Arena

A skill named in bold or in backticks, or run as /<name>, is the file ../<name>/SKILL.md beside this skill's directory, and a principle named without its prefix is ../principle-<name>/SKILL.md. Read the file and follow it. Most pstack skills are user-only, so your skill tool cannot load them. Every subagent is a Rex worker pane: spawn, wait and close it per ../rex-subagents/SKILL.md, with the defaults in the Subagents section of ../poteto-mode/SKILL.md.

Open a task list (your harness's task tool, or a checklist file at ~/.local/state/pstack/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. Use the arena runners line in ~/.config/pstack/pstack-models.md, one seat per entry, each passed to subagent spawn --entry as written. If the file or that line is missing, default to one claude seat and one codex seat at xhigh, each on its harness's default model. Say so and suggest /setup-pstack. An auto or inherit-parent entry in this line means your own harness on its default model. A family is the harness: claude or codex. If a configured entry's model is rejected, run that seat on its harness's default model and say so. If that fails too, drop the seat and say so. If dropped seats leave fewer than two runners, add runners until there are two, or one when the line has one entry. Run each added runner on a remaining runner's entry, or on your own harness when none remain, and say so. Spawn more runners when the arena covers multiple design directions. Use the same model N times when the work is generation-bound rather than judgment-sensitive. Pick each subagent's reasoning level and fast or standard speed for the job, per the Subagents section of the poteto-mode skill.
4. Give each candidate its own output location (a git worktree where possible, otherwise /tmp/arena-<slug>/candidate-<n>/), per the separate-before-serializing-shared-state principle skill. A candidate that writes code gets --worktree arena-<slug>-<n> --base <base commit>. Commit the base first, because uncommitted edits do not reach a new worktree.

Phase B: Fan out

Spawn all N candidates in one turn, one subagent spawn each named arena <slug> <n>, each with the task, the absolute path to the shared grounding, its own output path, and instructions to produce the artifact and a short rationale naming the alternatives it considered and what it rejected. End the turn. Each candidate's wake-up line starts your next turn, and subagent status shows each candidate's worktree.