Arena

---
name: arena
description: "Spawn N parallel candidates at the same task, pick a base, graft the strongest parts of the losers into it. Use for /arena, 'arena this', 'throw it in the arena', or when one attempt at a non-trivial artifact would lock in the wrong shape."
---

# Arena

Fan out N parallel attempts at the same task. Read every candidate end to end. Pick the strongest as the base. Graft the best ideas from the others into it. Verify the synthesized result.

**Dispatch contract.** Read [`provider-dispatch.md`](../poteto-mode/references/provider-dispatch.md) before fan-out. Configured values are provider-qualified descriptors, not host-native model slugs. The parent starts native and external lanes directly; children never route themselves. On Codex, resolve the remaining Claude tool names via [`codex-tools.md`](../poteto-mode/references/codex-tools.md).

## Start

Open a todolist with one entry per phase before launching anything.

1. Frame
2. Fan out
3. Cross-judge
4. Pick
5. Graft
6. Verify

## Phase A: Frame

The N candidates will receive the same prompt, so the prompt is the contract.

1. State the artifact each candidate is producing.
2. Derive the rubric. State what success looks like for *this* task, then turn it into 3-6 concrete gradeable criteria. The rubric is the picker's tool in Phase D. Candidates only see the task.
3. Pick the runners. Use `arena runners` from the current harness's pstack model sheet when present. Otherwise default to one each on `codex:gpt-5.6-sol@max`, `grok:grok-4.7@xhigh`, `claude:opus@max`. An `auto` or `inherit-parent` entry in this line or the cross-judge line uses the parent model through its native lane. Each seat follows provider-dispatch's saved-chain policy before every attempt. A terminal outcome the sheet's `# fallback` policy authorizes makes a read-only seat eligible for its next saved attempt (`usage-exhausted` only when no policy is saved); a writer advances only when the helper proves the workload never started or a recorded parent inspection clears it, and otherwise stops for inspection. Every other route failure is a dropout, not a substitution; record it and continue with the remaining seats. Spawn more when the arena covers multiple design directions. Same descriptor N times when the work is generation-bound rather than judgment-sensitive.
4. Assign output paths. Each candidate writes to its own location (a git worktree where possible, otherwise `/tmp/arena-<slug>/candidate-<n>/`), per the **separate-before-serializing-shared-state** principle skill.

## Phase B: Fan out

Start all N lanes in one fan-out phase through the provider-dispatch contract. Native lanes are background subagents. External lanes are direct background launcher processes with retained task/session handles, never foreground calls and never subagents supervising subprocesses. Give every lane the task, the path to the shared grounding, its own output path, and instructions to produce both the artifact and a short rationale.

Each rationale names the alternatives the candidate considered and what it rejected.

An external lane counts only when it satisfies [Completion and dropouts](../poteto-mode/references/provider-dispatch.md#completion-and-dropouts), including the assignment and model-argument checks for Codex, Devin, or Cursor `pinned-argv` evidence; a native lane counts when its tool transcript returns the assigned model's result. If the helper stops a candidate or its chain is exhausted, proceed with N-1 and note the exact dropout in the synthesis record. If it returns `inspect`, preserve the writer and inspect it before proceeding. A fallback that ran reports its actual descriptor; never describe it as the primary, and never replace a dropout with another provider silently.

## Phase C: Cross-judge

After every Phase B candidate chain is terminal, choose the judge descriptor from the `arena cross-judge pool` line in the current harness's pstack model sheet. If the sheet or that line is missing, choose from `codex:gpt-5.6-sol@max`, `grok:grok-4.7@xhigh`, `claude:opus@max`. Prefer a provider different from the parent and the likely base candidate. Dispatch one read-only judge through the provider contract. It sees the rubric and completed candidates by path label, scores each criterion, and recommends a base with rationale. It runs in parallel with the parent's reading in Phase D, not with the candidates themselves. Don't dispatch the judge while candidates are still writing.

## Phase D: Pick a base

Read every candidate end to end before picking.

Score each candidate against the rubric criterion by criterion, not on holistic feel. Compare against the cross-judge. Agreement on the base confirms the pick. Disagreement means one of you is biased or the rubric was ambiguous. Read both rationales before deciding.

Pick the base on which candidate a future maintainer can extend most easily without breaking invariants. Prefer the cleaner boundary or smaller API when two feel tied, per the Laziness Protocol.

Record the pick and the reason in a short synthesis note alongside the base artifact, including the cross-judge's verdict.

## Phase E: Graft

Walk each losing candidate once more and identify what is worth porting into the base. The signal is usually one or two things per candidate, not most of it.

Fold each graft in by hand, per the **redesign-from-first-principles** principle skill. Don't paste mechanically. The result has to remain coherent under one mental model.

Record what was grafted, from which candidate, and what was rejected and why.

When N candidates converge on the same shape, that is a strong agreement signal. Note the convergence in the record and ship the consensus shape. No graft is needed. When N candidates wildly diverge, Phase A was under-specified. Reframe and re-run rather than averaging the divergence.

## Phase F: Verify

The synthesized artifact has to hold up under the same scrutiny as any other output, per the **prove-it-works** principle skill.

If verification surfaces a problem the arena did not catch, either Phase A was wrong (re-frame and re-run) or one candidate caught it and you missed the graft (go back to Phase E). Don't paper over.

## Outputs

One synthesized artifact. One short synthesis note alongside, naming the base, the grafts (with source candidate), the rejections, the dropouts and any fallback attempts with their actual descriptors, and the verification result.