CourseAI System Design: The Complete Course · Module 9: Agentic Architecture · part 62 of 107
Part 62 · Module 9: Agentic Architecture

Lesson 2: Designing the tool surface the agent sees

34 min read·
Module 9 · Lesson 2 · About 60 minutes · Needs: Module 3 lesson 2 (tokens per call), Module 9 lesson 1 (the bounded agent). One optional hosted call; everything else runs with no key

The situation

Week 9, Monday 28 December 2026. Lesson 1 called the tool surface the agent's main design document; Gareth now asks what goes in it. Tallis's systems expose 22 API endpoints. Funmi wants to wrap each one as a tool. Ana asks what each tool costs per call; Leila asks what a result may carry into the model.

The running example is Cases task C007: Tallis requester t-tallis-u014 writes "My laptop screen is cracked, please arrange a replacement." The expected tools are hris.get_employee, assets.get_device and tickets.create.

One way in The tool surface Active flow 1 Processing step number Request C007 request cracked laptop Cases agent Cases agent model decides what to call Tool surface tools Northdesk designs Tallis systems Tallis HRIS run by the tenant Tallis Assets run by the tenant Tallis Tickets run by the tenant Tallis Access run by the tenant 1 2 3
C007 (green, untrusted text) reaches Tallis's four systems (grey, run by the tenant) only through the tool surface Northdesk designs (teal).

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