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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, so the safety profile is carried. The description adds genuinely new behavioral context: the result includes capital, an example number, the eFon rate page, the cheapest per-minute callback rate, and a scam warning for one-ring calls. It does not mention any rate limits or the shape of the failure case when a code is unknown, which keeps it short of a 5.

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 and scoped by the shared-code caveat in the first clause, then use cases, then the sibling hand-off. It is dense and somewhat run-on as a single long sentence, but every clause carries information an agent can act on.

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?

There is no output schema, yet the description enumerates what comes back (country, capital, example number, rate page, cheapest rate, scam warning), so return content is not a gap. Combined with full annotation coverage and full schema coverage, an agent has everything needed to call this correctly.

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 both parameters (including the language enum) are already documented in the schema. The description restates accepted code formats ('+36, 0036 or +1 876') that the schema description already gives, adding no new syntax or normalization rules. Baseline 3 is appropriate when the schema does the heavy lifting.

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 ('reverse lookup of an international dialing code') and immediately bounds the scope, including the shared-code case (+1, +7, +44). It also names the sibling it is not (get_call_rates for the full rate list), so an agent can separate it from the other rate tools without opening any 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 triggering queries ('which country is +36', 'who is calling me from +233', 'what does 00252 mean', a missed call from an unknown foreign number) and an explicit alternative with its condition: use get_call_rates for the full rate list. When-to-use and when-to-use-something-else are both covered.

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