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

Annotations only declare readOnlyHint=true, and the description adds meaningful context beyond that: a subscription prerequisite, a geographic limitation, and the specific 403 failure mode. It also enumerates the returned fields, which matters because no output schema exists. Rate limits or latency behavior are not covered, keeping 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?

Two dense sentences: the first front-loads the return value, the second carries the eligibility constraints, and the endpoint path is appended as a compact technical reference. Nothing is redundant or padded.

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 one-parameter read tool with no output schema, the description covers what is returned, who may call it, and the main failure condition. It is nearly complete; only secondary details such as error payload shape for non-403 failures or any rate limiting are absent.

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% and the single parameter is fully documented in the schema (international format, length bounds), so the baseline is 3. The description only echoes the number via the {msisdn} path placeholder, adding no format or validation detail beyond what the schema already provides.

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 names the resource (a phone number) and the exact payload returned (country, mcc/mnc, operator), so an agent knows precisely what this tool produces. No sibling tool performs a lookup, so it is unambiguously distinct from the get_/list_/send_ family.

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?

It states clear eligibility constraints: premium smsmode subscription required, and only French numbers are supported (403 otherwise). This tells the agent when the call will fail, though there is no alternative tool to route to and no positive 'use this when' framing beyond the implicit lookup use case.

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