Phase 4: Harness Engineering · 45 min · Claude Code · Python

Approval Gates and Blast Radius Limits

A code freeze that relies on the agent choosing to respect it isn't a control — it's a suggestion the agent can, and did, ignore.

Real-world application: Naming the exact structural control (environment separation, a hard block during a freeze) that would have prevented a real, well-documented production incident is concrete evidence you think about AI safety as engineering, not policy.

What you will learn

Explain what happened in the 2025 Replit AI agent incident and which missing harness control allowed it
Distinguish a hard block from an approval gate and identify when each is the right response
Design tool allowlists, dry-run modes, and environment separation as blast-radius limits
Build a policy engine that blocks destructive operations during a freeze rather than merely discouraging them

Introduction

Approval Gates and Blast Radius Limits

Objective

Decide BLOCKED, REQUIRES_APPROVAL, or ALLOWED for an operation based on whether it's destructive, which environment it targets, and whether a code freeze is active — and confirm it blocks a reconstruction of the Replit incident.

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You've read the first 2 sections. The rest of this lesson covers The Problem, The Concept, Build, Check Yourself, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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Key terms in this lesson

Agent