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

Beyond the annotations it discloses pricing and auth mechanics ($0.01/call via x402 USDC or a card key, first 25 free, failed calls free) and explains the non-idempotent cost implication: 'each call is charged, so repeats are not free'. It also characterises the output as heuristics rather than a verified blocklist, which is important behavioral context an agent cannot infer from readOnlyHint alone.

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?

The lead clause front-loads the action and the decision it supports, and the sentence is dense with distinct facts rather than filler. The closing 'Reads or computes only and changes nothing' largely restates readOnlyHint/destructiveHint, costing some tightness.

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, yet the description spells out the shape of the result (redirect hops, final URL, domain age, registrar, trust flags, HTTPS/security headers, named signals) so an agent can anticipate what it gets back. Combined with the cost model and safety profile, nothing material is missing for a one-parameter read tool.

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?

With a single parameter and 100% schema description coverage, the schema already documents that url is the link to check and must be http/https. The description adds only a worked example (url=http://github.com) and no additional format or edge-case guidance, 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?

The description states a specific verb and resource ('Check a link') and enumerates exactly what is returned: each redirect hop and final URL, domain age/registrar/trust flags, HTTPS and security headers, and named risk signals. This clearly distinguishes it from siblings like url_redirects (hops only), domain_report or pack_domain_intel (domain facts only) and pack_website_audit.

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?

It gives concrete usage contexts: 'before an agent opens or pays it', links in emails, chats, scraped pages, or a payee's website. It also sets a boundary by declaring this is 'heuristic signals from public facts, not a phishing blocklist'. It does not name a specific sibling alternative for narrower needs, so it falls short of a full 5.

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