CourseAI System Design: The Complete Course · Module 15: Communicating a Design · part 103 of 107
Part 103 · Module 15: Communicating a Design

Lesson 5: Debugging a design that got slower and dearer

33 min read·
Module 15 · Lesson 5 · About 45 minutes · Needs: Module 3 lessons 4 and 5 (cost per success, the first-token budget) and Module 10 lesson 6 (release bundles). No key and no model calls

The situation

Week 15, Monday 8 February 2027. Gareth, rehearsing the design review, hands you September's traces: "Ana says the bill jumped. Find why, find where, and tell me what a fix buys, in numbers." Module 13 lesson 2's drift alarm named the release; this lesson diagnoses it and prices the fix.

The running example is trace ask-1108: a Tallis Retail Group question at 15:34 UTC on 29 September, after prompt ask-v8 and config cfg-12 shipped as bundle b-639c719b (Module 10 lesson 6).

THE RUNNING EXAMPLE What ask-1108 read and wrote Tallis, 29 Sep 15:34 UTC TOKENS IN system 700 history 720 context, 12 chunks 1,920 3,419 in user 79 TOKENS OUT output 464 bundle b-639c719b: ask-v8 + cfg-12, small tier, 4,185 ms end to end
The running example read 3,419 tokens, 1,920 of them twelve retrieved chunks (amber: what the model reads and writes), and wrote 464 tokens in 4,185 ms.

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