Arena

Open a task plan with one entry per phase (Frame, Fan out, Cross-judge, Pick, Graft, Verify) before launching anything. By default, return one synthesized artifact. Comparison-only work returns a recommendation; production implementation follows only when requested. Preserve the caller's finish line.

Phase A: Frame

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

1. State the artifact each candidate is producing, the constraints it must preserve, and the evidence that would establish success.
2. Derive the rubric, 3-6 concrete gradeable criteria for what success looks like on this task. Share the requirements and criteria with the candidates and judge. Set them before seeing candidates.
3. Pick the runners through orchestration. Use its working selection for candidates and primary review selection for the cross-judge unless the caller specifies models. Honor a requested candidate count. Use at least two candidates for a comparison; add more when they test distinct directions. Independent attempts on the same model are useful too. Report any requested model diversity that could not be obtained.
4. Give each candidate its own output location: a task-owned worktree through the host's delegation tools for code changes, or a per-runner scratch directory for standalone artifacts. Follow the host's current workspace rules and orchestration's ownership and recovery rules. Give every candidate the same starting source. For isolated checkouts, confirm the starting revision and supply relevant uncommitted inputs explicitly rather than assuming they were copied.

Use prototype when a UI question needs observation on the target platform, and architecture experiments for technical assumptions. Keep product preferences distinct from facts a run can settle. Coverage partitions and races belong to orchestration; arena gives each runner the same artifact to produce.

Phase B: Fan out

Dispatch independent candidates through orchestration, each with the same task, criteria, 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. Keep candidates independent of each other's work. Run in parallel within capacity, or in waves with the same starting inputs.

If a candidate produces no usable output, note the dropout. Follow orchestration's bounded recovery. With fewer than two usable candidates, report the comparison as incomplete rather than claiming a winner.

Phase C: Cross-judge

After all Phase B candidates stop writing, spawn one read-only judge through orchestration. It sees the rubric and the candidates by path label, scores each criterion, and recommends a base with rationale. It runs in parallel with your reading in Phase D. Keep the candidates fixed while judging them.

The judge assesses evidence, not model reputation. An unavailable required judge is a coverage gap; retain the candidates and report it. Model settings and recovery come from orchestration rather than an arena-specific panel.

Phase D: Pick a base

Read every candidate end to end before picking. Score each against the rubric criterion by criterion, not on holistic feel, and compare with the cross-judge. Agreement supports the pick. Disagreement may indicate bias or an ambiguous rubric. Read both rationales and check the evidence before deciding; if the rubric needs clarification, apply it to every candidate before choosing.

Pick the base a future maintainer can extend most easily without breaking invariants. When two feel tied, prefer the cleaner interface or smaller public surface, per Laziness Protocol. Judge UI behavior on the target surface and measured claims on comparable workloads. A score does not replace the required proof.

For a comparison-only request, return the candidates, evidence, tradeoffs, and recommendation here. Preserve an explicitly requested user choice or checkpoint before synthesis.

Phase E: Graft (synthesis only)

Walk each losing candidate once more for what is worth porting into the base, usually one or two things per candidate, not most of it. Fold each graft in by hand, per Redesign From First Principles, so the result stays coherent under one mental model. Don't paste mechanically.

If the candidates converge on one shape, note the agreement and use the consensus shape. No graft is needed. If they answer incompatible interpretations of the brief, reframe and re-run rather than averaging the divergence. Preserve the invariants and accepted tradeoffs of the chosen base as you graft.

Phase F: Verify (synthesis only)

Verify the synthesized artifact like any other output, per Prove It Works. Run the checks the combined result needs; passing candidates do not establish that their combination works. A design sketch supports only the claims its caller checks or experiments exercised. Mark untested runtime claims explicitly.

If verification finds a problem the arena missed, revisit the frame or the graft that introduced it. Don't paper over it. For production changes, use verify for the real workflow and review-fix-loop for the complete integrated change, or let pr own that review when publishing the same scope.

Outputs

Comparison-only work returns the candidates, evidence, tradeoffs, and recommendation, including any dropouts.

Synthesis returns one synthesized artifact, with a short synthesis note alongside naming the base and why, the cross-judge's verdict, each graft with its source candidate, what was rejected and why, any convergence or dropouts, and the verification result. Include run instructions and remaining evidence gaps for either finish line. Retain the candidates until the decision and decisive evidence are recorded; clean up task-owned disposable work only when it is no longer needed.