Borrowing it
Nothing to install: this file belongs to tellahq/opensession. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/tellahq/opensession/main/.agents/skills/pstack-suite/skills/arena/SKILL.mdgit clone --depth 1 https://github.com/tellahq/opensessionWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/tellahq/opensession/arena)<a href="https://agentmods.dev/skills/tellahq/opensession/arena"><img src="https://agentmods.dev/badge/skills/tellahq/opensession/arena.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00060 | $0.01111 |
| Opus 5 | $0.00030 | $0.00556 |
| Sonnet 5 | $0.00012 | $0.00222 |
| Haiku 4.5 | $0.00006 | $0.00111 |
Grade A, and why
arena scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 3d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 72 lines — stays where its author put it; the contents beside it link to each section on GitHub.
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.
Start
Keep a checklist with one entry per phase before launching anything. The arena runs autonomously and the list keeps phases from silently disappearing.
- Frame
- Fan out
- Cross-judge
- Pick
- Graft
- Verify
Phase A: Frame
The N candidates will receive the same prompt, so the prompt is the contract. Get it right before spawning anything.
- State the artifact each candidate is producing.
- Derive the rubric. State what success looks like for this task, then turn it into 3-6 concrete gradeable criteria. Concrete:
Adds a --dry-run flag that skips writes. Vague:code is correct. The rubric is the picker's tool in Phase D; candidates only see the task. - Pick the runners. Use the current session or workspace model preset by default. Pass explicit models only when the user or preset provides valid configured ids. Spawn more when the arena covers multiple design directions. The same model N times is appropriate when the work is generation-bound rather than judgment-sensitive.
- Assign output paths. Each candidate writes to its own location (a git worktree where possible, otherwise
/tmp/arena-<slug>/candidate-<n>/). N candidates writing to the same path is shared mutable state and fails the the separate-before-serializing-shared-state principle skill test.
Phase B: Fan out
Discover the policy-gated Open Session session tools. Spawn all N children in parallel with spawn_task. Use ask mode for read-only artifacts and code mode with separate isolated worktrees for writes. Begin each brief with /pstack and include the task, shared grounding, exclusive output path, verification, and instructions to produce both the artifact and a short rationale.
The rationale is mandatory. Without it, the parent cannot tell whether a candidate's structure is principled or accidental, which makes Phase E grafting unreliable. Each rationale names the alternatives the candidate considered and what it rejected.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 3d ago First seen · 72 lines · 60 tokens per session scan A 03d3a1c4f35e
arena is a skill published in the GitHub repository tellahq/opensession (355 stars, last pushed yesterday), licensed MIT. It adds 60 tokens to every session and 1,111 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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