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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint, so the safety profile is covered by structured data. The description's mention of returned fields (price, network, payTo, scheme) largely duplicates the output schema, and it discloses nothing about auth, rate limits, or failure behavior (e.g., what happens when the URL is not a paywall).

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?

Three short sentences, front-loaded with the core capability and the fields reported. The trailing "Try GET /probe/sample" is useful but slightly promotional; nothing else 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?

For a two-parameter read-only probe with an output schema and full schema coverage, the description is essentially complete — the agent knows what it does, what it returns, and roughly when to use it. Only the sibling routing and non-paywall edge behavior are unaddressed.

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?

Schema description coverage is 100%, so baseline is 3; both url and method are already fully documented. "Try GET /probe/sample" adds a small amount of practical guidance about exercising the tool, but no syntax or format detail beyond the schema.

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?

States a specific verb (probe) and resource (public URL) plus exactly what the report contains: x402 paywall presence, price, network, payTo, scheme. This is far more specific than a tautology, but it never references the closely related siblings (x402_echo, can_pay) that an agent would need to differentiate against.

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?

"Shop paywalls before you spend" implies the use case (checking cost before paying), which is genuine context beyond the schema. However, it gives no explicit when-not guidance and does not name an alternative tool such as can_pay or x402_echo, leaving the routing decision to inference.

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