plug for Every Tool
Every AI app kept rewriting the same integrations. MCP turns M × N of them into M + N. Count the savings, watch a session on the wire, catch a poisoned tool.
Tool Calling: How a Text Machine Pushes Buttons
A model can only write text. Watch it ask for the weather in JSON, see who really runs the tool, and find out where the security has to live.
RAG: Giving a Model an Open-Book Exam
A model has never read your refund policy. RAG looks it up and pastes it into the prompt. Follow one question through search, chunks, citations and failure.
Embeddings & Vector Databases: Searching by Meaning
Give every text coordinates so similar meanings sit close together, and search becomes geometry. Measure fruit, search by meaning, watch an index hop.
Quantization: Squeezing a Model Into Fewer Bits
Round almost all of a model's billions of weights to one of 16 levels and it still works. Round some yourself, then find out what fits on your machine.
LoRA: Steering Billions of Weights by Training a Few Million
LoRA fine-tunes a giant model by training two skinny matrices per weight matrix. Count the 0.39%, watch an adapter train, and fit a run on one GPU.
Reinforcement Learning & RLHF: Learning From a Score, Not an Answer
A model can learn without ever seeing the right answer, only a score. Train one, rate answers yourself, then watch a loose leash find the loophole.
Fine-tuning: Teaching a Know-It-All to Be Helpful
A base model knows Paris but answers with another quiz question. Fine-tuning fixes it with the same loop on new data. Mask the loss, then watch a model forget.
Pre-training: Learning Everything by Guessing the Next Word
Every LLM starts as random numbers, then guesses the next token trillions of times. Bet on the next token, train a tiny model live, and see why scale pays.
Transformers & Attention: How Words Look at Each Other
Watch “bank” pick a meaning, then build attention by hand: queries, keys, values, softmax, and why long contexts cost n² compute and gigabytes.
Weights, Parameters & Inference: A Model Is a Pile of Numbers
An LLM is billions of learned numbers. Fit a house price with one, build a neuron, make it bend, count to 8 billion, and see why answers trickle out.
Tokens: The Alphabet a Model Actually Reads
Models never see letters, only numbered chunks of text. Train a real tokenizer in your browser and see why counting the r's in strawberry is so hard.
What an LLM Actually Does (and Why It Remembers Nothing)
An LLM guesses the next token, appends it, and guesses again. Roll its dice, then see why it remembers nothing and why long chats cost so much.
A Field Guide to Today's AI Models
A walk through the major families of AI models — what they do, how they work, and what it takes to train them — for people who want intuition, not a textbook.