What is vendor-prompting?

A reference guide for writing and auditing prompts for Anthropic Claude and OpenAI GPT-5.6 agents. It connects each recommendation to a rule, source, and validator check.

Use it to create or review .agent.md and .prompt.md files, investigate prompt-lint findings, and choose a suitable model family.

It helps you apply vendor guidance consistently and trace why a prompt was flagged. It also reduces guesswork when choosing patterns for agent files.

vendor-prompting — Skill for Claude Code and Codex by jonathan-vella

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add jonathan-vella/apex-accelerator --skill vendor-prompting

Clone the repo

git clone --depth 1 https://github.com/jonathan-vella/apex-accelerator

Made for: Claude Code, Codex.

Wrote 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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/jonathan-vella/apex-accelerator/vendor-prompting/github.svg)](https://agentmods.dev/skills/jonathan-vella/apex-accelerator/vendor-prompting)

Your own site

<a href="https://agentmods.dev/skills/jonathan-vella/apex-accelerator/vendor-prompting"><img src="https://agentmods.dev/badge/skills/jonathan-vella/apex-accelerator/vendor-prompting/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/jonathan-vella/apex-accelerator/vendor-prompting"><img src="https://agentmods.dev/badge/skills/jonathan-vella/apex-accelerator/vendor-prompting.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 94 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 2,239 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

NVIDIA SkillSpector pass 7 Sept 2026

How audits are shown

Origin original No closer match found in the catalogue.

Token cost

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	$0.00094	$0.02239
Opus 5	$0.00047	$0.01120
Sonnet 5	$0.00019	$0.00448
Haiku 4.5	$0.00009	$0.00224

Measured 8d ago against content hash 22ccd8016b37, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-12, from the pricing page.

Security

Grade A, and why

vendor-prompting 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 8d 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.

.github/skills/vendor-prompting/SKILL.md · 151 lines

How it starts

The opening of the file, as written. The whole thing — 151 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Vendor Prompting Best Practices

Audit-grade reference for the prompting patterns published by Anthropic (Claude family) and OpenAI (GPT-5.6-Terra family). Used to author and audit .agent.md and .prompt.md files in this repository.

The machine-readable source of truth is rules.json — every rule has an ID, source citation, severity, applies-to, and validator-check binding. The skill prose, the thin enforcement instruction vendor-prompting.instructions.md, and validate-agents.mjs all reference rule IDs from that file.

When to Use This Skill

Authoring a new .agent.md or .prompt.md and wanting the right vendor patterns up front.
Auditing an existing agent against vendor best practices (the audit procedure is in audit-procedure.md).
Investigating a finding from npm run lint:vendor-prompting — every finding includes a ruleId that maps to a rule in rules.json and back to a reference here.
Choosing the right model family for a new agent (decision rules in family-support.md).

Do NOT load this skill for routine edits where the format is already known. The thin instruction vendor-prompting.instructions.md auto-loads on *.agent.md / *.prompt.md edits and carries the hard-rule shortlist.

Decision Tree

I am editing or reviewing a *.agent.md / *.prompt.md ...
├── Which model is in the frontmatter?
│   ├── Claude Opus / Claude Sonnet → load references/claude-best-practices.md
│   ├── Claude Haiku                → load references/claude-best-practices.md (warn-only)
│   ├── GPT-5.6-Terra               → load references/gpt-5-prompting.md
│   ├── GPT-5.4                     → load references/gpt-5-prompting.md (shared OpenAI cohort)
│   ├── GPT-Codex / GPT-4o          → reviewer-only; minimal automated rules
│   └── Unknown / missing           → ERROR: force explicit model: in frontmatter
│
├── Is this a .prompt.md (single string model:) or .agent.md (array)?
│   ├── prompt → load references/checklists.md "prompt" column
│   └── agent  → load references/checklists.md "agent" column
│
└── Want the full audit procedure (5-15 min, produces written report)?
    → load references/audit-procedure.md and assets/audit-template.md

Read the full file on GitHub · 151 lines

Files

What ships with it

13 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

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.

8d ago First seen · 151 lines · 94 tokens per session scan A 22ccd8016b37

Subscribe to this mod's changes

vendor-prompting is a skill published in the GitHub repository jonathan-vella/apex-accelerator (50 stars, last pushed 5d ago), licensed MIT. It adds 94 tokens to every session and 2,239 once invoked, about $0.0005 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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vendor-prompting

Getting it into your agent

Wrote this? Show the measurements

What it costs to keep this loaded

Grade A, and why

Nothing flagged

How it starts

Vendor Prompting Best Practices

When to Use This Skill

Decision Tree

What ships with it

What this file has done since we first saw it

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