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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds a genuinely useful behavioral fact beyond them: that the end user's country cannot be inferred over MCP, which explains why 'country' may be demanded. No output format or rate-limit detail, but the bar is lower with annotations present.

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 tight sentences, no filler. The primary purpose and its return values are front-loaded, with the parameter constraint following as supporting detail.

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 2-parameter, read-only tool with no output schema, the description enumerates the return fields the agent can expect and covers the one conditional input rule. Nothing needed to invoke it correctly 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 coverage is 100%, so the baseline is 3. The description goes slightly beyond the schema by explaining the rationale for the conditional 'country' requirement (no country inference over MCP), reinforcing why the parameter matters rather than merely restating it.

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 (validates and formats) and resource (phone number), then enumerates exactly what is returned: validity, country, line type, and E.164/international/national formats. It is unmistakably distinct from siblings like email_validate or iban_validate.

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?

Clearly states the condition that governs the second parameter: numbers without a leading + require 'country'. It gives no explicit alternatives or when-not guidance, but this is a single-purpose tool with no near-identical sibling, so the context is sufficient.

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