List custom fields

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 fully covered by structured data. The description adds only a brief returns note ('The field names in use'), with no detail on pagination, ordering, or what 'in use' means. Useful but modest beyond the annotations.

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?

Four short, labeled lines with the purpose front-loaded and zero filler; each sentence carries distinct information (purpose, exclusion, return, timing). The bolded header scaffolding is slightly formulaic but costs nothing in length.

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-parameter read tool with no output schema, the definition covers what it does, what it returns, what it is not, and when to reach for it, while annotations carry the safety profile. Only minor gaps remain, such as the return shape or ordering.

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?

The tool takes zero parameters, so there is nothing to disambiguate and the baseline is 4. The description correctly implies a no-argument enumeration with no filtering options to document.

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+resource ('List the custom data fields in use across contacts') and explicitly distinguishes itself from the nearest sibling ('Not for: The values stored in a field, which is plunk_get_field_values'). An agent can separate this from plunk_get_field_values and plunk_get_field_usage without opening a schema.

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?

Provides explicit when-to-use ('Before writing a segment filter, so the field named is one that exists') and an explicit exclusion pointing at the alternative tool. Both the routing condition and the anti-pattern are stated, leaving nothing to inference.

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