How do I know the agent I shipped last week still behaves the same way today?

Model providers continuously update weights, swap quantization levels, and reroute traffic across hardware without changing the endpoint name or issuing a changelog. In April 2026, a wave of Claude Code quality complaints was traced to product-layer changes with no model version bump, no notification, and no way for builders to detect the regression without measuring outputs themselves. A product built on a specific model checkpoint can drift in refusal patterns, tool-call behavior, or output format weeks before anyone notices. No tooling exists to take a behavioral fingerprint of a deployed agent against a live endpoint and alert when that fingerprint shifts, even though this is exactly the regression test every software team runs on every library dependency upgrade.