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Phase 17 — Capstone: An Enterprise Agentic Platform, End to End

Answers these JD lines: all of them at once — this is the Staff/Principal system-design interview. It composes the whole track into the "secure, multi-tenant, durable, evaluated, observed agent platform" that Citi, Docker, Cohere, and Wolters Kluwer are hiring someone to own.

Why this phase exists

You've built seventeen mechanisms in isolation. Real value is in composition — knowing the order of the layers, the trust boundaries between them, and how each one's guarantee stacks into a platform. This capstone makes you wire them into one request lifecycle and defend every placement, which is exactly what the senior interview probes: not "can you build a ReAct loop" but "can you architect and reason about the whole system."

Concept map — the request lifecycle

Durability (08) and sandboxing (09) wrap the runtime; MCP (03) is the tool transport; the reliability and cost math (00) governs the whole thing.

The lab

Deliverables checklist

• Lab 01 green under LAB_MODULE=solution pytest and your own lab.py.
• You can draw the lifecycle and name each layer's phase from memory.
• You can explain why each layer sits where it does (order matters).
• You can point to where reliability, security, cost, eval, tenancy, and observability live.

Key takeaways

• A platform is layers composed in the right order with explicit trust boundaries — not a pile of features.
• The five load-bearing truths (trust boundary, reliability compounds, injection is architectural, cost/latency are engineered, evaluate-or-don't-ship) all show up in one lifecycle.
• The senior interview is about this — the whole system and its tradeoffs. Use system-design/ and interview-prep/ to rehearse it.