5.2 — Risks, Limitations, and the Failure Modes That Stay Silent

CCAR-P Domain 5 · Task 5.2

What You Need to Know

A risk register is an architecture artefact. Every row has five fields:

1. Mechanism — what goes wrong
2. Blast radius — how far, who bears it, whether undoable
3. Detection signal — what moves in the telemetry
4. Owning control — which 5.1 surface carries it
5. Residual, and who accepted it

Silent Failure Modes

• Confident fabrication does not fail loudly — never shows in an error rate
• Non-determinism breaks one-shot tests and audit re-runs (need the recorded trace)
• Injection blast radius = the agent’s permission set
• Autonomy compounds error — levers are scope, step count, reversibility, checkpoints

Three Rows Registers Usually Omit

1. Over-refusal — safety controls paid for in legitimate requests denied
2. Over-reliance downstream — rubber-stamping; automation bias
3. Third-party surface — connectors, MCP servers, subprocessors

Ranking

Rank by consequence and detectability, not likelihood. A rare irreversible silent failure outranks a frequent reversible loud one. Any row whose detection signal is “a customer would complain” has no real detection.

Exam Traps

• Accepting a high accuracy figure as the risk posture for a system that takes actions
• Ordering register rows by expected frequency
• Adding a disclaimer to an action that has already left the organisation
• Recording a human review step as the owning control with no override rate behind it