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

With readOnlyHint=true already covering safety, the description adds semantic context the annotations cannot: that listed numbers are opt-outs that never receive SMS, that a single-number lookup returning `{empty: true}` means the number is not blacklisted, and that no matches yields an empty result. It does not discuss pagination defaults 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?

Three compact sentences, front-loaded with the primary bulk-list behavior before the lookup variant. The trailing REST endpoint note is useful provenance, though 'Returns an empty result when the blacklist is empty' is mildly redundant with the empty-result behavior already implied.

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 read-only, 6-parameter list tool with no output schema and full schema coverage, the description supplies the missing semantic layer: what the blacklist means and how to interpret the single-number response. Only return-shape details for the paged list mode are left unaddressed, which is minor given the annotations.

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 paging/sort/date parameters are already documented and the baseline is 3. The description still earns above baseline by explaining the semantic outcome of the `number` lookup param (`{empty: true}` means not on the list), which the schema's one-line description does not convey.

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 ('List the numbers on the account's opt-out blacklist') and immediately clarifies what membership means (no SMS is ever sent to them), so an agent understands the tool's scope without opening the schema. It also distinguishes the bulk-list mode from the single-number lookup mode, separating it from the add_to_blacklist 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 'or look up one number' clause tells the agent the alternate mode and the condition that triggers it, which is real routing guidance. It stops short of explicitly contrasting against siblings like add_to_blacklist or warning about when not to use pagination params.

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