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

Beyond the annotations, the description adds that processing is 'ephemeral' and that the tool does not query live carrier databases. This is valuable contextual information about the tool's offline, non-validating nature, going beyond what readOnlyHint and idempotentHint already convey.

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 description is two concise sentences, with the primary purpose front-loaded in the first sentence and supplementary behavioral details in the second. There is no redundant wording or unnecessary information, making it efficiently structured.

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?

The description is complete for this simple tool: it covers purpose, privacy (ephemeral processing), and scope (carrier-query exclusion). Given the full schema, annotations, and output schema, no critical information is missing for an agent to decide whether and how to invoke the tool.

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?

The input schema provides 100% coverage for both parameters with clear descriptions. The tool description does not significantly expand on the parameters, merely implying that countryCode supplies country context. Since the schema already does the heavy lifting, the description adds little parameter-specific value.

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 uses a specific verb ('Normalize') and clearly identifies the resource ('a phone number') and the outcome ('international format'). It also mentions 'reviewed country-specific rules,' which distinguishes it from the sibling tool that merely looks up country calling codes, avoiding confusion with validation or live lookup.

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 explicitly states what the tool does not do ('does not query carriers, HLR/HSS, MNP databases, or live subscribers'), providing a clear boundary for when not to use it. However, it does not explicitly name the sibling tool as an alternative, so the guidance is clear but not fully explicit.

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