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

Annotations already declare readOnly, openWorld, idempotent, and non-destructive behavior, so the description doesn't need to repeat those. It adds useful behavioral context: it performs a 'live line-type read', returns approximate local time and a calling-window flag, and restricts itself to deliverability/reputation signals while excluding legal compliance data.

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 front-load the core question, then enumerate the returned signals, then add a necessary limitation. Every sentence adds value and there is no filler or redundant restatement of the tool name.

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 single-parameter tool with rich annotations and an output schema, the description is complete: it explains what the verdict answers, what fields will come back, and what the tool intentionally does not cover. No important guidance is missing for an agent deciding whether to call it.

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%: the only parameter, phone_number, is already documented as a 'US/Canada number, any common format.' The description's 'North American number' phrase aligns with the schema but adds no new parameter-level guidance. Baseline 3 is appropriate because the schema carries the parameter documentation burden.

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 states a specific purpose with a concrete question: 'can this North American number receive SMS, and is now a reasonable time to send?' It names the output fields (sms_capable, VoIP flag, spam reputation, local time, calling-window flag), clearly distinguishing this as a combined deliverability-and-timing check rather than a generic lookup.

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 clearly frames this as the tool for 'AI outreach' and sets an expectation boundary with 'not a legal compliance determination (no DNC/reassigned-number/litigator data).' It does not explicitly name sibling alternatives or say when to prefer vri_spam_check vs this tool, so it stops short of a 5.

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