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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered by structured data. The description adds only the filter semantics and says nothing about result shape, default breadth of the unfiltered call, or pagination behavior (which lives in the schema).

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 tight sentences with the core action front-loaded and the filter guidance immediately following. Nothing is padded, though the second sentence could be slightly more explicit about the mutual-exclusivity or combination of the two filters.

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 simple, zero-required-parameter read-only list tool with no output schema, the description covers purpose and the two primary filter intents while the schema handles pagination defaults. Complete enough to invoke correctly, with only minor gaps in result expectations.

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, per_page, entity_id, and field_definition_id are all documented in the schema. The description restates entity_id's meaning slightly more concretely (all custom fields for a specific record) but adds no syntax or format detail beyond the schema, so the baseline 3 is appropriate.

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 ('List custom field values') and immediately explains the two filter dimensions, which separates it from the singular sibling get_custom_field_value. It does not explicitly name alternatives, but the resource and behavior are unambiguous.

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 when each filter applies ('to get all custom fields for a specific record, or by field_definition_id'), which is genuinely useful context. However, it never states when not to use this tool or points to the sibling list/get tools, leaving routing to inference.

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