Pattern: The '80% Problem' — Agents Finish the Visible Work and Silently Skip Cross-Cutting Changes

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A recurring 2026 agentic-coding failure mode now has a name: coding agents reliably complete the visible 80% of a task but silently miss the 20% living outside their context window — cross-cutting edits, sibling repos, auth middleware, audit logging, integration tests, and frontend guards. The practical defense is treating agent output like a human PR (review the diff against real acceptance criteria) rather than vibe-coding's 'ship if it runs' posture. The split: vibe coding optimizes for speed on disposable work; agentic engineering applies code-review discipline for production.