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 adds the returned field set, which is useful, but says nothing about pagination, result ordering, empty-list behavior, or auth requirements expected of an external API endpoint.

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 sentences plus the endpoint path; the purpose is front-loaded and the downstream usage hint follows. The raw REST path is slightly redundant for an agent but is short and harmless.

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 compensates by enumerating the returned fields (key, name, type), which is the key thing an agent needs to plan a follow-up add/update call. Minor gap: no indication of response envelope or field ordering.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; the baseline of 4 applies. Schema coverage is 100% and the description correctly signals a no-argument call by mentioning nothing optional.

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+resource: subscriber custom field definitions, and names the returned fields (key, name, type). It is clearly separable from siblings like list_subscriber_categories or list_campaign_categories, though it doesn't explicitly say 'all fields, no filtering.'

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?

"Use the keys when adding or updating subscribers" gives concrete downstream context that ties this tool to enginemailer_add_subscriber and enginemailer_update_subscriber. No explicit when-not or exclusion, so it stops short of a 5.

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