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

Annotations already declare readOnlyHint=true, idempotentHint=true and openWorldHint=true, so the safety profile is covered without prose. The description adds one genuinely new behavioral fact — a signed XDR-1 receipt is returned — but says nothing about what happens when validation fails (error vs. result field) or any rate limits.

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 sentences, zero filler, and the core definition is front-loaded before the situational rationale. Nothing would be lost by keeping it exactly as is.

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 one required parameter, full schema coverage and an output schema present, the description needn't explain return shape, and it correctly frames the check as structural rather than deliverability. The one gap is failure behavior — whether an invalid number raises an error or returns a negative verdict.

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 parameter is documented with an example, so the baseline is 3. The description goes beyond the schema by defining validity precisely (+CC prefix, 8-15 digits, no leading zero), which is real meaning the schema does not contain.

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 (Validate) and resource (E.164 phone number) and even spells out the structural rule set (+CC, 8-15 digits, no leading zero), so the agent knows precisely what operation is performed. It does not, however, distinguish itself from potential overlap with siblings such as batch_validate, which likely performs the same check in bulk.

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 phrase 'before dispatching SMS alerts or OTPs' gives a concrete triggering context, which is stronger than implied usage. It stops short of explicit exclusions or naming alternatives (e.g., batch_validate for multiple numbers, or a deliverability lookup for reachability).

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