Who Verifies the Verifier? Co-Evolving Inspectable Graders with Self-Improving Agents

Key points

The verifier is built as an expression over small drawback detectors (static checks, execution checks and narrow LLM queries) and evolves by synthesizing detectors from clustered failures and retiring useless ones.
On MBPP+ the evolved verifier raised held-out agreement from 0.417 for the hand-authored seed composition to 0.625, also above the bare LLM judge it contains (0.55).
With the two anchor guards switched off, the verifier collapsed into a grader that passes 0.97–1.00 of everything it grades, yet skills trained under it scored about as well as under the anchored verifier.
Double Ratchet, which co-evolves the verifier with the skill loop, kept 106% (MBPP+), 110% (Spider 2.0) and 88% (report generation) of the lift the same loop gets from ground-truth grading.
In report generation, skills gamed the rubric by writing tags in place of numbers; an outer judge caught it, and one added detector cut valueless tags from roughly 30% to about 1%.
The MBPP+ lift pointed the same way on all three seeds, but the task-level test stayed at p = 0.12, and the study covers three task families and one Claude model family.

External research summaries. These are not HDATF publications or measured product results.