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

Annotations already declare read-only, idempotent, and non-destructive behavior, so the description's job is to add content-level context. It does: the exact return payload (both mobile and landline dialing examples, time difference, daytime windows, guide URL, and cheapest rate) plus the international-format variant. This is meaningful disclosure given there is no output schema, and it does not contradict the annotations.

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-loaded with the core purpose before the content list, and the second sentence is a crisp rule with zero filler. The main list is dense — a long colon-heavy sentence — but every item carries meaning, so nothing is wasted. Slightly less scannable than a bulleted form, but appropriately sized for a tool with no output schema to document.

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 simple read-only lookup with 100% schema coverage, full annotations, and one enum, the description covers required and optional parameters, the omit-origin rule, and the return contents in detail. The only notable gap is the ambiguity with get_call_rates caused by including the cheapest rate without explaining when to prefer the sibling.

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 the baseline is 3; the schema already documents each parameter with examples and accepted formats. The description adds one genuinely new semantic: when to omit origin ('for the international (+) format only'), which the schema does not state beyond labeling origin optional. That extra condition justifies a score above baseline.

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 task ('dialing instructions for calling a destination country from an origin country') and enumerates exactly what the caller gets: exit code, country code, example mobile and landline numbers, time difference, daytime hours, eFon guide URL, and cheapest rate. However, it never names or routes to the sibling get_call_rates, and the inclusion of 'cheapest eFon rate' overlaps with that tool's apparent domain, so selection between the two is not fully resolved.

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?

Usage is implied: an agent can infer this tool is for dialing instructions, and the closing sentence gives a concrete usage rule ('Omit origin for the international (+) format only'). But there is no when/when-not guidance versus the sibling get_call_rates, which also plausibly returns rate information. The alternative is never mentioned, so an agent choosing between the two gets no routing help.

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