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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so safety behavior is covered structurally. The description goes beyond that by disclosing the enriched payload returned (carrier, line type, location) and an inline invocation example, which is meaningful given there is no output schema. It does not mention rate limits or error behavior, so it falls short of a 5.

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?

Two tight sentences with the capability and return fields front-loaded before the example. Efficient overall, though the embedded example call slightly duplicates the schema's own examples array and could be trimmed.

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?

For a three-parameter read-only lookup with no output schema, the description supplies the essential missing piece: what fields come back. Combined with the annotations and full schema coverage, an agent has enough to invoke it correctly; only edge-case behavior (invalid numbers, errors, quotas) is 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%, so the schema already documents phone, _apiKey and country thoroughly, including the ISO2 fallback rule. The description repeats an example call rather than adding new semantics (e.g. acceptable formats beyond the schema's examples). 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+resource ('Validate and enrich a phone number') and immediately enumerates the concrete outputs (validity, international/national formats, carrier, line type, country/location). This is far more than a restatement of the name and lets an agent know exactly what capability it is selecting, with no sibling in the list offering anything comparable.

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 implies the use case (validate/enrich a phone number) but never states when to reach for this tool versus an alternative, nor any prerequisites or exclusions. With no sibling covering phone validation, the absence of routing guidance is tolerable, but the tool itself offers no explicit when-to-use framing.

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