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

Annotations already establish readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds meaningful behavioral context: it enumerates what verification returns and explains why 'country' cannot be inferred in the MCP environment, which goes beyond the 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?

Two compact sentences, with the core action and outputs front-loaded and the important conditional requirement stated immediately. Every sentence earns its place; there is 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?

Despite the absence of an output schema, the description lists the expected result categories, and the input schema covers both parameters. The main gap is error behavior for invalid numbers, but for a read-only verification tool with full parameter coverage, the definition is nearly 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 coverage is 100%, so the schema already documents both parameters. The description essentially restates the conditional requirement for 'country' rather than adding new semantic detail, matching the baseline of 3.

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 uses a specific verb ('Validates and formats') plus a precise resource ('a phone number') and enumerates concrete outputs: validity, country, line type, E.164, international and national formats. Even among many sibling validation tools, the phone-specific scope makes it unambiguous.

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?

The description makes the main usage condition explicit: numbers without a leading '+' require the 'country' parameter, because the end user's country cannot be inferred over MCP. It does not name alternatives or exclusions, but the tool's purpose and conditional prerequisite are clear enough for selection.

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