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

The readOnlyHint annotation already establishes this as a safe read, so the description's added value is the disclosure of what the payload contains (contact data, addresses, total spent) — genuinely useful since there is no output schema. It stops short of noting behavior on a missing id or any auth/permission requirements.

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?

A single front-loaded sentence with no filler; the returned-field list is packed into the same clause rather than a separate sentence.

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 fetch with no output schema, the description covers purpose and return content adequately. Only minor gaps remain (behavior on not-found, whether 'total spent' is scoped by currency or period).

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 the single parameter already documents its type, positive-integer constraint and provenance ('from recurpay_list_subscribers'). The description adds no parameter-level meaning beyond the schema, so the baseline 3 applies.

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 ('Get one subscriber'), disambiguates the domain term with '(customer)', and enumerates the returned data. The singular 'one' implicitly separates it from recurpay_list_subscribers, but no sibling is named 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?

Usage is only implied: fetching a single subscriber by id. There is no explicit statement of when to prefer this over recurpay_list_subscribers or recurpay_update_subscriber, and no prerequisites or failure conditions are mentioned.

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