CourseAI System Design: The Complete Course · Module 4: Architecture Selection · part 29 of 107
Part 29 · Module 4: Architecture Selection

Lesson 1: The architecture ladder, one rung at a time

32 min read·
Module 4 · Lesson 1 · About 50 minutes · Needs: Module 3 (token and cost arithmetic); Module 2 lesson 1's deterministic baseline helps

The situation

Week 4, Monday 23 November 2026. Joel wants Ask rebuilt "as an agent, like Cases". Helen wants the simplest design that meets the bar. Gareth sets the test: "Show me every rung below the agent, what each costs, and what each one gets wrong."

You trace one question up the ladder. A Tallis Retail Group employee (t-tallis-u035, golden question G028) asks "How many days of annual leave do I get?". The 2026 policy (t-tallis-d002) says 30. The 2025 policy it replaced (t-tallis-d001) still says 29.

THE LADDER Eight rungs for one question RUNG MODEL CALLS PER REQUEST 1 deterministic code none 2 single call 1 3 call plus retrieval 1 4 prompt chain 2 5 workflow with routing 2 6 bounded agent 4 7 multi-agent 5 8 fine-tuned step 1 Up the ladder: more calls, more ways to fail
The ladder for Ask's leave question: deterministic code (blue) at the bottom, model rungs (amber) above, agents and fine-tuning (grey) at the top; each rung up adds calls and new ways to fail.

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