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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world behavior, so the safety profile is fully covered. The description adds that a built-in dialing-code table is used, hinting at deterministic offline behavior, but that largely restates openWorldHint=false and gives no detail on output format or error cases.

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 sentences with zero padding: the usage trigger is front-loaded, followed immediately by the mechanism. Every clause earns its place.

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?

With a full output schema, return values need no explanation, and the three required params are schema-documented. The description is adequate for a simple pure-function tool, though it could note country-format expectations.

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 all three parameters (from_country, to_country, phone_number) are already documented in the schema, including the local-format example. The description adds no extra meaning such as accepted country formats or number normalization rules, so baseline 3 applies.

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 ('Formats a local phone number') and the scope ('calling between two countries'). No sibling tool overlaps with phone-number formatting, so ambiguity is low, but the description never explicitly differentiates itself from siblings.

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?

Opens with an explicit usage cue ('Use when the user needs to dial a local number internationally'), which gives clear context for invocation. It offers no exclusions or named alternatives, but no sibling is close enough to require routing guidance.

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