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

Annotations already cover read-only/idempotent/openWorld=false, so the bar is lower, and the description still adds real behavior: rates depend only on the number called, not caller location, and premium/satellite/special-service ranges cost more than standard. It also mentions free-credit coverage and a dialer link in the response, though it does not add auth or rate-limit caveats.

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-loads the core purpose and the sibling routing early, then handles parameter semantics. The sentence is dense and long, but each clause carries distinct information rather than padding.

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 an output schema present, return values need not be spelled out; the description covers purpose, routing, parameter interactions, and pricing behavior. Everything an agent needs to call and interpret this correctly is present.

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, but the description goes beyond the schema by explaining the interaction between the two params: pass `destination` for the standard rate, `number` for the exact-number rate, and a local-format number also requires `destination`. That mutual-dependency rule is genuinely useful selection guidance.

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 (returns the per-minute price of calling a country or number) and explicitly distinguishes itself from its sibling get_dialing_instructions. An agent can route correctly without opening either schema.

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?

Gives explicit trigger conditions ('when someone asks what a call abroad costs or how to call a phone abroad from a computer') and names the alternative for the adjacent need (digit dialing) with the selecting condition.

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