Lesson 1: Which custom work belongs in the product
18 min read·13 Oct 2026
Module 13 · Lesson 1 · About 40 minutes · Needs: Module 3 (the hidden rules), Module 4 (matching TMS and CRM customers), the St Mary's incident from Module 5; basic Python
The situation
Tuesday 26 May 2026, week 11. The engagement ends on Friday 5 June. Ines Varga, Larkfield Assist's product manager, is planning the next quarter and asks every forward deployed engineer one question: "What did you build on site that the product should have done for you, and what should it never do?"
Over eleven weeks you built a lot for Harbourline: a working-day count with bank holidays, the hidden rules in config, an outbound redaction gate, the St Mary's same-day rule, a release gate. Some of it is Larkfield Assist's next feature. Some of it must never leave Harbourline. This lesson sorts it, with evidence.