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

With no annotations, the description carries the full burden and succeeds: it explains accepted input variants, multi-format output (E.164 + national), country-resolution behavior, defaultCountry fallback, and the explicit limitation around mobile operator identification (portability, original allocation caveat, warnings on partial numbering plans). That is unusually rich behavioural disclosure for a validation endpoint.

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 message is front-loaded, but the final paragraph on mobile-operator semantics is lengthy, reads like product copy, and repeats the allocation caveat twice. The 'one thing this endpoint deliberately does not claim' framing is distinctive but spends a substantial share of the description on a non-core behaviour.

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 validation tool with no output schema, the description covers input formats, defaulting behaviour, output shape, and explicitly flags edge cases (portability, partial numbering plans, warnings). It is thorough, though field-level detail of the returned object beyond E.164/national/warnings/originallyAllocatedTo is left implicit.

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 two parameters are already documented. The description adds real context (defaultCountry is applied whenever no international prefix is present, and its default is US) that supplements, but does not exceed, 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?

The description opens with a specific verb (validates and normalises), a specific resource (phone number) and a precise output format (E.164). It clearly differentiates from its closest siblings (email_verify, iban_validate) by naming the target data type and canonical form.

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 names real contexts ("signup and checkout flows that need to store one canonical form and reject typos early"), making the when-to-use clear. It does not name email_verify or iban_validate as alternatives, but the per-type nature of the sibling set makes that unnecessary.

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