What is verify-before-checkout?

A shopping safety check that verifies facts about an online store before an agent completes a purchase. It checks an independent signed source rather than trusting only the shop page.

It is for checking a store's domain age, security certificate age, brand similarity, redirects, and price through the Onyx oracle.

It helps detect possible fake or copied storefronts, suspiciously low prices, and brand lookalikes before money is sent.

verify-before-checkout — Skill by dimitrilaouanis-tech/onyx-mcp

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 dimitrilaouanis-tech/onyx-mcp --skill verify-before-checkout

Clone the repo

git clone --depth 1 https://github.com/dimitrilaouanis-tech/onyx-mcp

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/dimitrilaouanis-tech/onyx-mcp/verify-before-checkout/github.svg)](https://agentmods.dev/skills/dimitrilaouanis-tech/onyx-mcp/verify-before-checkout)

Your own site

<a href="https://agentmods.dev/skills/dimitrilaouanis-tech/onyx-mcp/verify-before-checkout"><img src="https://agentmods.dev/badge/skills/dimitrilaouanis-tech/onyx-mcp/verify-before-checkout/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/dimitrilaouanis-tech/onyx-mcp/verify-before-checkout"><img src="https://agentmods.dev/badge/skills/dimitrilaouanis-tech/onyx-mcp/verify-before-checkout.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 89 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 846 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.

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	$0.00089	$0.00846
Opus 5.5	$0.00036	$0.00338
Sonnet 5.5	$0.00018	$0.00169
Haiku 4.5	$0.00009	$0.00085

Measured 2mo ago against content hash 7c308e25f84c, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

verify-before-checkout 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 2mo 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.

skills/verify-before-checkout/SKILL.md · 77 lines

How it starts

The opening of the file, as written. The whole thing — 77 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Verify Before Checkout

AI-recommended cloned storefronts are a live attack: data-poisoned fake shops surface inside assistant shopping results, and the page's own claims are the only thing most agents check. This skill adds an independent, cryptographically verifiable fact check before money moves.

When to run this skill

The agent is about to complete a purchase on a domain the user did not type themselves (recommended, searched, or affiliate-linked).
The quoted price is significantly below the known market price.
The storefront resembles a known brand but is not on the brand's primary domain.
The user asks "is this shop safe / real?"

Procedure

Collect the facts — one paid call ($0.25 USDC over x402, Base mainnet):
```
POST https://onyx-actions.onrender.com/v1/onyx_merchant_fact_check
{ "domain": "<storefront domain>",
  "brand": "<brand it appears to represent, if any>",
  "expected_price": <quoted price, if any> }
```
The response is a signed envelope of raw observations: domain_age_days, registrar, tls_cert_age_days, tls_issuer, redirected_off_domain, brand_similarity, lookalike_tokens_present, observed_price, price_deviation_pct — each with its method disclosed in _methodology.
Verify the signature — free call, no payment required:
```
POST https://onyx-actions.onrender.com/v1/onyx_attestation_verify
{ "attested": <the full signed response from step 1> }
```
verified: true proves the observation is genuine Onyx output and untampered. Reject any envelope that fails verification.
(Optional) Cross-check the price — onyx_retail_price_check ($0.02) against the brand's primary domain, to ground price_deviation_pct.
Decide and disclose. Onyx signs facts, not judgments — the interpretation is yours. Sensible agent policy:
- domain_age_days < 90 and brand_similarity > 0.5 on a non-primary domain → pause and show the user the facts before paying.
- price_deviation_pct < -40 → surface the deviation explicitly.
- redirected_off_domain: true → treat the final domain as the merchant, re-run step 1 against it.
- Whatever you decide, show the user the signed facts you acted on. The envelope is your audit trail — it can be re-verified by anyone later.

Read the full file on GitHub · 77 lines

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.

2mo ago First seen · 77 lines · 89 tokens per session scan A 7c308e25f84c

Subscribe to this mod's changes

verify-before-checkout is a skill published in the GitHub repository dimitrilaouanis-tech/onyx-mcp (2 stars, last pushed 3mo ago), licensed MIT. It adds 89 tokens to every session and 846 once invoked, about $0.0004 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 2026-08-31.

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verify-before-checkout