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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds the outcome semantics (found = blacklisted, 404 = not), which is genuinely useful context beyond the structured fields, though it does not cover auth requirements or 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 tight sentences. The purpose is front-loaded and the endpoint/outcome detail follows without waste.

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 read tool with no output schema and annotations covering safety, the description supplies the essential outcome semantics. It lacks explicit usage-vs-alternative guidance, which holds it short of 5.

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%, so the E.164 format and the phone parameter are fully documented in the schema. The description adds no parameter-level detail beyond what the schema already says, so baseline 3 is appropriate.

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?

Specific verb+resource ('Check whether one phone number is on the account's blacklist') and clearly distinguished from siblings add_to_blacklist and list_blacklist. The scope note 'one phone number' contrasts with the listing sibling.

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 behavior of the underlying API (found vs 404) is disclosed, which implicitly signals use context, but there is no explicit when-to-use vs alternatives like list_blacklist or add_to_blacklist. Adequate but with a clear gap.

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