Model choice cannot rescue a weak production system. "The model is interchangeable. The harness is not." LangChain's coding agent improved from 52.8% to 66.5% on Terminal Bench 2.0 without changing model parameters. The system around the model changed. A production harness controls context, execution, memory, and the boundaries between agents. Before an agent touches production: 1. Keep the root instruction file small. Load task-specific context only when needed. 2. Give each agent a narrow role and only the tools that role requires. 3. Store decisions, failures, and session state outside the model's context window. 4. Require separate steps for research, planning, execution, and verification. 5. Enforce spending limits, approved tool lists, test gates, rollback paths, and a complete action log. 6. Do not ship unless you can stop and reverse a bad action, then reconstruct it from the audit trail. Without this harness, each session forgets context and compounds the same failures.