An agent is only as good as the system engineering around it.

Anthropic published this postmortem after users experienced a noticeable drop in Claude Code quality. That’s the important setup: the issue was real, but the root cause mostly wasn’t the base model. Three levels shape the outcome: Model — the base intelligence Context — what the agent remembers, what gets dropped, and how state carries across turns Harness — the prompts, tool wiring, defaults, and guardrails around the model In this case, the quality drop mostly came from layers 2 and 3: less re