list_vpn_catalog

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

No annotations are provided, so the description carries the full burden. It usefully discloses the data source ('X402_BASE_URL discovery') and that the result is a list of purchasable items, implying a safe read, but it says nothing about auth requirements, rate limits, caching, or failure modes.

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

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

One dense sentence, front-loaded with the verb and the primary resource, with the returned fields tucked into a compact parenthetical. Nothing is wasted.

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

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

With no output schema and no parameters, the description compensates well by enumerating the return contents (id, USD price, duration, country codes). It is close to complete for a simple discovery call, though auth/access expectations for the X402_BASE_URL endpoint remain unstated.

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

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

The schema declares zero parameters, so there is no parameter semantics to convey and the baseline of 4 applies. The description correctly signals a no-argument call.

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

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

The description gives a specific verb ('List') plus a precisely scoped resource ('purchasable VPN plans ... and supported server country codes'), and even names the fields returned. This is clearly distinguishable from the write/payment siblings (buy_vpn, sign_and_pay) without opening any schema.

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

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

Usage is only implied: an agent can infer this is the pre-purchase discovery call, especially given the buy_vpn sibling, but the description never says when to call it or that it precedes a purchase. No alternatives or exclusions are named.

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