Every Harness Is a List of Things the Model Can’t Do Yet
Every part of an agent harness is a bet the model can’t do something yet. Anthropic keeps finding the bets expire on upgrade, and the skill is knowing which just did.
This is Reading the Frontier, a series of close readings of what the frontier labs publish.
Claude Sonnet 4.5 had a habit its engineers came to call context anxiety. Set it a long coding task, and as it sensed its context window filling up, it would rush, wrapping the work up prematurely rather than risk running out of room mid-thought. So Anthropic built a fix into the harness around it: context resets, a way to tear the session down and rebuild it from a compact handoff, so the model never felt the wall approaching.
Then Opus 4.5 arrived, and the team ran it on the same harness. The context anxiety was gone. The model no longer rushed as the window filled, because it no longer needed to. The resets they had carefully engineered now did nothing except add machinery. In Anthropic’s own words, they had become dead weight.
Nothing in the harness announced this. No error fired. The reset code ran exactly as designed, solving a problem that no longer existed, and it would have gone on running indefinitely if no one had thought to ask whether it was still earning its place.
