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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds real behavioral value beyond that: the exact set of possible statuses (available, backorder, rent to own, with another vendor, in use, carrier/block-issued info for locals) and the fact that links to history and purchase are returned. No auth or rate-limit context, which is the only notable gap.

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?

The lead sentence is well front-loaded with scope and accepted formats. However, the third sentence is a long enumeration of return statuses and links, which is largely redundant given that an output schema exists; that space would be better spent on usage exclusions or pricing/auth caveats.

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 a full output schema available, the description does not need to enumerate return values, and with all four annotations set it does not need to explain safety. It covers scope, accepted input formats, and intent-based triggers, leaving only minor gaps such as explicit sibling routing and any cost/access constraints.

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 description coverage is 100% and the single parameter's schema description already documents the 10-digit or vanity-spelling formats, including the same examples. The description reinforces the dual format but adds no syntax or validation meaning beyond the schema, so the baseline of 3 is correct.

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 (check a phone number) with clear scope: one number, US or Canadian, toll free or local, accepted as digits or vanity spelling. It is distinguishable from the search-oriented siblings (find_business_numbers, search_vanity_numbers) by being a single-number lookup, and it references history as a separate concern, though it never names a sibling explicitly.

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?

"Use it when someone asks whether a number is available, what it costs, or what it is today" gives explicit triggering conditions tied to the user's intent. It stops short of stating when NOT to use it or naming the alternative tools to pick instead for bulk or search-style lookups.

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