PRIOR: PRECEDENT RECORDS INVALIDATE OPERATIVE RULES

An agent's past decisions can weaken its adherence to an explicit policy that remains in context. We call this phenomenon precedent erosion. We introduce Ledger, a controlled suite of binary governance decisions with synthetic histories, and measure actions, stated-policy identification and reports of the operative policy. Length-matched controls, sham subtraction and five validity checks address measurement confounds. In the primary low-risk setting, one permissive record after five compliant records yields an action-level sham-subtracted double difference of $+0.75$ in Qwen3-4B, while the recovery criterion is first met after five corrective records. Qwen3-14B shows no positive erosion under the same conditions. Violations coexist with correct identification of the stated policy: recognising a rule does not ensure following it. In Qwen3-4B, late-layer precedent shifts align with the activation direction defined by removing the policy sentence. Injecting this direction induces or reverses violations, whereas norm-matched controls do not produce these flips at the tested dose. Controlled post-training experiments on Qwen3-4B-Base produce substantial erosion after response distillation from Qwen3-14B, while token-matched SFT, preference optimisation and self-distillation controls show little or none. Together, these results characterise precedent erosion, establish a causal handle and show that a teacher's resistance to precedent does not guarantee a resistant student.