Lesson 1: A small neural network, one mechanism at a time

Module 4 · Lesson 1 · About 60 minutes · Needs: Module 3 (the ticket split, bag-of-words counts, train versus validation); no model key needed

The situation

Dev forwards a training log. A contractor replaced BrightCart's ticket classifier with a neural network, and the log ends with forty lines of loss: nan. An earlier run scored 100 percent in the notebook, then sorted real tickets barely better than chance. Dev asks: "What is this thing doing, and how would we know when it is broken?"

This lesson builds a network small enough to check by hand on the 40 shared tickets, with one ticket traced through every step, so that you can read Dev's log.