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

Annotations already cover safety (readOnlyHint true, destructiveHint false), so the description only needs to add context. It does add the return payload (id, status, custom field values), but says nothing about what happens when the email is not found or whether the lookup spans all subscriber categories in this open-world API.

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?

One short, front-loaded sentence naming the key and the returned fields. The trailing raw endpoint path adds little for an agent but is not costly.

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, single-parameter lookup with no output schema, the description usefully enumerates the returned fields. Only the not-found / error behavior is missing.

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 coverage is 100% with a single email parameter already fully documented (format, pattern, max length). The description restates 'by email' without adding semantics beyond the schema, so 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 plus the lookup key: fetch "One subscriber by email" returning id, status and custom field values. That clearly separates it from add_subscriber, update_subscriber and list_* siblings, though it never names an 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?

Usage is only implied by the verb 'get by email' – there is no statement of when to prefer this over list_subscriber_categories or update_subscriber. For a trivial single-record lookup that is serviceable but not explicit guidance.

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