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

With no annotations provided, the description carries the full disclosure burden and does well: it declares 'Read-only', specifies the external API queried, and explicitly states this is not a probe of the target, which tells the agent no packets reach the domain's own infrastructure. It omits rate limits, auth requirements, and error behavior, which keeps it out of the top score.

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 primary purpose is front-loaded in the first clause, followed by the data source and return fields. The middle sentence explaining preload's HTTPS guarantee is genuinely useful orientation but is the longest element and edges toward background exposition rather than invocation guidance.

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, and the description correctly compensates by enumerating the return values (status preloaded/pending/unknown, bulk-imported flag, parent domain). For a one-parameter, unauthenticated lookup that is nearly sufficient; only edge cases such as invalid input or API failure handling are unaddressed.

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 there is only one trivially-named parameter, so the schema already does the documenting work. The description adds no format constraints, sub-domain handling rules, or examples beyond the schema's 'Domain, e.g. example.com'.

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?

Names a specific verb and resource ('Check whether a domain is in the browsers' HSTS preload list') and goes further to state the data source (hstspreload.org's keyless status API) and the exact returned dimensions (status, bulk-imported, parent coverage). This is clearly distinguishable from siblings like tls_posture or security_txt, which check live headers rather than preload membership.

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?

The description clarifies the nature of the operation ('A membership lookup, never a probe of the target') and contextualizes it against header-based checks ('a stronger guarantee than the Strict-Transport-Security header alone'), which implicitly steers the agent to header tools when a live probe is wanted. However, it never explicitly names an alternative tool or states when not to use this one, so it falls short of a 5.

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