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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, non-open-world). The description adds real context beyond them: processing is done 'locally', 'sends no SMS', and it does not confirm subscriber ownership. That last point is useful scoping an agent could otherwise misjudge.

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?

Front-loads purpose, then usage, then scope exclusions in three short sentences with no filler. The opening fragment 'International phone format with normalization' is slightly clipped but still functional as a lead.

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, so the description is the only place to learn what a call returns — and it never says whether the result is a boolean, a normalized string, or an error shape. For a single-param validator this is the main remaining gap; safety behavior is otherwise well covered.

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% and the single 'phone' parameter is documented with an example (+66812345678), so the schema carries the parameter burden. The description adds only the notion of E.164 normalization and does not clarify syntax, accepted variants, or how invalid input behaves — 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: checking/normalizing a candidate phone number into E.164 style, with the explicit trigger 'before storing a test record'. It is clearly distinct from the generate-* siblings by being a validation/normalization tool, though it never names a sibling 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?

'Use to check a candidate international number in E.164 style before storing a test record' gives a concrete usage context, and the note that it does not verify subscriber ownership defines a boundary. No alternative tool is named for the case where you need generation (generate-phone-number) rather than checking, so it stops short of explicit routing.

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