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

Annotations already declare readOnly/openWorld/idempotent/non-destructive, and the description adds genuinely new behavioral facts: the target is contacted but not paid, the tool itself costs $0.01 USDC per call via x402 (eip155:8453), and it reports a list of spec problems. The cost and no-payment guarantees are exactly the sort of context an agent needs before invoking.

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?

Dense and front-loaded — the question-form opener establishes intent and the second sentence enumerates the return payload. Minor waste: the opening clause duplicates the tool title verbatim, and the field list is a long run-on, but nothing is padded.

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, the description correctly carries the burden of describing return values in detail (version, options, payTo, latency, spec problems). The one real gap is parameter coverage — the 'method' enum is left entirely to the schema.

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 0% for two parameters. The description only obliquely implies 'url' via 'Calls a paid URL' and never mentions the 'method' parameter (GET/POST/HEAD) or its default, so it fails to compensate for the undocumented 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?

Specific verb+resource: 'Probe an x402 endpoint' followed by a concrete enumeration of what is inspected (402 challenge, payment options, latency, spec problems). It is unmistakable what the tool does and does not do ('Nothing is paid'), but it never names or distinguishes itself from the sibling x402_search.

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 implied by 'Calls a paid URL without paying' — an agent can infer you use this to vet an x402 endpoint before paying. However there is no explicit when-to-use/when-not, and no routing between this and x402_search or the web_* siblings.

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