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

Annotations already declare readOnly, idempotent, non-destructive, so safety is covered; the description adds real value by explaining what the returned field_value actually is (the key used in personalisation) and flagging that custom-field segmentation is currently UI-only. It omits pagination behavior and whether a total count is returned, but that is a modest gap against the annotation coverage.

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, front-loaded with the purpose and no filler. The trailing UI navigation detail '(Audience > Segments)' is slightly more than an agent strictly needs, but it earns its place by clarifying where the unsupported workflow lives.

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 returns — and it does, noting that each item includes its field_value key. Pagination semantics are left entirely to the schema, which covers them, so nothing critical for correct invocation 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 description coverage is 100% — page and limit (including the 10-100 range and default 20) are fully documented in the schema. The description mentions neither, so it adds nothing beyond the structured field data; 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?

Specific verb + resource: 'List the custom contact properties/fields defined in your account', with the returned payload characterized (each field's key, field_value). It is clearly distinguishable from create_custom_field / update_custom_field / delete_custom_field, though it never names those siblings 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 description implies the primary use case (fetching field_value keys for personalisation) and adds a scope caveat that segments on custom fields must be built in the Mailercloud UI. However, it never states when to call this versus the create/update/delete custom-field siblings, and the segment remark is a vague limitation rather than an explicit 'do not use for X'.

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