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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds real context beyond that: the union-of-org-and-project return semantics when projectId is set, the trimming effect of scope='project-only', and the plan-feature prerequisite.

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 dense sentences with the core purpose front-loaded and no filler. Every clause (entity-type scoping, union/scope behavior, plan requirement) contributes actionable information.

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 zero-required, read-only list tool whose annotations carry the safety profile and with no output schema to explain, the description covers purpose, parameter interaction, and the gating plan feature. Missing only an explicit pointer to sibling tools for the neighboring definition/value operations.

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 the baseline is 3, but the description adds value by explaining how projectId and scope interact (union of org-wide and project-scoped fields unless scope='project-only'), which the schema only conveys per-parameter in isolation.

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 states a specific verb and resource ('List custom-field definitions') and scopes it by entity type (TASK/PROJECT/PROPOSAL), so an agent knows exactly what is returned. It does not explicitly distinguish itself from the sibling set (create_custom_field_def, update_custom_field_def, get_custom_field_values), though the 'definitions' vs 'values' contrast is implied by the name.

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?

It explains the conditional behavior when projectId is set and notes the PRO+ customFields plan requirement, which is useful precondition context. However, it never states when to reach for this tool versus alternatives like get_custom_field_values or the create/update definition siblings, 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.