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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, covering the safety profile. The description adds extra behavioral context: numbers are added via opt-out link or manually, and sends to them are rejected at no charge. This goes beyond the annotations and gives useful operational understanding. No contradictions.

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 description is concise, using only two short sentences. It front-loads the core purpose ('List opt-out numbers') and adds a clarifying second sentence about what the numbers are and the consequence of sending to them. No filler or repetition, ideal structure.

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?

Given the tool is a simple list with two optional pagination parameters (fully described in schema) and has an output schema (so return format is covered elsewhere), the description is complete. It explains the meaning of the list and the consequence of sending to those numbers, which is all an agent needs to call it correctly.

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%: both limit and cursor are described in the input schema. The description does not mention parameters at all. According to the rubric, when schema coverage is high, a baseline of 3 is appropriate, and the description does not add any parameter-specific semantics beyond the schema.

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 clearly states the tool lists opt-out numbers and explains what those are: numbers this account will never send to, with sends to them rejected at no charge. This is a specific verb+resource (list + opt-out numbers) and distinguishes it from siblings like add_to_blacklist/remove_from_blacklist which modify the list, and list_contacts which lists different entities.

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 provides clear context on what the tool returns (the blacklist) but does not explicitly mention when to use it versus alternatives or when not to. It implies usage: when you need to view opt-out numbers. Since it's a read-only list operation and sibling add/remove tools exist, the purpose is clear, but there is no explicit guidance or exclusion. That warrants a 4 per the rubric for 'clear context, no exclusions'.

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