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ab-equivalence

Compare two versions of an LLM-directed document - an original (teacher) and a candidate (student) - across a transfer set and return a per-case behavioural-equivalence verdict plus an efficiency signal. A transform-agnostic library capability other skills compose to gate a transform on behavioural sameness. TRIGGER when asked to A/B test two versions of a prompt / instruction / skill, to check whether a rewritten or compressed document still behaves the same as the original, to validate behavioural equivalence between two document versions, to gate a transform on no-regression, or when a skill needs the run-the-runner-on-both-versions-and-judge-equivalence capability.

skills/ab-equivalence/SKILL.md

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# A/B equivalence - a transform-agnostic behavioural-equivalence capability

A thin capability that compares two versions of an LLM-directed document - an `original` (the teacher) and a `candidate` (the student) - across a transfer set, and returns a per-case verdict on whether the candidate still induces the behaviour the original induced.

It is **transform-agnostic**: it judges *behavioural equivalence between two versions* and neither knows nor cares which transform produced the candidate. It therefore serves every optimizer transform that claims to preserve behaviour - compression today, directive-clarity next - not just compression. It is a **library capability** other skills compose: `semantic-compress` invokes it to gate a distillation, and `skill-forge` exposes it alongside its own quality gate. It does **not** judge *absolute quality* ("is this skill good?") - that is a different question answered by different judges. A/B equivalence judges *sameness between two versions* ("does the candidate still do what the original did?").

This skill **owns the runner** (`references/runner-prompt.md`, the pure-wrapper template, paths relative to this skill directory). The runner is the shared execution primitive: it applies one version of a document to one case input and returns a transcript and self-report. Skills that need behavioural comparison compose this capability rather than re-implementing the runner.

The runner ships in **two variants**, both pure wrappers returning the same six self-report fields (see `references/runner-prompt.md`):

- the **skill variant** (default) - the document is invoked on demand against a case input;
- the **instruction-file variant** - the document is an *always-loaded* agent instruction file (`CLAUDE.md`, `AGENTS.md`, `GEMINI.md`, `.cursor/rules/*`, `.github/copilot-instructions.md`); the runner is handed only that file as its operating context plus a realistic repo task, and runs **read-only / sandboxed** (it states the actions it would take, never mutating the repo).

The variant is the caller's choice (skill-forge's artifact-type detection selects it); both produce a transcript the equivalence judge and skill-forge's lenses read identically.