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 bar is lower. The description adds genuinely non-structured context: the number is used for lookup only and is neither stored nor returned (a privacy guarantee), plus the longest-prefix matching rule.

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?

Three short sentences, front-loaded with the core action; the forwarded-call clarification and the non-storage note each earn their place as disambiguation and behavioral context. No filler.

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?

An output schema exists, so return values need not be explained, and the single, fully documented parameter plus rich annotations cover most needs. It stops short of saying what happens when no prefix matches (error vs. fallback) or the currency/units of the returned price, which are the remaining gaps.

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% for the single parameter, including the E.164 pattern and an example, so the baseline is 3. The description adds only the marginal note that the number is not persisted, not any format or matching detail beyond the schema.

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 gives a specific verb+resource (per-minute price of a call to one phone number) and names the resolution mechanism (longest matching prefix). It is clear, but it does not explicitly distinguish itself from the sibling get_forwarding_rate, which it partially overlaps with by covering 'a call forwarded, to one phone number'.

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 rather than stated: the tool is for looking up a rate for a specific number. There is no explicit 'use this instead of get_area_pricing/get_forwarding_rate when...' routing, and the overlap with get_forwarding_rate is left unresolved for the agent.

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