Lesson 2: Changing the architecture without stopping it
32 min read·
Module 14 · Lesson 2 · About 50 minutes · Needs: Module 3 lesson 3 (re-embedding cost), Module 6 lessons 1 and 3 (embedding choice, the score floor, the alias swap), Module 7 lesson 4 (model change management). No key and no model calls; two embedding models download from Hugging Face
The situation
Week 14, Monday 1 February 2027. Gareth asks: "Which of today's choices will you regret in a year, and what does undoing each one cost?" Helen wants one of them undone now, as routine work: the embedding model. MiniLM reads 256 word pieces, so Module 6 capped chunks at 180 words, while real documents average 6,000 tokens.
The candidate is BAAI/bge-base-en-v1.5 (MIT licence, read on the Hugging Face Hub on 19 October 2026): 768 dimensions, 512 word pieces. The running example is G028: t-tallis-u035 asks "How many days of annual leave do I get?"; the answer is 30 days, in t-tallis-d002.