Conciseness

Every sentence is operational decision logic (model fallback chains, path conventions, tie-break rules) — nothing explains concepts Claude already knows, and it assumes competence (never defines 'worktree' or 'graft'). At ~49 dense lines for a six-phase orchestration protocol, every token earns its place; the 4 anchor's 'minor instances of over-explanation' do not appear.

Actionability

Guidance is highly concrete ("run_in_background: true", "/tmp/arena-<slug>/candidate-<n>/", specific model slugs, todolist-first instruction) but has minor gaps: N is never given a default, and "the path to the shared grounding" in Phase B refers to an artifact no phase creates or defines. This fits the 4 anchor ('concrete guidance with minor gaps') better than 5, and clearly above 3 since nothing is pseudocode or hand-wavy.

Workflow Clarity

Phases A–F are explicitly sequenced with a required upfront todolist, and validation is built in: Phase F verifies with feedback loops ("If verification finds a problem... go back to Phase E" / "re-frame and re-run"), plus dropout handling, tool-rejection fallbacks, and cross-judge disagreement resolution. This matches the 5 anchor's explicit validation steps and error-recovery loops.

Progressive Disclosure

The skill is under 50 lines, needs no external reference files, and is cleanly organized into one section per phase plus an Outputs section — squarely the rubric's simple-skill exception for a 5. References to sibling principle skills (prove-it-works, redesign-from-first-principles) are clearly signaled by name, not buried.

Specificity

The description lists multiple concrete actions covering the full pipeline — "Spawn N parallel candidates at the same task, pick a base, graft the strongest parts of the losers into it" — with no vague filler. All three verbs are specific and jointly comprehensive for this workflow, matching the top anchor rather than the 'minor gaps' level below.

Completeness

Both parts are explicit: 'what' is "Spawn N parallel candidates... pick a base, graft the strongest parts" and 'when' is the full "Use for /arena, 'arena this', 'throw it in the arena', or when..." clause with concrete trigger phrases. This matches the 5 anchor exactly; the 4 anchor ('when' could be more explicit) does not apply.

Trigger Term Quality

Triggers are the natural phrases a user would say: "/arena", "'arena this'", "'throw it in the arena'", plus the scenario "when one attempt at a non-trivial artifact would lock in the wrong shape". Coverage includes the command, colloquial synonyms, and a model-side trigger condition; nothing common is missing.

Distinctiveness Conflict Risk

"arena" is a clear niche with distinct invocation phrases; no other common skill (review, refactor, test) would fire on these triggers. The scenario trigger is scoped to multi-candidate generation, keeping conflict risk minimal.