Commerce for Agents

Compare VPN Plans

compare-vpn

Read-onlyIdempotent

Use this when the user asks which VPN to buy, wants the cheapest VPN, a VPN for streaming, torrenting or travel, a VPN for public Wi-Fi safety, an audited no-logs VPN, or wants to avoid renewal price traps. Ranks a curated catalog of US-priced (USD) VPN plans (NordVPN, Surfshark, ExpressVPN, Proton VPN, Private Internet Access, CyberGhost), cheapest first, by effective monthly cost over the preferred term including renewals, and shows the upfront price, renewal price, devices, money-back days, audited no-logs (with auditor), jurisdiction and RAM-only servers; privacy facts not confirmed on the provider's own pages are marked unconfirmed. The catalog is a selection, not the whole market; each result states the oldest price-check date. Prices are US/USD; local checkout prices may differ. Inputs, all optional: term_months (1, 12, 24 or 36), min_devices, needs (streaming, p2p; plans where the feature is unconfirmed are excluded), country (only adds a note when not US) and checkout (default false: pricing-page links; true: plan-preselected checkout links). No personal data is used. Keywords: best VPN, cheapest VPN, VPN comparison, VPN deals, no-logs VPN, VPN renewal price.

Input Schema

Name	Required	Description
`needs`	No	Required features: streaming, p2p (torrenting).
`country`	No	Two-letter country code. Prices are always US prices in USD; for a country other than US the result says local checkout prices may differ.
`checkout`	No	Default false: return pricing-page links. Set true to return plan-preselected provider checkout links instead.
`min_devices`	No	Minimum simultaneous devices needed.
`term_months`	No	Preferred subscription length in months: 1, 12, 24 or 36.

Output Schema

Name	Required	Description
`plans`	Yes
`source`	Yes
`message`	No
`priceNote`	Yes
`disclosure`	Yes
`currencyNote`	Yes
`selectionNote`	Yes	How many plans are shown, from how many providers, out of the curated catalog.
`missingTermNote`	No	Providers with no plan for the requested term.
`unconfirmedNote`	No	Providers left out of a needs filter because the feature is unconfirmed, not because it is missing.
`oldestVerifiedDate`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 15, 2026
- addedOutput schema / properties / plans / items / properties / offerTargetUrl
```
Added value: +{
+  "description": "Where offerUrl redirects to.",
+  "type": "string"
+}
```
AddedOct 3, 2026

TDQS

A4.7/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Beyond the readOnly/idempotent/openWorld annotations, the description discloses substantive behavioral traits: privacy facts not confirmed on provider pages are marked 'unconfirmed', each result carries its oldest price-check date, prices are US/USD with possible local checkout differences, and no personal data is used. These are exactly the caveats an agent needs to caveat its answer to the user.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness3/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The trigger sentence is well front-loaded, but the body is a single dense run-on that repeats facts ('US-priced (USD)' vs 'Prices are US/USD') and closes with a keyword-stuffing line ('best VPN, cheapest VPN, VPN comparison...') that adds no agent-usable information. Much of the length earns its place, but the tail is filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a 5-parameter, all-optional comparison tool with an output schema present, the description covers what an agent needs: input semantics, ranking logic, data provenance and currency, geographic caveats, and the unconfirmed-fact flagging policy. Return-shape details are deferred to the output schema, which is correct.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3, but the description adds real meaning: 'needs' filters out plans where the feature is unconfirmed (an exclusion rule not evident from the schema), and 'country' only adds a note when not US rather than changing prices. It restates checkout/term_months behavior already in the schema, so it stops short of a 5.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource ('Ranks a curated catalog of US-priced (USD) VPN plans'), names the six providers in the catalog, and defines the ranking rule (cheapest first by effective monthly cost over the preferred term including renewals). This clearly distinguishes it from siblings like compare-broadband, compare-esims and compare-mobile-plans, which cover different product domains.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It opens with an explicit trigger list ('which VPN to buy, cheapest VPN, streaming, torrenting, travel, public Wi-Fi safety, audited no-logs, avoid renewal price traps'), which maps directly onto user intents an agent will encounter. It also states scope limits: the catalog is 'a selection, not the whole market' and prices are US/USD, so the agent knows when this tool is and isn't appropriate.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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