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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false. The description adds valuable context beyond that: the call is 'Free', the result is exhaustive ('every' service), and the response contents are enumerated. It does not mention pagination or rate limits, but those are not strongly needed for a simple listing tool.

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?

Two dense, front-loaded sentences with no filler. 'Free' and 'Call this first' are immediately useful, and every clause contributes either scope, output format, or usage guidance.

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?

For a zero-parameter, no-output-schema catalog tool, the description is complete: it states what is listed, what fields are returned, the cost, and when to call. There are no missing details that would prevent correct invocation or interpretation.

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 tool has zero parameters, so the input schema fully covers parameter semantics. Per the rubric, a zero-parameter tool receives a baseline of 4, and the description correctly avoids inventing parameter details that do not exist.

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 states a specific verb ('List'), a precise resource ('every paid x402 service this gateway sells'), and the exact output fields (name, description, price in USD, HTTP method, live path). This clearly distinguishes it from the sibling tools, which focus on analytics, routing, or research rather than catalog discovery.

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?

'Call this first to discover what can be bought here' gives explicit when-to-use guidance and positions the tool as the discovery entry point. It does not list alternatives or when-not-to-use cases, but for a zero-parameter catalog tool the primary usage context is clear.

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