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

Beyond the annotations (openWorldHint=true, idempotentHint=true, destructiveHint=false), the description adds two genuinely useful behavioral facts: the lookup is live/number-portability sourced, and it costs 3 credits with no charge when data is unavailable. It does not address the bypass_cache behavior or latency, but the cost and live-source disclosure is real added value.

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?

A single dense sentence with scope and returned fields front-loaded and no filler. The telegraphic compression ('ported, port date, current line type and VoIP') is efficient but slightly terse to parse.

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 no output schema, the description usefully enumerates what comes back, and annotations already cover the safety profile, so the main remaining gap is the two undocumented optional parameters. For a 3-param read-style lookup this is close to complete.

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 0%, so the description must carry parameter meaning, yet it never mentions bypass_cache or default_country. Only the required number parameter is indirectly explained by the US/Canada scope note; the two optional parameters remain opaque in both schema and description.

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 states a specific verb and resource (carrier lookup for US/Canada numbers) and enumerates the returned facts: current carrier, ported status, port date, line type, VoIP. It is clearly distinguishable from siblings like validate_phone or reverse_phone, though it never names an alternative explicitly.

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?

There is no guidance on when to choose this over validate_phone or reverse_phone, nor any stated prerequisites or exclusions beyond the implicit US/Canada scope. The credit note is cost context, not usage guidance, so the agent must infer the use case from the purpose alone.

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