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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false. The description goes beyond them by disclosing the data source (Calmido directory), the geographic coverage, and accepted input formats, which builds trust about result scope. It omits rate limits, auth requirements, and coverage gaps within the directory.

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?

Three sentences, cleanly ordered: what is returned, when to invoke, and how to format input. Nothing is padded and the highest-value content (data returned and trigger conditions) comes first.

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?

An output schema exists, so return values need no prose explanation, and annotations carry the safety profile. The description supplies coverage area, source, input formats, and the country fallback rule — everything an agent needs to call this correctly.

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%, so the baseline is 3, but the description adds a decision rule the schema does not state directly: 'pass `country` for a national number outside the Netherlands.' It also restates both number formats with concrete examples, reinforcing the schema's guidance for the required parameter.

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 ('Look up who is behind a phone number') and names the scope (Netherlands, Belgium, Luxembourg) plus the data source (the Calmido phone directory). It also enumerates the returned content — community reports, spam/scam categories, trust verdict, registered business — so an agent knows exactly what this tool yields. No siblings exist, but the definition is self-sufficient.

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 explicit trigger conditions: 'whenever someone asks who called them, whether a number is safe, spam or a scam, or which company a number belongs to.' That maps intent to tool well. It stops short of an explicit when-not clause or alternatives, though with no sibling tools there is little to route against.

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