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

Annotations already declare the safe read-only/idempotent profile, yet the description adds genuinely new behavioral facts: results are derived offline with no assignment or reachability check, line type is returned only where ranges are unambiguous and is otherwise null with a reason rather than guessed, plus auth model (no API key), rate limits (1 free call/day) and pricing ($0.002 via x402). That is well beyond what the structured fields convey.

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 scope in the opening clause, with limitations and edge-case policy following in a logical order. Slightly long, and the trailing pricing/auth sentence is close to boilerplate, but each sentence still carries usable information.

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?

An output schema exists, yet the description still covers the two things an agent must know to call it correctly: what the result means (length match, conditional line type) and what it explicitly does not guarantee (assignment/reachability). For a two-parameter read tool with a documented output, nothing material is missing.

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% for both parameters, so the baseline is 3. The description restates the required input ('Required input: phone') but does not clarify the optional 'country' fallback parameter beyond what the schema already says ('Calling code, if the number has no +'), adding little semantic value.

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 (normalise a phone number to E.164) and enumerates exactly what it returns: country, calling code, national number, length validity, and conditional line type. An agent can distinguish it from adjacent validation siblings like email_verify or tax_id_check without opening the 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 clear usage context and a strong 'when not' signal: 'Offline: it does not check that the number is assigned or reachable.' It does not, however, name an alternative tool or route the agent elsewhere for reachability checks, so it stops short of full when/when-not/alternatives guidance.

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