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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the low-risk profile is covered. The description's main addition is the $0.01 per-call price, which matters because 'readOnly' can mislead an agent into thinking the call is free. However, the two-step 402-challenge-then-pay workflow, the biggest behavioral surprise here, is disclosed only in the schema, not in the description.

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 compact sentence, front-loaded with the core question and followed by the invocation form and price. The 'GET /x402/x402_check?url=' fragment is mildly redundant with the schema but does anchor the call shape and cost.

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 tool whose real complexity is a paid 402-challenge handshake, the description omits the flow entirely and does not say what a result looks like (valid/invalid, challenge details). No output schema exists to cover returns, so the description leaves the agent to discover the payment dance from parameter docs alone.

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 the schema fully documents both url and the optional payment proof. The description repeats only the url query parameter and adds no semantics the schema doesn't already carry, which is the baseline 3 case when the schema does the heavy lifting.

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 names a specific resource and check: determining whether a URL is a valid agent-payable paid resource under x402 v1 and v2. 'x402 Doctor' is jargon but the parenthetical explanation disambiguates it from the check_* siblings, which target DNS, headers, CVEs, and so on.

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 the purpose (validate/pay against an x402 endpoint) but the description never states when to prefer this over alternatives or what preconditions exist, such as needing a URL that actually serves an x402 challenge. $0.01 gives a cost signal but no exclusion criteria.

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