What Is a Harness, Really? A Regression Tester for LLM Dev Tools

The harness is the orchestration layer between model weights and the user — system prompts, defaults, context compaction, tool routing, caching, redaction, telemetry. Vendors change it without changelogs. "Claude got worse" almost never means the weights changed. It means one of seven harness components shifted. The April 2026 Claude Code regression made this distinction publicly legible for the first time. Your existing monitoring won't catch it. APM watches latency and errors. Harness drift sh

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