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

Annotations cover read-only and open-world access, and the description adds genuinely new behavior: a 2-credit cost per successful call with a CAD-cent conversion, and the guarantee that schema-invalid input is rejected before billing. It stops short of describing failure modes (DNS failure, timeout) or what 'present: false' costs, but the billing semantics are well disclosed.

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 core action and target path are front-loaded in the first clause, followed by the return shape and then billing. Nearly every sentence earns its place; the return-field list is compact, though the two enumerations of fields ('Contact and Expires' then the response keys) slightly overlap.

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?

No output schema exists, so the description correctly takes on the job of enumerating the response object and explaining each field's meaning. With a single fully-documented parameter and annotations covering safety, the only real gap is error/timeout behavior and whether a missing file is billed.

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?

Only one parameter at 100% schema coverage, so the schema already carries the constraint set (length bounds, IP-literal and private-suffix refusal). The description only echoes 'domain' as part of the return object and adds no input interpretation beyond the schema; baseline 3 is appropriate.

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 (fetches) and resource (/.well-known/security.txt) plus the RFC 9116 standard, and immediately names the fields reported. It is trivially distinguishable from adjacent siblings such as robots-check, dns-lookup or mail-auth because the target artifact is named explicitly.

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 only implied by the artifact fetched — an agent can infer this is the tool for checking a domain's security.txt, but there is no explicit 'use this when…' or routing away from siblings like robots-check or header-audit. The one explicit rule given is about billing/rejection, not about when to select the tool.

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