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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description's real contribution is the scoping qualifier 'active (subscribed)', which tells the agent unsubscribed/inactive contacts are excluded from the count. It says nothing about cost, rate limits, or whether the count is exact or eventual.

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 short sentences, front-loaded with the meaning and followed by the endpoint reference; no filler. Slight deduction only because the first sentence is a noun fragment rather than a stated action, which reads marginally less like an instruction.

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 one-parameter read-only counter with full schema coverage and annotations covering the safety profile, the description supplies enough to call it correctly. A return-value note is unnecessary absent an output schema, though a brief hint that the response is a numeric total would have closed the last gap.

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?

There is a single parameter with 100% schema description coverage ('NewsMAN list id.'), so the schema carries the semantics. The description adds nothing further about list_id, e.g. whether a numeric id and string id are interchangeable or what happens for an unknown list — 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?

The description states a specific resource and metric: the count of active (subscribed) subscribers for a given list, plus the underlying endpoint GET /v2/list/count. The phrase 'of a list' implicitly separates it from newsman_count_segment_subscribers and newsman_list_subscribers, though it never names those siblings.

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?

Usage is only implied: 'of a list' suggests using this instead of the segment-level counter when a list-wide total is wanted, and 'active (subscribed)' signals it is not a full roster lookup. There is no explicit when-to-use, when-not-to-use, or named alternative.

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