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

Annotations already establish readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds the E.164 standard and the 500-number cap, but leaves 'checks their potential validity' undefined and gives no hint about what the output contains or whether external lookups occur.

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 with the core purpose front-loaded. Minor redundancy in repeating the 500-number limit that maxItems already encodes, but nothing wasted.

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 should say what comes back for each number (valid flag, reason, E.164-normalized value) — 'checks their potential validity' is too vague to substitute. The input side is fully covered by the schema, so the gap is on the return-value side.

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 single phoneNumbers parameter is fully documented in the schema, so the baseline is 3. The description restates the 500-item cap already present in maxItems and adds no format examples or normalization guidance beyond what the schema provides.

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 ('Validates the format of given phone numbers') plus the standard applied (E.164) and a batch scope. It implicitly distinguishes itself from the sibling verifyWhatsAppNumberExists by being a format/validity check rather than an existence check, though the sibling is never named.

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?

Usage is only implied — an agent can infer this is a pre-flight format check, but the description never says when to prefer it over verifyWhatsAppNumberExists or what to do with the results. No explicit exclusions or prerequisites are given.

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