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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds real behavioral context on top: the response payload composition (member counts by status, custom fields, preferences, paging) and a cost signal ('cheap'), which matters for quota-sensitive agents. It does not discuss rate limits or the 1-1000 page_size ceiling's practical effect.

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 plus a compact endpoint mapping (GET /lists/{Page}/{PageSize}.json). The verb, scope, return contents, and practical use case are all front-loaded with no filler or repetition.

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?

There is no output schema, so the description carries the burden of describing the return value — and it does, enumerating counts, custom field definitions, preferences, and paging. Combined with the 100% schema coverage on inputs, an agent has everything needed to call this 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 description coverage is 100% with two enum-constrained params (sort_by, sort_method), so the schema already carries the parameter meaning. The description only alludes to 'paging info' without documenting the page/page_size interplay or defaults, so it adds little beyond the schema — the baseline 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 (List) and resource (active email/mailing lists in the account) and enumerates what comes back: member counts by status, custom field definitions and preferences, and paging info. The scope qualifier 'active' and the 'find list IDs' framing implicitly separate it from moosend_get_mailing_list, which fetches one list.

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?

Gives explicit usage context ('a cheap way to confirm the API key works and to find list IDs'), which tells the agent when this is the right entry point rather than jumping straight to a get_* sibling. It stops short of naming an alternative or an exclusion (e.g., 'for a single list's details use moosend_get_mailing_list').

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