Getting it into your agent
This one installs as part of its plugin. Adding the marketplace and installing the plugin brings it with everything else the plugin ships.
/plugin marketplace add AppsVortex/arness/plugin install arn-codeWrote 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/appsvortex/arness/arn-code-plan)<a href="https://agentmods.dev/skills/appsvortex/arness/arn-code-plan"><img src="https://agentmods.dev/badge/skills/appsvortex/arness/arn-code-plan/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/appsvortex/arness/arn-code-plan"><img src="https://agentmods.dev/badge/skills/appsvortex/arness/arn-code-plan.svg" alt="Reviewed on agentmods" width="80" 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 | $0.00139 | $0.03716 |
| Opus 5.5 | $0.00056 | $0.01486 |
| Sonnet 5.5 | $0.00028 | $0.00743 |
| Haiku 4.5 | $0.00014 | $0.00372 |
Grade A, and why
arn-code-plan 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 1mo 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 — 290 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Arness Plan
Generate an implementation plan from a Arness specification by invoking the arn-code-feature-planner agent. The plan is written to disk as a PLAN_PREVIEW file, presented to the user for review, and iteratively refined based on feedback until approved. The approved plan then feeds into /arn-code-save-plan for structuring into phases, tasks, and reports.
Pipeline position:
arn-code-init -> arn-code-feature-spec / arn-code-bug-spec -> **arn-code-plan** -> arn-code-save-plan -> arn-code-review-plan -> arn-code-taskify -> arn-code-execute-plan
Prerequisites
If no ## Arness section exists in the project's CLAUDE.md, inform the user: "Arness is not configured for this project yet. Run /arn-planning to get started — it will set everything up automatically." Do not proceed without it.
Workflow
Step 1: Load Configuration
Read the project's CLAUDE.md and extract the ## Arness section to find:
- Plans directory — base path where project plans and PLAN_PREVIEW files are stored
- Specs directory — path to the directory containing specification files
- Code patterns — path to the directory containing stored pattern documentation
If ## Arness is not found, inform the user: "Arness is not configured for this project yet. Run /arn-planning to get started — it will set everything up automatically." Do not proceed.
Step 2: Find the Specification
The user may provide a spec name as an argument (e.g., "arness plan FEATURE_websocket-notifications" or "plan the spec websocket-notifications").
If an argument was provided:
- Look for
<specs-dir>/<argument>.md(exact match) - If not found, try
<specs-dir>/FEATURE_<argument>.mdand<specs-dir>/BUGFIX_<argument>.md - If not found, try matching files in
<specs-dir>/that contain the argument text in their filename - If still not found, list available specs and ask the user to choose
If no argument was provided:
- List all
.mdfiles in<specs-dir>/ - If only one exists, use it automatically
- If multiple exist, show the list sorted by modification date (most recent first) and ask the user to choose
- If none exist, inform the user: "No specifications found in
<specs-dir>/. Run/arn-code-feature-specor/arn-code-bug-specto create one first."
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.
- 1mo ago First seen · 290 lines · 139 tokens per session scan A 76180261b806
arn-code-plan is a skill published in the GitHub repository AppsVortex/arness (33 stars, last pushed 2mo ago), licensed MIT. It adds 139 tokens to every session and 3,716 once invoked, about $0.0006 per session on Opus 5.5. 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 2026-08-30.
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