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

Annotations already declare readOnly and non-destructive, so the description is free to add value elsewhere—and it does by enumerating what is returned (status and dates, name, IP, custom properties, segments), which matters since there is no output schema. It does not say what happens when the email is not found in the list, which is the main remaining gap.

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 resource and lookup key, with the endpoint as a compact trailing reference. The opening fragment ('One subscriber of a list by email address:') is slightly clipped but still efficient.

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?

With no output schema, the description steps in to list the returned fields, which is the key missing information an agent needs. Only not-found/error behavior is absent, which is a minor gap for a read-only lookup.

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%, and both parameters (email, list_id) are documented there. The description only reinforces that the lookup is list-scoped and keyed on email; 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?

States a specific verb and resource—retrieving one subscriber of a list by email—and even names the underlying endpoint (GET /v2/subscriber/getByEmail). It implicitly differentiates from newsman_get_subscriber_by_id via the 'by email' qualifier, but never names the alternative explicitly.

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 email-based lookup key is stated, which hints at when this tool applies versus an ID lookup, but there is no explicit guidance on choosing between this and newsman_get_subscriber_by_id or newsman_search_subscribers. Usage must be inferred from the sibling names.

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