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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuine value by disclosing exactly what the caller receives (exit code, country code, example numbers, time difference/daylight overlap, guide URL, cheapest rate) and how omitting origin changes the output format.

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 purpose is front-loaded and the second sentence carries the one invocation rule. The middle enumeration of returned fields is long and slightly run-on, but with no output schema it earns its place by telling the agent what it will get.

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 no output schema, the description correctly carries the burden of describing return content, and it does so thoroughly, including units like time difference and daylight hours. The remaining gap is routing relative to the three sibling tools, which is left 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 coverage is 100%, so baseline is 3, but the description adds behavior the schema does not: omitting origin switches the result to the international (+) format only. That is a meaningful semantic rule about the optional param that an agent cannot infer from the schema alone.

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 states a specific resource (eFon dialing guide) and its scope (calling a destination country from an origin country), with the exact contents enumerated. It is clear what the tool returns, but it never distinguishes itself from siblings like lookup_calling_code or get_call_rates, and the mention of the 'cheapest eFon rate' overlaps with rate-focused siblings without clarification.

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 only guidance is 'Omit origin for the international (+) format only', which explains one invocation mode but not when to choose this tool over lookup_calling_code or get_call_rates. Usage is implied by the purpose rather than stated, with no exclusions or alternatives named.

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