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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so safety is covered. The description adds real value beyond them: it discloses the exact return shape (id, caption, type with allowed values) despite no output schema existing, and cites the underlying endpoint GET /v1/lists/{listId}/fields. It stops short of mentioning pagination or empty-list behavior.

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 plus endpoint reference, with every clause carrying information (return fields, type domain, write-key role). The telegraphic fragment style with no leading verb is efficient but marginally less readable than a full 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 single-parameter read tool the description covers the resource, the returned fields with their type domain, and the practical purpose of the ids, which compensates for the absent output schema. Missing only error/empty-state behavior and explicit sibling routing.

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?

With one parameter and 100% schema description coverage, the schema already documents list_id, its pattern, length bounds and even cross-references sarbacane_list_lists. The description adds nothing specific about list_id, 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?

The description names the specific resource (custom fields/columns of a contact list) and enumerates the returned attributes (id, caption, type plus the type enum values), so an agent can tell it apart from sarbacane_list_lists or sarbacane_list_contacts at a glance. It lacks an explicit contrast sentence with those siblings, keeping it just short of a 5.

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?

'Field ids are the keys used when writing contacts' implicitly tells the agent to call this before create_contact/upsert_contact, which is genuine usage context. However, it never explicitly states when to use this versus alternatives (e.g. sarbacane_list_lists to obtain a list_id first) or any exclusions, so guidance remains inferred.

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