Arena

Principles named in bold are in the principles skill: call the Skill tool with principles once, then read each named principle's file.

Open a task list (your harness's task tool, or a checklist file at ~/.local/state/ox/checklists/<repo>-<branch>.md when it has none) with one entry per phase (Frame, Fan out, Cross-judge, Pick, Graft, Verify) before launching anything.

Phase A: Frame

Every candidate gets the same prompt, so the prompt is the contract.

1. State the artifact each candidate is producing.
2. Derive the rubric, 3-6 concrete gradeable criteria for what success looks like on this task. It is the picker's tool in Phase D. Candidates only see the task.
3. Pick the runners from the panel line of ~/.config/ox/models.md, one candidate per entry, passing each entry's model and effort to your subagent tool. If the file or the line is missing, or the file's # harness line names a different harness, run three candidates on your own model, say so, and suggest the user run /setup-ox models: the candidates then differ by sampling, not by model. If the tool rejects an entry, run that seat on the closest model it lists and say so. Add candidates when the arena covers several design directions. Use the same model several times when the work is generation-bound rather than judgment-sensitive.
4. Give each candidate its own output location, per Separate Before Serializing Shared State. A candidate that writes code gets its own git worktree: commit the base first, because uncommitted edits do not reach a new worktree, then run git worktree add <path> -b arena-<slug>-<n> <base commit>. Any other candidate writes to its own directory, arena-<slug>/candidate-<n>/ under the system temp directory.

Phase B: Fan out

Start all N candidates at once. Each brief carries the task, the absolute path to the shared grounding, the candidate's own output path as the only place it may write, and instructions to produce the artifact and a short rationale naming the alternatives it considered and what it rejected. Wait for every candidate to finish.