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

The annotation already declares readOnlyHint=true, so the safety profile is covered. The description adds that the result set is page-paginated and discloses the underlying endpoint (GET /fields), which is modest extra context but no detail on default page size, ordering, or total counts.

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?

Three short sentences, zero filler, and the resource plus its optional narrowing are front-loaded before the pagination and endpoint notes.

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 paginated list tool, the description plus the readOnlyHint annotation and fully-documented schema give an agent everything needed to call it. It does not mention return shape, but with no output schema that is a minor omission for a simple list operation.

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%, so page, limit, type and keyword are all documented in the schema, including the filter[...] mapping and the type enum values. The description only restates the filtering idea, adding no syntax or format meaning beyond the schema.

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 (list) and resource (custom/default subscriber fields), plus what the returned set can be narrowed by. The resource is clearly distinct from siblings like list_groups, list_segments and list_subscribers, though no sibling is named as an alternative.

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 phrase 'optionally filtered by type or keyword' implies when the filters are useful, but there is no explicit when-to-use vs when-not, no prerequisite or scope guidance, and no named alternative among the many list_* siblings.

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