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

Goes well beyond the readOnly/idempotent annotations: discloses that the lookup is free, that operator detection is a heuristic based on number ranges, that MVNOs resolve to host networks, and that a wrong-network voucher delivers but is non-refundable. That last point is critical risk information the agent cannot get from structured fields.

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 purpose and cost, then the risk warning. Five sentences, all doing work. Slightly dense in the final sentence, but 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?

For a single-parameter read tool with no output schema, the description covers the return content (country, operator, offers, other_brands) and the main failure mode well. Missing details on offer shape or pricing format are minor at this complexity level.

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% and the E.164 format is documented in the schema, so baseline is 3. The description adds useful context by naming `other_brands` (presumably a response field, since no output schema exists), but it is phrased as something to 'use', which could be misread as an input parameter that does not exist in 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?

States a specific verb and resource: identify country/operator of a phone number and list top-up offers for it. Clear enough for an agent to grasp without opening the schema, but it never differentiates itself from the sibling list_offers tool, leaving ambiguity about which to call for an offer list.

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 clear pre-purchase workflow guidance: read the detected brand back to the human and get confirmation before buying. However, it offers no explicit when-not or alternative-tool guidance (e.g., versus list_offers or quote), so the routing decision between siblings is left to inference.

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