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

Annotations already establish read-only, idempotent, open-world, non-destructive behavior, and the description still adds substantial context: HEAD-then-GET confirmation strategy, robots.txt compliance, the exact User-Agent used, refusal of private/internal addresses, and the charge/no-charge rules. This is exactly the beyond-annotations detail that helps an agent predict outcomes.

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?

Front-loaded with purpose and result vocabulary, then mechanics, then payment rules in a logical order. It is dense but nearly every clause carries information an agent needs (cost limits, retry path, no-charge conditions); only the User-Agent brand string is arguably gratuitous.

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 full burden of describing returns, and it does: status, redirect chain, final URL, and the classification enum. It also covers the failure mode (isError + PaymentRequired object) and the remediation path, so an agent can handle both success and payment-required outcomes.

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% and both parameters are already well documented in the schema (bare-domain semantics, size limits, x402 payload vs. private key warning). The description restates the _meta['x402/payment'] preference and the URL count cap but adds no new parameter-level meaning, so the baseline 3 applies.

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 ('Check whether links are alive') with an enumerated output contract: HTTP status, every redirect hop, final URL, and a classification vocabulary. This clearly separates it from siblings such as robots_txt_check, page_to_markdown, or website_screenshot, which do different things to a URL.

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?

Gives clear operational context: free for 1 URL when the free web tool is enabled, up to 10 URLs per paid call, and an explicit retry path when a paid call is rejected. It does not, however, route the agent away from siblings (e.g., 'use robots_txt_check to inspect robots rules in detail' or 'use website_screenshot for visual capture'), so 5 is not warranted.

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