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

Annotations already cover read-only/idempotent semantics, and the description adds the costly-to-discover facts: it performs a live unpaid call against the target plus fetches discovery files, charges 0.005 USDC via x402 with no account or API key, refuses invalid input before payment, never charges a failed run, and returns PaymentRequired when unpaid. Those are exactly the billing and failure traits 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?

One front-loaded paragraph that leads with what is checked and what is returned before pricing, which is the right order. It is dense but nearly every clause earns its place; the trailing HTTP URL and pricing repetition are slightly promotional overhead.

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?

There is no output schema, so the description carries the return burden and does so: networks, USD prices, and 'every problem with its fix.' Combined with payment handling, refusal-before-payment, and the free-sample pointer, an agent has everything needed to call it and interpret the result.

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 coverage is 100% and both parameters carry descriptions, so the schema does the heavy lifting and a 3 baseline applies. The description adds nothing about `url` or `method` semantics beyond an example query string, though it does implicitly show a full-URL shape.

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 and resource ('Validate any x402 endpoint') and then enumerates exactly what is validated: the 402 challenge in both v2 header and v1 body form, every offer field, CORS, and discovery files. That specificity makes it unmistakable against neighbors like url_safety_check or site_audit, which an agent can rule out without opening a 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?

Establishes the use case concretely ('the way buyer agents see it') and routes the agent to get_example for a free sample answer, plus gives an HTTP mirror. It does not explicitly say when to prefer site_audit or url_safety_check instead, so the comparison remains implicit.

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