Focused but Fragile: Diagnosing Tail-Rule Loss under Recursive Agent Skill Compression

Compression can preserve an agent skill's routine while omitting the rule needed for an exception. We study this failure mode in an artifact-only maintenance regime, where each rewrite sees only its predecessor and cannot revisit the source evidence used to create the skill. We introduce TailSkills, a controlled benchmark of 208 task variants derived from 54 base tasks and 14 exception types. Each variant pairs an ordinary environment with an exception-triggering environment and is admitted only after reference-oracle execution and deterministic verification. The benchmark reveals a selective durability gap. Among the 56 variants with detectable registered markers, unit-weighted S4 retention is 48.7% for regular workflow units but only 15.7% for tail recovery units. A separate high-precision analysis over all 208 variants reproduces the artifact-level separation across four pipelines: at S4, regular retention ranges from 31.2% to 43.9%, whereas tail retention ranges from 2.1% to 15.2%. On a fixed set of 52 tail variants, S4 success is lower than S1 in three of four tested pipelines, with endpoint drops of 3.8–7.7 percentage points; the remaining pipeline follows a non-monotone trajectory and ends at its S1 level. Paired execution audits further show that artifact loss can become task failure in some executor–task settings, but does not produce a uniform downstream drop. These findings indicate that ordinary-task success is insufficient to assess the durability of compressed procedural skills: recovery rules require direct evaluation.